@ WORLD HEALTH ORGANIZATIONORGANISATION MONDIALE DE LA SANTE EXPERT COMMITTEE ON EPIDEI,EOLOGY OF ONCHOCERCIASIS BroLoGy, EcoLoGY AND VECToR CHARACTERTSTTCS 0F IASIS VECTORS OF IHE WEST AFRICAN SAVANNAS by ilippon, D. Qui116v6r6, Y. S6chan and B. Pendriez eas Scientific and Technical Research Office (ORSTOM) Paris, France oUd oNcrro/IiP/75.20 ORIGII{AL: FRENCII Geneva, 1O-18 November 1975 Provisional agetlle ileot !12 gt. L,ONq, F. l lLt l-i)_ \) oti'o 1 2 3 CONTENTS STI'DY ZONES . . IDEMITY OF ITIE VECfi)RS . . VECTOR DISIts.IBUTTON . 3.1 General distribution 3.2 Distribution of cytotypes . . IdECIDR BIOLOGY AND ECOLOGY . . 4.L Preimaginal bio-ecology . 4.2 Bio-ecology of the females 4.2.L Gonotrophic cycle . . 4.2.2 Daily biting cycle 4.2.3 Resting places 4.2.4 Movement 4.2.4.1 Dispersal . 4.2.4.2 Migration... 4.2.4.3 MovemenE and O. volvulus parasitism . . . 4.2.5 Longevity . . 4.2.6 Feeding preferences: zoophily 4.2.6.1 Disagreement between preimaginal population and that of biting females The issue of this document does not constitute formal publication. lt should not be reviewed, abstracted or quoted without the agreement of the World Health Organization. Authors alone are responsible for views expressed in signed articles. density Page 3 3 4 4 4 5 5 6 6 8 8 9 9 11 11 L2 13 4 13 I ( Ce document ne constttue pas une publtcation. ll ne doit faire I'objet d'aucun compte rendu ou rdsumd ni d'aucune citation sans l'autorisation de l'Organisation Mondiale de la Sant6. Les opinions exprim6es dans les artrcles signCs n'engagent que leurs auteurs. i I 0NCH0 /WP/75.20 Page 2 4.2.6.2 4.2.6.3 4.2.6.4 4.2.6.5 4.2.6.6 Numerical study of the relationship between feeding preferences and parasitism of females by O. volvulus Identification of blood meals of the fetnales Page 13 L4 L4 L4 t5 15 L6 r6 L6 16 L6 L6 16 L7 l8 19 L9 2T 27 a Presence of developing parasite origin in fernales . . Direct observation . Conclusions. larvae of non-human 5. VECTOR POTENTIAL 5.1 Factors 5.1.1 REFERENCES TABLES influencing the parasitism rates Factors connected with the parasite reservoir . . 5.1.I.1 Endemicity rate . . 5.1.1.2 Average microfilarial load of onchocerciasis cases 5.1.1.3 Man-vector contact 5.L.2 0 volvu1us cycle in S. damnosum s. l.; quantitative date . 6 7 5.L.2.1 Number of microfilariae 5.!.2.2 Parasite output . . 5.2 Vector/parasite complexes . . VECTOR POIEMIAL AND IRANSMISSION INTENSITY CONCLUSIONS . d . ingested 0NcH0 /r^rPl7 5 . 20 Page 3 1. STIIDY ZONES This document is a condensation of a large number of studies made over the last decade in different onchocerciasis foci of the savannas of French-speaking WesE Africa. Consequently, the zone considered stretches from easEern Seaegal in the r^resttothe Niger- Upper Volta frontiers in the east. Its southern border corresponds roughly to latitude 8o north (liroits of Ehe forest-savanna mosaic zone or pre-forest sector) while towards the north it reaches in places the 15th parallel, the extreme limit of the northwards spread during the rainy season of the simuliid vectors of onchocerciasis in West Africa. Proceeding from south to north, this bett covers the bioclimatic regions of the Guinea savanna, the Sudan savanna and the southern fringe of the sub-Sahelian zone. A11 these regions have in conmon a savanna or steppe type of vegetation where the densiEy of tree cover decreases considerably on going from south to north, I a climate characterized by a dry season alternati-ng with a rainy season whose duration gradually shortens from six Eo four months on advancing from the south towards the north, at the same time as the annual rainfall decreases. Apart from rainfall, the rainy season is characterized by high htrmidity and comparatively low temPeratures while the dry season can be divided into the cold dry season (winter, relatively 1ow night temPeratures) and the hot dry season (spring, very high day and night temperatures). The foci considered are located in the hydrological basins of the Niger (the Faya, Bamako, Bandiagara and Farako focl in Mali; the upper Bango6 foci in the Ivory Coast; the foci along the right bank tributaries of the river in Niger), the Volta (foci in the upper basin of the Black Volta, and along the Tansilla, Bougouri-Ba and White and Red Volta rivers), the Senegal (Kayes-Bafoulab6-Y61iman6 region in Mali), the Bandama (upper and niddle basin) and the Como6 (upper basin). 2. IDENIITY oF THE VECToRS A11 known West African vectors of Onchocerca volvulus be long to the Simulium (Edwardsellun) damnosum Theobald, 19O3, complex and to the Nile subgroup of this complex, as defined by Dunbar & Vajime (tglZ). Cytotaxonomic studies carried on for nearly 1O years (Dunbar, 1966, L969' 1973, L974; Dunbar & Vajime, 197L, L972; Qui116v€r6, L974, 1975) have all shown, infact, that S. damnosum comprises a complex of morphologically non-differentiable species which can be distinguished solely by the arrangement of chromosome inversions, at present onlydetectable in the giant chromosomes of the 1arva1 salivary glands. According to the recent classification by Vajime & Dunbar (L975), the eight potenEial vector species of nanely S. dannosum S. yahense and S. onchocerciasis defined by these authors are present in the zone considered, s.s., S. sirbanum S. sudanense S. sanctipauli, S. soubrense, S. squamosum, However, this specific terminology will noE be followed in the present paper, since the authors regard its adoption as prernature, in the absence of any possibility of breeding, crossing, and chromosomal or other identification of the adults. In line with Dunbar &Vajime (197L, 1972) and Qui116v6r6 (L974, 1975) rhe auEhors will continue to classify rhedifferent chromosomal rtentities" of the complex in chromosomal forms or rcytotypestt defined by a special sPectrum of floating inversions, some of which cytotypes may be regroupedinpairs in accordance with inversions they have in conmonl these pairs could rank as species. Thefollowing pairs will therefore be considered (see vajime & Dunbar, L975): thepairNile-sirba, 1 In partlcular, the gallery forests which are dense and continuous in the Guinea savanna, become disconEinuous and shrink in the Sudan savanna, to finally disappear more tothe north. dleguerense. oNcrro/I.IP/75.20 Page 4 formed by the cytotyPes Nile ( = S. daronosum Th.) and Sirba (= S. surbanum * S. sudanense); Banda.na (= S. sanctipauli)2 and Soubr6 the cytotypes Bi1le (= S. squarnosum) and 1 the pair Bandama-Soubr6, formed by the cytotyPes S soubrense ; the pair Bille-Yah, formed bY(= vah (= S. yahense); and Ehe cytotyPe Di6gu6ra (= S. dieguerense). At present very few morphological characteristics are known which would enable cytotyPes to be identified macroscopi.cally. It should be noted however that Yah larvae are characterized by reroarkably developed, sharp, spiny tubercles and an extremely dense cuticular covering of large elongated scales, so that, in particular, they can be differentiated by the naked eye from Bandama larvae whose dorsal tubercles are almost non- existent and whose abdominal scales are smaller, rounded and much less dense (qui116v6r6, L974; Garms & Vajine, L975). So far, no morphological or anatomical characteristics are known whereby the larvae of the other forms can be distinguished, but research is proceeding along these 1ines, making use, in particular, of the scanning electron microscope. 3. VECTOR DISTRIBUTION 3. I General dist.ribution Within the savanna belt considered, the S. damnosum complex i-s represented along a1I watercourses where the ecological conditions necessary for preimaginal develop,ment of members of the complex are found in combination (cf. Le Berre, 1966). The northern limit seems to be constituted by the absence of watercourses with a sufficiently prolonged and regular flow Eo maintaj.n stable preimaginal S. damnosum s.1. colonies (ptritippon & Balay, L967, L972). Since there are nuoerous temporary rivers in the savanna zone this northern limit varies considerably with the season. In the dry season it coincides with the northern limit of the perennial watercourses (from west to east: Gambia, Senegal, Baou16, Niger, Baning, Black Volta, middle lJhite Vo1ta, Oti, Niger); but in the rainy season the limit shifts 2OO to 4OO km northwards along the tributaries of the Senegal, Niger, Banki and Vo1ta rivers so that at several points it comes close to the 15th parallel (frontier between Mali and Mauritania, Bandiagara plateau, I"1a1i-Niger-UPPer Volta borders) (ehilippon et al. , L969a). It should be noted that although the bend of the Niger includes several areas of rapids it is free from S. damnosum s.1. which, in the north is not found beyond the approximate latitudes of Sa*ako on thJ ascending branch and Niamey on the descending branch. 3.2 Distribution of cytotypes The Di6gu6ra cytotype is known only from a few specimens coming from the breeding place where it was described in west,ern Mali (Bafing river)' The Nile-Sirba pair seems to be the chief representative of the complex in the savanna region and is present in all the foci previously listed (Vajime & Dunbar, 1975) as well as in Guinea and northern Liberia (Garms & Vajime, L974, L975). Moreover its distribution area spreads wetl beyond the region considered in the south along the rivers (Como6, Bandama' cavaI1y, Cestos, saint-John). The cytotyPe rePresented in the southern fringe of the S. sirbanum and S. sudanense are differentiated only in the female larvae (and not in an absolute manner) s egnent. 2 It ,horrld be noted that according to Quill6v6r6 (L975), Bandama and Soubr6 might form one and the same cytotype in the Ivory Coast, because of the frequency of heterozygous hybrids. 1 by the nature (standard or inverted homozygous) of the IS/3 chromosomal oNcHo/wP/7s. 20 Page 5 Guinea savanna belt, the pre-forest sector and the forest is essentially Nile, although the latter can also reach the northern limit of distribution of the complex; on the other hand, Ehe Sirba cytoEype seems Eo be predominant in the more northern savannas and, in particular, it is well represented in temporary watercourse zones; its frequency decreases in the Guinea savanna and iE becomes exceptional more to the south. Al1 maps of the distribution of members of the complex show that the Bille-Yah and Bandama-Soubr6 pairs are characteristic of the humid, southern regions (Va5ime & Dunbar, L975; Garms & Vajime, 1975; Quill6v6r6 & Pendriez, 1975b). They are, horvever, Present in the sout,h of the savanna belt considered (Guinea savanna) and in the Ivory Coast, Guinea and Liberia. In the south-east Ivory Coast, the distribution area of Yah, Bandama and Soubr6 coincides exactly with the map of the annual isohyets (qui116ver6 & Pendriez, L975b), with as a result, an upward shift of these cytotypes along the Guinea frontier towards the lOth para11el. Along the Ivory Coast rivers (Como6, Bandama) a more or less sPoradic upward shift of Bandama and Soubr6 seens to be more frequent and intense in the rainy season and can reach as far as the Ivory Coast-Upper Volta fiontier. In the zone considered the Bille cytotype is known only in the Gu6na region (south-west of Upper Volta, north of the Guinea savanna, about 11'N) through a small, isolated and extremely limited focus enclaved in the large zone at Nile-Cereba and following the upper Black Volta. 4. VECIOR BIOLOGY AND ECOLOGY 4.L Preimaginal bio-ecology The biology of preimaginal s darnnostro s.1. populations will not be dealt with here since it has been considered by many auftroiJ-(cE. WHO, 1973). Our knowledge concerni.ng the differen- tial biology of each member of the complex is at present zero, particularly as regards speed of development, food and respiratory requirements, movement, etc. The ecological fact.ors which govern (tolerable current speed, presence of available and acceptable suPports, quality and quantity of larval food carried by the current) and influence(hydrological variations, larval food, water temperature, vital competition and anthropic factors) the existence and balance of preimaginal breeding places of S. damnosum s.1. have been covered in detail by Le Berre (1966). Various recent observations are comencing to clarify the differential distribution seen in the savanna zone of the various mernbers of the complex. Thus, Yah is found exclusively along smaIl, fast flowing and shady \^ratercourses in wooded and hi11y regions, in some southern Guinea savanna zones (west and south-west Ivory Coast, northern Liberia, southern Guinea), and in certain forest sectors (Garms & Vajime, 1975; Qui116v6r6 & Pendriez, L975a); in this type of river Yah has never been found in symPatry with other members of che complex. 0n the other hand, Bandama, a characteristic feature of the large forest rivers (Garms & Vajime, L9741L975), is found in the same biotopes in the Guinea savanna when its distribution area shifts into that zone. Soubr6 shares the same biotopes (larval associations of both cytotypes are frequent on the sErme supports in theforest as in the Guinea savanna or the pre-forest sector) but it may also be found on rivers of average size in the Guinea savanna. In the Gu6na region, Bi11e colonizes average or sma11 juvenile, permanent watercourses which are clear and well oxygenated and flow on the Cambrian sandstone plateau. Ihe Nile and Sirba cytotypes (frequently associated on the same supports in the middle savanna zone) seem much more ubiquiEous and can make do with very varied types of lratercourses large perennial rivers flowing on the crystallophyllian base, small or average juvenile PermanenE rivers on the sandstone plateau, teoporary tributaries of all sizes, spillways of lakes and ponds etc. In the south of the Guinea savanna, as in the pre-forest sector and Ehe forest, Nile is often sympatric wiEh Bandama and Soubr6 in the large rivers. oNcHo /wP/75.20 page 6 At Di6gu6ra, where the cytotype of the same nErme was caught, the Bafing is a large river(upper course of the Senegal river) with a rugged course, which flows over a sandstone plateau with very scanty plant cover. Examination of the nature, depth and isolation of the supports at the micro-breeding places has so far not thrown any light on these ecological preferences. Study of the chemical composition of the v/ater seems more promising (qui1l6v6r6 & Pendriez, 1975a): in the Sudan or north Guinea savanna (ttiIe-Sirba biotopes) the water has a neutral or slightly basic pH and a relatively low organic matter content whatever the size of the river. In the forest, the pre-forest sector and the south of the Guinea savanna, smal1 waEer courses rising in these regions (Yah biotopes) have an acid pH (close to 5) related to the large amount of suspended organic matter, while the large streErms and rivers (Bandama and Soubr6 biotopes) originating in the savanna have an intermediate pH (lying between 6.5 and 7.2) as well as an average organic matter content because of the mixture of the Lraters of savanna tributaries and of those from more southerly zones. This evident relationship between the physico-chemical properties of the water and the larval localization of cytotypes might help to explain why Nile can be found both in the savanna and the forest, why Bandama and Soubr6 shift up into the Guinea savanna at certain seasons, and why the only sites where these two cytotypes have been caught on small water- courses (upper Sassandra basin, Quil16v6r6 & Pendriez, L975a) are situated in a sector where the pH of the r^/ater (6-Z) is slightly more acid than that of the rivers in the surrounding Guinea savanna. OEher factors, whether physico-chemical or not, must certainly be taken into considera- tion but this zoning of west African cytotypes in accordance with latitude must be compared with the zoning of the east African cytotypes of the same complex according to altitude, detected along Ugandan and Tanzanian rivers by Dunbar & Grunewald (1974) and related by these workers to a longitudinal variation in the physico-chemical properties of the water(uPstream -+ downstream increase in pH and SiO2 content, and decrease in CO2 and NH3 content) . 4.2 Bio-ecology of the females Under this heading only aspects liab1e to affect the dynami will be considered. csofO volvulus transmi-ssion 4.2.L Gonotrophic cycle The rhythm of blood meals is of course an important factor since, when considered in parallel with the duration of the parasite cycle, it governs the possibilities and intensity of retransmission of infective parasites by a biting population. Le Berre (1966) found that the length of the gonotrophic cycle is, on the average, four days in nulliparous and six days in parous S. damnosum s.1. females in l^Iest Africa. By studying the age of O. volvulus larvae naturally parasitic on a balanced population of S. damnosum s.1. the authors were able to confirm these data both in the Guinea savanna and in the Sudan savanna, I where the ratio between potentially infective parous females (those coming to take their third or subsequent blood meal) and all parous fernales agrees well with the survival tables given by Le Berre et al. (L964) (cf. Table l). It is possible that this duration may , at least in some seasons , be a day and a half shorter than shown in Le Berrers figures (1966). But the difference then observed may also be attribut,able to a longer average length of parasite development under natural conditions than under experimental ones. 1 oNCHO/WP/75. 20 Page 7 I{owever, this method does not give concordant results in the forest region, which seems to show either that the survival rate Ehere is higher than indicated by Le Berre et al. (L964)(cf. Sectiorr 4.2.5), that parasite development is more rapid there, or thaE the gonotrophic cycle is shorter there; the latter hypothesis is in line with observations made in the Cameroon forest by Duke (f968b) on the basis of a study of the average age of the Parasites(duration of three days in the hot season to four days in the cold season for each Sono- trophic cycle) and by Thompson (L974) on the basis of markings. Following many other workers, the authors have confirmed that the development cycle of O. volvulus lasts, on the average, six to eight days in the S. damnosum s.1. female in West Africa. Table 2 shows, however, that at comparable ambiant temperatures Ehis cycle is appreciably shorter in the forest region than in the savanna region, which confirms the observations of Duke (I968b) in the Cameroon forest; it has also been confirmed EhaE a fall in temperature helps to lengthen the cycle considerably. Nevertheless, it seems impossible in Ehe savanna region, no matter what the conditions of ternperature, bioclimatic environment or of a cytotaxonomic nature may be, for a complete parasite cycle to take place between the first two blood meals of a female; Ehe same is probably true in the forest, where the shortening of the parasite cycle is counterbalanced by a shortening of the gonotrophic cycle (Duke, 1968b). On the other hand, the hypothesis of a complete parasite cycle taking place between two subsequent and successive blood meals in multiparous females is plausible in the savanna under opEimal temperature conditions. A study of the regression of a population of biting females aft.er destruction of preimaginal breeding places by insecticides seems to confirm this in the Guinea savanna (Lu6na focus, Bil1e cytotype, dry season: cf. Table 3), since the infectivity rates increase considerably as from the moment when the population contains only multiParae; this increase cannot be explained by better vector potential of the old Parous females since the infection rates of parous females remain consEant. The size of the multiparous fraction concerned is very difficult to estimate, and it should be stressed that the s€une experiment when repeated in another Guinea savanna focus (L6raba, cold dry season) and in the Sudan savanna (Kouoro, hot dry season) did not reveal any significant increase in infectivity as from the third blood meal. In regard to drop in temperature, the second column in Table 2 corresponds to extreme but real condi.tions occurring every winter, although for a limited period in the savanna region. It appears that lor^r ternperatures considerably lengthen the duration of stages I and II of parasite development and retard the appearance of infective larvae although without causing any appreciable mortality among the parasites or changing the vecEor potential of the populations, since almost all females reach the infective stage before their third blood mea1. Finally it should be noted that the retention of egg masses, which could theoretically modify the intensity of onchocercal transmission by increasing Ehe length of the gonot,rophic cycle in relation to that of the parasite cycle, plays only an insignificant role: caEches of gravid biting females are reported only exceptionally, eiEher on human bait at breeding places and in the presence of very high densities of ovipositing fenales (Marr, I971, northern Ghana), or in very limited numbers in light traps (Odetoyinbo, 1970). The authors have accidentally recorded such catches, involving very small numbers, in Ehe Gui.nea savannain the dry season, at Poi.nts remoEe from productive breeding places and distant from any running water; in such situations the females are, moreover, able to make do with sEearnlets and currenE speeds very much less than those necessary for larval develop,ment. oNCHO/WP/75.20 Page 8 4.2.2 Daily biting cycle Le Berre (fSO6) established that, both in the savanna and in the forest, temperature is the principal factor limiting the biting activity of S. damnosum s.1. females: above 35-37'C bites become rarer and this is shown by a two-peak daily biting curve (one peak in the morning and the other more marked in the evening, separated by a midday trough) in the savanna during the dry season. Below this threshold the daily biting curve is unimodal, with a single afternoon peak: the case of the Guinea savanna and, more rarely, the Sudan savanna during the rainy season and in the forest at all seasons. There is also a lower threshold; the authors have never observed biting females below l8oC, temperatures encountered at the beginning of the morning in the Sudan and northern Guinea savanna zones during the co1d, dry season, and which delay the first bites to about 9 or even l0 a.m. Le Berre (1966) also noted that S. damnosum s 1. females react differently to tempera- ture variations according to physiological age: parous females are less affected by high temperatures and bite mostly in the morning and at midday, whereas the majority of nulliparae bite when the high midday temperatures fall, accounting essentially for the peak at the end of the afternoon. The younger nulliparae (sti11 containing meconium in the stomach) do not bite significantly more early than the older females. As regards parous females, the potentially infective orr.",1 and consequently themultiparae tendto bite later (especially at the beginning of the afternoon) than those which are not multiparous (primiparae), although among the latter the older ones ("old paraerr, determined by the condition of the abdominal fatty body and in parEicular of the l"lalpighian tubes) bite last. Horaever, since these o1d parae are always few in number, the general tendency is that the younger the parae the earlier they come to biEe, but the difference between the various age-groups is never more than two hours. These results differ from those of Duke (1968a) and Disney (1972), obtained in the Cameroon forest, which led the latter author to believe that the older the S. damnosum females the earlier they bite, \^rhatever their physiological age may be. By simultaneous catches on man and on donkeys in the sub-Sahelian region (Kayes, cf. 4.2.6) the authors were able to show that in this zone the hourly biting cycle of zoophilic females (parous and nulliparous) is much more regular than that of anthroPoPhilic fernales; anthropophilic populations are significantly older than zoophilic ones at midday (which agrees with the findings of Disney, 1972 concerning ornithophilic populations in the Cameroon forest) but this is not associated with any significant difference between the average ages of the two populations in view of the small numbers of females who come to bite at these hours of the day in such latitudes. 4.2.3 (a) Resting places Nature It has become a truism to say that the resting P Iaces of S. damnosum s.l. are practically unknown in the savanna region, for although various authors have reported catches of fasting or gravid females in low vegetation (Davies, 1962; Le Berre, 1966; Balay, 1968; Disney & Boreham, L969), high vegetation (Marr, 1971; Walsh, L972), deeP holes in rocks (Crisp, 1956; Ovazza et al., 1958), both near and far from breeding places, catches of engorged females during maturation of the egg masses are extremely small and exceptional (Disney & Boreharn, 1969; Marr, 1971). I quantity calculated from the ratio In/m, where n is the nurnber of infective females ( containing infective O. volvulus larvae) , m the number of infected females (containing developing al. , L972) . 0. volvulus larvae) and 1 the total nr:mber of parous fernales (Garms, in Duke et oNcHo/I^IP/75.20 Page 9 The auEhors have seen, c r one occasion only, considerable numbers (several hundreds) of engorged females resting during the rainy season in the sub-Sahelian region (Kayes), near large preimaginal breeding places, in shelters under rocks. This population included fasting females, gravid females and engorged females; the number of gravid fernales(engorged with sweeE juices) decreased at the end of the day (time of oviposition) but some of them sPent the night in the shelter. Developnnent of the ovaries and digestion of stomachal blood had at most commenced in Ehe engorged females, corresponding to a survival period of 24 hours, and they were plentiful in particular at the beginning of the morning and the end of the day. Such a site seems to be a temporary resting place where engorged and gravid fernales can stay for several hours and shelter during the night following the blood meal or preceding oviposition. But no more such si.tes were found, despite numerous searches in similar places; the site described was not positive during the following rainy season and it must be considered that the true resting places still rernaj.n to be found. (b) Resting places and nutrition other than blood Like many Nematocera females, those of S. damnosum s. l. feed on plant juices in addition to their periodic blood neals. It is generally agreed that this nutrj-tion is based on the nectar of flowers (Lewis & Domoney, 1966), but the method of biting or sucking is still unknown and Iutarr (L97L) has also seen parous, nulliparous and gravid females feeding on leaves in the Guinea savanna. Le Berre (1966) showed that the females always take a sweet juice meal before each blood meal but the findings of Marr (1971) and those described in (a) above seem Eo indicate that the females take more than one such meal per gonotrophic cycle. Such nutrition in the form of sweet plant juices may play an important epidemiological role since it can be taken that infective O. volvulus larvae are able to escape when theplant juice is ingested (bearing in mind tE;GE"-savanna they reach maturity several days before being retransmitted) which would correspondingly decrease the vector potential of infective females. A comparative study of the infectivity of surviving females and of wild feoales caught at the seme time on human bait before biting shows, taking into account that the mortality of the surviving feroales and the larvae they contain is 1ow, that this phenomenon plays an appreciable role in regions where the average parasite load of the females is high; however, its effect is practically unnoticeable in savanna regions where the parasite loads are low (cf. Table 3). 4.2.4 Movement A11 authors agree in recognizing that S damnosum s.1. fernales have a great power of fllght in lIest Africa; the maxiotrm distances recorded sometimes reach several scores of kilornetres (Le Berre,1966; Noamesi, L966). Distinction should be made between dispersive flights, which are acti.ve and not motivated by physiological needs, flights motivated by those needs (search for sweet juices, for blood meals, for oviposition sites; it may be noted in this connexion Ehat the observation rePorted under 4.2.3 (a) suggests that the resting female may move about to change the place of shelter or obtain sweet juices), and flights of a migratory nature, which are more or less Passive. 4.2.4.1 Dispersal Patterns The dispersive flight takes place before the blood meal, at each of the gonotrophic cycles (Le Berre, 1966). It is relatively rapid: Le Berre loc. cit. ) estimates it at less than 24 hours and, i.n TanzanLa, Hausermann (1968) observed, by means of marking, stretches of 5 km covered in six hours and 3.5 km in nine hours. oNcHo/wP/75.20 Page 10 Dispersal is influenced essentially by plant cover (in the forest movement is possible, but not in the savanna), cloudiness (in the absence of favourable plant cover its lack prevents dispersal, either of these two factors alone being adequate) and humidity, increase in which favours dispersal. By study of the alternate or combined effects of the first two factors Le Berre (f966) was able to define two types and different kinds of dispersal areas, depending on the West African seasons and bioclimatic zones. In the savanna plant cover is never sufficlent by itself to ensure dispersal excePt inside gallery forests. In the Sudan savanna dispersal is consequently linear (occurring only along wat.ercourses in gallery forests, when these exist) during the greater part of the year and only becomes radial (dispersal radiating in all directions from the water- courses) when cloud cover provides protection for the fernales (short periods during the rainy season). Further north the areas of localization of the females become smaller aud smaller and limited to the inunediate neighbourhood of preimaginal breeding places(ptrilippon & Balay, 1967, L972). In the Guinea savanna, dispersal is alternately linear in the dry season, for the sEutre reasons as above (the female then remaining confined to the gallery forests) and radial in the rainy season, because of the development of cloud cover ouEside the gallery forests. It should be remembered that, in the forest, dispersal is radial at all seasons because favourable plant cover is always present. Range The average range of maximum dispersal decreases from soutb to north in the West African savanna region. It may attain some 30 km in the Guinea savanna (Le Berre, 1966), t.his distance is halved near the 12o parallel and it does not exceed 7-8 km further north(ehilippon & Balay, L967). Here again, dispersal is linear since in any case, especially during the dry season, the females move preferably along the beds of wooded tributaries even if the latter are dry. Whatever the type of savanna, in the absence of linear dispersal, the densities of biting females very rapidly decrease as the distance from preimaginal breeding places increases, and become negligible beyond about 3 km. However, in the south Guinea savanna during the rainy season, the numbers caught may stiI1 be as much as 26% of. those found near breeding places 10 km away, and L4/" at a disEance of 20 km; these figures are close to those obtained by Le Berre (1966) in the forest zone. Dispersal according to ase of the females Le Berre (1966) stressed that nulliparous females are more strongly affected by dispersal, since their proportion in catches increases with distance from preimaginal breeding places. Duke (tgtS) obtained the same results in the Guinea and Sudan savannas of Cameroon and showed that the average physiological age of populations which disperse linearly is also lower than that of populatios close to breeding places; he also noted that among parae the older fernales disperse least, whether linear or radially. The authors did not find in the forest region the difference observed by Duke (1975) in Cameroon €unong forest populations, where tendency to disperse shown by parous females is relatively more marked than that shown by nulliparae. It would seern, rather, that forest females of all ages disperse Eo a considerable extent (distinctly. more than in the savanna) and that consequently the proportion of parous females remains relatively high (but always lower) as compared with that in breeding places, even at a condiderable distance from the latter, but that there is, however, a gradual decrease in the proportion of the oldest females as distance from the breeding places increases (cf. Table 5). It must be stressed, moreover, that the average age of populaEions that disperse linearly is difficult to establish because of the possibility that dispersive females may find places suitable for oviposition (even if these are unsuitable for larval development) and carry out their gonotrophic cycle on the spot in the absence of any indigenous population. oNcHo/'/JP/75.20 PaSe ll 4.2.4.2 Migration At present i-t seems that S. dannosum s.1. females are able to move over disEances as much as 150 ro 2OO km (Le Berre, 1968, I97O). Ihese flights are thought to be of a migra- tory nature and largely passive, being considerably favoured by Ehe prevailing winds. Probably they occur at a certain height (cf. catches of females in the interception Erap nearly IO m above the ground, Walsh, L972). In the savanna region there are two major migratory movements, one north-east/south-west in the dry season, connected with the harmattan, and the second north-south at the beginning of the rainy season, connected \dith Ehe norEhward shift of the intertropical monsoon front (Ovazza et al. , L967). This second migratory movement is the origin of the annual repopulation of temporary breeding places in the north Sudan savanna and the sub-Saheliatl zote (Lamontellerie, L964; Ovazza et al., L965; Le Berre & Balay, 1967) from permanent breeding places more to the south. Together with the harmattan it is responsible for the concentration of females at certain special points of foci treated with insecticides (case of Badikaha, on the middle Bandama, Le Berre, 1968) and may be involved in the seasonal shift of certain cytotypes from southern zones to the savanna region. 4.2.4.3 Movement and 9._ volllllus parasitism Despite results obtained by Lebied (1950) it has not been confirroed in the l^Iest African savanna that, by changing the structure of Lhe flight muscles, O. volvulus parasitism can reduce the flight range of S. darnnosum s.1. females The maximum distances from breeding places aE which infecEed females have been capEured in various Guinea savanna zones range from 12 to 20 km (Kirk, L947; Lewis, 1953; Le Berre, L966); as regards infective females, these distances lie between 7 km (Ouke et a1., Lg75) and 19 km (Le Berre, 1966). The authors findings in the Guinea and Sudan savannas indicated a dispersal of infective females limited to a maximum distance of 5 km, while infected females could reach 20 km. DesPite these relatively large figures the densities of fernales parasitized by O. volvulus are usually very small, even at a short distance from the breeding places, essenEially because the numbers of dispersive females are comparaEively small and their average age much lower than that of sedentary females. The infective females disperse distlnctly less than the infected ones, which seems to bear a relationship to their age rather than to the parasitism itself, for there is no difference bet\reen the averageparasite load of dispersive females and of sedentary ones. Ihe ratio: infective parae/ infected Parae can, moreover, be a good index for the average age of a population, provided that the man-vector contact is constant everywhere and that the numbers are sufficient. The situation is the same in the more southerly savanna regions where cytotypes may be involved (Bandama, Soubr6, Yah) able to tolerate much higher parasite loads than the cLassic savanna cytotypes (cf. Section 5 and Table 4). It follows from these observations that the dispersal of the females, although considerable, is not the direct cause of the spread of onchocerciasis foci and that usually new cases of human infection result from hurnan movement urithin the radius of action ofinfective fernales (near rivers colonized by S . da.urnosum s. 1 . ) rather than from the movement of infective fernales towards human comunities in the interior. It should, however, be considered that the dispersal of the feurales is the origin of the annual repopulation of the preimaginal breeding places which form during the rainy season on the temporary tribu- taries of the p4in watercourses of the savanna: in many onchocerciasis foci between the 8th and 12th paraIlels human beings keep completely our of the way of the simuliidpopulations of these rivers by maintaining an uninhabited belt sometimes a few dozenkilometres wide on each bank. However, in such circr:mstances the main part of onchocerciasis transmissi-on is ensured by temporary blackfly populations issuing from the first ones which settle during the rainy season on tributaries, in the innnediate proximity of human couunpnities. oNcHo/l^iP/75.20 page 12 4.2.5 Longevity The longevity factor is clearly of speeial importance from the epidemiological viewpoint since, taking into account the length of the gonotrophic cycle and of the parasite cyc1e, it governs the quantity of females which reach the epidemiologically dangerous age and can retransmit the parasite at each of their subsequent blood meals. Most auEhorsl agree with Le Berre (1966) that the maximum longevity of S. damnosum s.1. females does not exceed a month, corresponding to not more than five gonotrophic cycles. Le Berre et al. (1964) have shown that there is a gradual increase in the longevity of S. damnosum s.1. females from the south towards the north (from the forest to the Sudan savanna) and they have drawn up a table giving the survival rate of these fenales in terms of the parous/nulliparous ratio calculated in accordance with an annual cycle based on balanced populations (cf. Table 1). Le Berre (1966) explains these variations in longevity by the action of ecological factors (c1imate, vegetation) which promote maximum activity Ermong the forest females, whereas in the Sudan savanna dispersal activities that are much more restricted because of the unfavourable environment lead to maximum longevity. In the Guinea savanna there is an alternation of populations with longevity of the forest type in the wet season and populations with longevity of the Sudan savanna type in the dry season. The auEhors r/ere able to check Le Berre's data (1966) in the Sudan and Guinea savanna regions by determining, for balanced populations in different foci, the parous/nulliparous ratio and the ratio of potenLially infective females to total parous females (based on parasitism of fernales by O. vo!yq\g) (cf. Table 1); the average longevity seems to be highest in the most northern foci and it increases noticeably from the south towards the north in the savanna region. It should be noted that the Gu6na foci (Bille cytotype) does not differ in this respect from other Nile and Sirba cytotypes of the Guinea savanna. It should be pointed out, nevertheless, that the agreement becomes much less distinct in the pre-forest sector and in the forest. I^lhile various authors (Garms, 1973; Garms & Vajime, L974 Lo Liberia; Philippon et aI. , L97O Lr Zaire) have found very low parous rates in the forest, signs of a very small average longevity comparable to that established by Le Berre et al , (L964), other data show in Cameroon (Duke, f968b) and in lvory Coast (Yah cytotype foci in the south-east, Bandama and Soubr6 cytotype foci in the Pre-forest sector and the south-\^rest) that the average longevity in certain forest sectors may be of the same order of size as in the Guinea savanna. It therefore seems likely that, in addition to the effect of ecological factors, genetic facEors have some local effect. It should be stressed t.hat che life tables show an average survi-val rate and that the latter is subject to variations during the life of the female (probable decrease in relation to age). The nature of the factors influencing the survival rate is certainly complex (1ack of food resources or oviposition sites, role of parasites: Duke, 1968b; Garms, L973); in ection 5 the influence of 0. volvulus parasitism on the longevity of the female will be examined. It should be mentioned that while Mermithidae parasitism does not aPPreciably influence the gonotrophic mechanism of S. darnnosum s.l. females (Le Berre, 1966, L97L), it very appreciably reduces the longevity of the imagos: out of a female population 3O-5O% parasitized by Mermithidae and which has fed on an onchocerciasis case, mortality approaches 5O% on the second day after the blood meal, exceeds 75% by the fourth day and is close to 1OO% on the sixth day (figures which are reached in non-parasitized females or ones infected 1 ,Oran the exception of Noamesi (L966) who reports the caPture of marked females four months after marking; but the extraordinarily high proportion of marked females among those recaptured makes these results very questionable from the statistical viewpoint. S 0NcH0/wP/7s.20 Page 13 \,/ith 0. volvulus alone only on the fourth, sixth and tenth days, respectively), orre-half to two-thirds of the morEality being caused by the breaking out of the Mermithidae larvae. Ihe maximum survival period of the parasiEized females is not more than seven days but MermiEhidae parasitism does not prevent the developmen t of O. volvulus as far as the infective stage.(Results obtained in the Guinea savanna during the rainy season in the extreme north of the Ivory Coast: L6raba focus. ) 4.2.6 Feeding preferences: zoophily In West Africa, S damnosr:m s f. is generally regarded as a very anthropophilic species, because of its extreme aggressiveness Eor^rards man in many regi.ons. However, in various sectors a few scattered observations also show it to be zoophilic. 4.2.6.1 Disagreement between preimaginal population density and that of biting fernales A precise relationship is often difficult to establish, except when balanced and extremely abundant preimaginal populaEions exist and catches on human bait at the sarne place are almost completely absent. The authors found such disagreement in the sub-Sahelian region, in the north Sudan savanna (Senegal Valley, Phillppon et aI. , l97O; and Upper Garbia, Pendriez & Sdchan, 1971)1 and in the forest region (along the Cavally and Bandama rivers). Factors such as lack of sources of blood meals (Lewis, 1960) or the "dilution'r of females because of intense dispersal (Le Berre, 1966) cannot explain the phenomenon i,n any of these cases, but, on the other hand, it was subsequently proved that zoophily existed in several of these sites. 4.2.6.2 Numerical studv of the relati onshi p between feeding preferences and parasitism of females by O. volvulus The method consists in establishing, on the basis of the dissections of wild females and survivors of caught females engorged on onchocerciasis cases (both operations taking place simultaneously at Ehe same place), the theoretical maximum O. volvulus infection which can be expected in a biting population of females and comparing it with the actual infection rate of the rvild population. In this way the authors showed total anthropophily to exist among feroales in the Sudan savanna during the hot, dry season (Samanddni, Nile and Sirba cytotypes), in the Guinea savanna during the rainy season (L6raba, Nile and Sirba cytotypes dominant) and in the forest during the dry season (south-west Ivory Coast, Yah cytotype), although in all these cases there llas a possibility of choice between a human population and domestic and/or wild animal populations which were both abundant and accessible. Conversely, the various authors found that there was practi.cally complete zoophily in the L6raba Valley (Guinea savanna, frontier between Upper Volta and Ivory Coast), during thedry season. In this sector the valley was deserted by man many generations ago because of onchocerciasis, over a belt 15-20 ko wide on both sides of the river. The disease is hypdrenderuic and very serious in the villages bordering on this deserted belt, although theinhabitants never go towards the river because of the taboos laid by religion and custom on the abandoned zone. 0n1y a few foreign hunters and fisheruren frequent this deserted belt. In the dry season, the caPtures show that there is no contact between man and simuliids because the climatic and botanical conditions are unfavourable for dispersal of the females. However, caPtures made in the gallery forest of the L6raba reveal abundant and balancedblackfly populations which are very aggressive towards man, but whose survival, in the absence of autogenesis, can be explained only by blood nutrition derived from animals (at this spoE the wlld fauna is very rich in maurnals and birds). The 0. volvulus parasitism 1 In th" s€une zones, Balay (1968) had already reported the sarue phenomenon along the temPorary right bank tributaries of the descending branch of the Niger River. oNcHoA^rP175.20 Page 14 rate among parous females confirms this absence of man-vector contact, since it does noE exceed O.10 to 0.30 for infected females and 0.15 to 0.30 for infective females, raEes 40-80 times lower than those in a comparable zone frequented by man. In the same focus there are possibilities of dispersal during Ehe rainy season, so that these females colonize rapids forroing on temporary tributaries that fill up, and daughter- populations setEle in the inrnediate vicinity of first line villages; these blackflies behave very aggressively tor.rards man and suffice by themselves to ensure the intense onchocercal transmission which is the cause of the endemicity and seriousness already mentioned Gt. 4.2.4.3). Thus in such a region there is an alternaEion between anthropophilic dry season populations, which are zoophagous from necessity, and ant.hropophilic and anthropophagous :'ainy season populations derived from them (ehilippon et al., I969b). 4.2.6.3 ldentification of blood meals of the fernales Unfortunately it is difficult to apply this method to S. damnosum s.1. because engorged females are rarely found in nature. The few specimens of stomachal blood analysed (Boreham determination) among the authorst catches revealed the presence of blood of human and avian origin in the Sudan savanna. This confirms the results obtained in the forests using the same method by Disney & Boreha:n, 1969 (Cameroon) and Garms d Voelker, 1969 (Liberia). 4.2.6.4 Presence of developing parasite larvae of non-human origin in females Duke (1967a) in the Cameroon savannas and forest and Garms & Voelker (1969) in the Liberian forest observed filarial larvae of clearly non-human origin infecting S. damnosum s.1. females. In Liberia, four different types of filariae (some of thern certainly avian filariae) have been reported, and the infectivity rate due to some of these filariae can bejust as high as that due to O. volvulus particularly in the case of the Bandarna cytotype(Garms, 1973). In Cameroon, three types of filariae of animal origin have been listed by Duke (1967a), one of which (type O) is relatively abundant in the Sudan savanna. The adult filaria (and consequ ently the host infecting the S. damnosum females) could not be identified from the larvaI morphology in any of these cases. The authors found larvae of filariae of animal origin of different types in several bioclimatic zones (Tab1e 5); it follows from this table that these larvae are specially frequent in Sahelian and north Sudan zones as well as in pre-foresE sector and forest regions. They are less abundant in the Guinea savanna and rare in the Sudan savanna. The only focus observed where parasitism by animal filariae reached the same intensity as O. volvulus parasitism is the Inga focus (Zaie), where the results are comparable with those of Garrns(f973) in the Liberian forest. 4.2.6.5 Direct observation There are very numerous observations of S. damnosum s.1. females bitin g wild or domestic animals (exclusively mammals and birds), but in West Africa these observations seem much more frequent in the sub-Sahelian and north Sudan fringe (Balay, 1968; Pendriez & S6chan, 1971; Philippon et al., 1971b) and in the forest (Blacklock, L926; Le Berre, 1966; Garms & Voelker, 1969) than in the Sudan and Guinea savannas (Crisp, 1956; Crosskey, 1955). Outside the forest zones, the present authors have directly observed considerable zoophily only in the sub-Sahelian zone (Kayes region) where the females (Nile and Sirba cytotypes) vigorously bite cattle, donkeys and sheep, and, to a lesser extent, poultry. Systematic captures made aE the same time on man and on animals showed that, during Ehe same period of time, the females collected on donkeys hrere 2.5 to 40 times more numerous than those biting man while for bovines they were 4 to 5O times more numerous. The lower the global density of biting females, Ehe more and more relatively numerous Ehe females caught on donkeys, the distribution between the two baits seeming to be more equal in the oNCHO/WP/7 5 . 20 Page 15 case of a very great abundance of biting females. The hourly and daily variations in density were the same for man and animals and the differences between the two baiEs constant, whaEever the distance separating thern. The average physiological age and Ehe O. volvulus parasitism rates of the two samples of catches did not show any significant differences; feeding preferences are consequently not changed by age and the females seem to bite man or animals at each of their blood meals with equal avidity. 4.2.6.6 Conclusions It therefore seems that S. dnmnosum s.l. is not reall y exclusively anthropophilic in any West African focus. 0n the contrary, there does not seem fo exist in this region any focus where complete zoophily i.s associated Trith a definite genetic type as it is i.n East Africa(MacCrae, 1968, L969; Dunbar&Vajime, L972). While in the extreme latitudes of distribution of the complex (forests in south and sub-Sahelian zone in the north) extensi.ve zoophily mayprevail, the Sreater part of the savanna zones seem to be the domain of largely domj.nanE anthroPoPhily, although zoophily is found there everywhere, even if it is not always very aPParent (because of the difficulty in assessing its relative i.mportance) and may be locally dominant in case of necessity (zoophagy). In the extreme northern zones where zoophily becomes dominant even when there is a choice of host, it does not seem to be linked either Eo cytotyPes or to the physiological age of the fenrales. However, these characteristj-cs cannot be extended to the forest zones, where, in particular, the Banda.ma and Soubr6 cytotypes seem to be associated with a high zoophily rate (Garms & Vajiroe, 1975) and where Lhe possibiliEy cannot be excluded that zoophily may lead to a certain percentage of errorin ascertaining the average age of anthropophilic populations (Disney, L972). In a region such as the Kayes one considered above, zoophily is, in any case, the reasonfor a considerable drop in the intensity of onchocercal transmi.ssion potential (cf. Section 6) since the major part of the biting population (a large fraction of which is potentiallyinfective) does not come into contact with man, as confirmed by the epiderniological facts. 5. VECTOR POTEMIAI Ihe most direct way of assessing the vecEor potential of a population is to find whatfraction of that population is parasitized by lnfective O. volvulus larvae. Since nulli-parous females cannoE b fraction is based solel particularly i of trmorphologi- In the savanna regj-on the data are rather fragmentary and irregular as regards their mode of Presentation but indicate in general (when simuliid populations are close topreimaginal breeding Places and in close contact with infected human populations) rates forinfected Parous females, on the one hand, and infective fenales, on the other (the twofigures being usually fairly close) Iying between 1 and 157" (Crosskey, 1954, L956, L957;Lewis, 1958; Davies et al., L962 in Northern Nigeria; Lewis et al., 1961 and Marr, tgOS inNorthern Ghana; Lamontellerie, 1964, in the east of Upper Vo1ta; philippon et a1., 196g inthe Ivory Coast-Upper Volta frontier area; Philippon et al. , L97Lb in Northern Mali;Pendriez & S6chan, 1971 in eastern Senegal). The extreoe values seem to be 0.307. (almost complete absence of man-vector contact) and 25%, the mean being around 5% (philippon, 196g). For purposes of standardization, all females containing infective o. vglvulus larvaewill be termed'rinfectivet',1 while all feroales containing dlvetopi.rg ito.""ic larvae of the same parasite will be terraed rrinfectedrt and those containing infecEive and/or developinglarvae will be termedrrparasitizedrr. The authors regard as infect,ive larvae all stage IIIlarvae which have left the Ehoracic muscles and are free in the haemocoelom of the held, 1 The problem of identification of O. volvulus 1arvae 1S e parasitized and show considerable variations in time and space thisy on the parous females. n regions where the females are markedly zoophil someti.mes a difficulE one and, ic, it might be wise to speakcalIy non-differentiable O vo lvulus larvaerr. oNcHo/I,IP/75.20 page 16 thorax and abdomen, while a distinction is made between infective larvae and stage III intra- muscular, non-infective larvae without a functioning anus (Bain, 1969). 5. 1 Factors influencing the parasitism rates 5. 1. 1 Factors connected with the parasite reservoir 5.1.1.1 Endemicitv rate It is evident that the frequency of microfilariae carriers has a direct influence on the chance of females becoming infected with o. volvulus. In this respect the limiting figures for L6raba (Guinea savanna) may be mentioned: 0.307" of infective Parae when the man-vector contact is almost completely absent and, along the same river at the same season and at the same latitude, but a few score kilometres upstream in the Presence of a human population with an endemicity rate of close to lOO%, rates almost 4O times higher (Philippon et al., 1968). 5.1.1.2 Average micro filarial load of onchocerciasis cases This factor does not seem to play an important part in the Guinea savanna in view of the great reduction in the number of parasites ingested and the consequently very small average parasite load of the parasitized fernales (Philippon & Bain, 1972). The results are of the sr-e order in the Sudanese savanna (cf. Table 7). 5. 1. 1.3 Man-vector contact The dispersal pattern of the females at the different seasons in the various types of savanna foci fully explains why the infectivity rates of the females are extremely variable and, especially in the Sudan savanna, why those rates decrease very rapidly wit'h increasing distance from the preimaginal breeding places. This is, firstly, because at a distance from the breeding places these females come i.nto contact with human PoPulations Elmong whom the prevalence of the disease is much lower and, secondly, because the average age of dispersive females is less than that of sedentary ones. It is, however, also necessary to take into account the way of life of these populations and their movements; in the Guinea savanna' in particular, Ehe infection rates often rise at the beginning of the rainy season, a period marked both by increased dispersal of biting females and by a human population movement towards cultivated fields that are close to watercourses' FinalIy, it should be renembered that in certain circumstances Ehe facultative zoophily of the females can be an imPortant factor in reducing the man-vector contact and the parasitism rates. 5.L.2 0. volvulus cYcle in S. damnosum s.1.: quantitative data 5.L.2.L Number of micro fi lariae es ted The number of microfilariae ingested by a S. damnosum female is, both in the savanna and in the forest (Duke, 1962a, b) extremely variable from one female to another with reference to the same parasite source. It may attain several thousands and depends on many factors: (a) like Duke (1962b), the authors have noted a direct relationship between the microfilarial load of the subject and the quantity of microfilariae ingested, r^7ith low and average dermal microfilariae densities; (b) the horizontal and deep distribution of microfilariae in the skin, related to cutaneous onchocercal symptoms and the localization of the bites of the vector' is also an imPortanE factor; oNcHo/wP/75.20 Page 17 (c) there is a direct correlation between the number of microfilariae ingested and the volume of blood absorbed as well as the duration of the blood meal; (d) rhis number is independent of the physiological age of the engorged females; (e) the authors have not noted any hourly variations in this nr.rmber related toItperiodicityrr of the microfilariae or the biting cycle of the females. 5.L.2.2 Parasite output This output may be characterized by three ParEutreters: (i) the proportion of ingested microfilariae which reach the infective stage, a value which is very variable because of differences in the average number of microfilariae ingested from different onchocerciasis cases, but which lies between 0.5 and 10% in the case of savanna vectors and parasite carriers; (ii) the proportion of females parasitized when Ehey reach the infective age, Iying between 30 and 5O% both in the Guinea and the Sudan savanna; (iii) Ehe average parasite load of infected or infective females, which is about the same in both females kept in captivity and in wild females; i.e. slightly more than two. The last t\ro parnmeters depend, in particular, on the survival period of parasitized females and of developing larvae. The authors found, like Duke d Lewis (f964), that larval morEality during the parasite cycle is very low (about 1%), as well as that of mature larvae. There is thus practically no deparasitization and the reduction in the number of parasites lngested is caused mainly by the formation of the peritrophic membrane; the latter, which forms at a very early stage at the time of the blood meal, cannot be Penetrated by the microfilariae and only an extrenely small number succeed in passing through it (Lewis, 1953; Bain & Philippon, 1969). In the forest, Duke (L962a) has reported hypermortality emorg females ingesting high concentrations of microfilariae, and the authors oade the same finding in the savanna zone; however, only females which ingest very large numbers of microfilariae die (more than 10O0 to 15OO); in such cases the peritrophic membrane either does not form, or forms only incompleti:ly. Certain rare females presenting a few dozen intramuscular parasites after the migration of the microfilariae to the thorax also die before the forty-eighth hour. Subsequently, the mortality of parasitized females does not differ from that of Lhose free from parasites. As regards the values of Ehese three par€rmeters, the Bille cytotyPe focus of Gh6na is an exception, since the following figures are obtained there: 30.L%, 66.7% ar.d 4.7%, resPectively. These figures are of the same order as those cited for the Qsmeroon forest region (35 to 45% for the first parameter: Lewis, 1960; Duke, 1962b; Duke & Lewis, 1964; from 4 to 6 for the third: Duke, 1962b; Duke et aL., L972) and as those the present authors found in the forest and pre-forest sector of the Ivory Coast arld ZaLte (percentage of microfilariae reaching the infective form ranging from 30 to 507"; Percentage of positive feurales at the infective age, close to 9O%; average parasite load of infectious females between 6 and 8). These data explain Ehe sometimes high parasitism rates of Parous females (ruore than 107. Ermong infective parae; more than 20% among infected parae) which may be found in the forest and they agree with the average parasite load of females found in nature, namely an average of 8 or 9 developing larvae per infected female and 4 to 6 infective larvae Per infective female, figures which agree with those of Garms (1973) (about 6 in the Liberian forest). oNCHO/WP/75.20 Page 18 5.2 yegtor/parasite comp The experiments of Duke et a1. (L966, L967b) showed that O. volvulus strains from a given bioclimatic region can be transmitted only with difficulty, if at all, by females originating in another bioclimatic zorLe, and these authors found that two main vector/parasite complexes exist, one in the Sudan savanna and the other in the forest-pre-forest sector-Guinea savanna, while Iocal variants of the two complexes can exist in intermediate secEors or in zones of contac t. The present authors have found in similar experiments that: their parasites perfectly, in- females of each bioclimatic region clearly transmit accordance r^rith the numerical norms set out above; - females of the L6raba region (Guinea savanna, NiIe and Sirba cytotypes dominant) transmit the parasite of the Samand6ni region (Sudan savanna, same cytotypes), and v].ce versa in the same r^/ay as the parasite from their own focus; - females of the Gu6na region (Guinea savanna, Bille cytotype) transmit the Samand6ni parasite in the same r47ay as the Gu6na parasite (parasite outpuE of the forest type in both cases); - females of the Lamto region (pre-forest sector, Bandama cytotype dominant) transmit the Samand6ni parasite with the sa.me intensity (parasite outputs of the forest type) as the Lamto parasite; - females of the Wa region (forest, single Yah cytotype) transmit practically no Samand6ni parasites (the same onchocerciasis paEient as in the previous case); - females of the Nab6r6 region (Sudan savanna, Nile and Sirba cytotypes) also transnie practically no Wa parasites. In reality, in the t\^ro cases of incompatibility the phenomenon of elimination of the parasites can be broken down into two phases: the passage of the microfilariae into the haernocoelom takes place along the same lines and with the same intensity as in the case of a vector and parasite originating in the same focus, and it is only after leaving the haemo- coelom that practically all the microfilariae die, most of them before they have reached the Ehoracic muscles. The transmission (or its absence) of an O volvuIus strain by a g. dq4,nosrun strain therefore seems to be linked to two factors: the first is of a purely entomological nature and seems to depend on the way in which the peritrophic membrane is formed. The latter, whose developmenE is particularly rapid and which the microfilariae are unable to penetrate in the savanna, seems to form more slowly in the forest region where, moreover, it is frequently incomplete or less resistant. It can be surmised that this difference is of genetic origin (the ttile and Sirba cytotypes are thought to have a peri- trophic membrane of the first type and the Bandama, Bil1e and Yah cytotypes one of the second type) but additional studies are necessary to confirm this hypothesis. Ihe second factor is related to the possibility or impossibility of O. volvulus micro- filariae surviving and developing in a given strain of S. damnosum; it is certainly more complex in nature and thorough studies will be necessary to determine the part played by parasite strain, vector strain, and parasite-vector co-existence in these compatibility and incompatibility phenomena. It should be observed that in no case has the origin of the parasite strain seemed to affect the longevity of the parasitized fernales. oNcHo/wP/75.20 Page 19 6. VECTOR POTENTIAI AI{D TRANSMISSION INTENSITY Transmission intensity is estimated in terms of the number of infective 0. volvulus larvae received by man and by year (Duke, 1968a; Philippon et al. , L969c) on the basis of systematic, standardized and perlodie catches on human bait. This value is theoretical and excessive (since any catcher undergoes maximum exposure), is largely dependent on htrman factors and choice of catching site, and shows large inter-annual variations' Particularly in the savanna zone where the dynamics of simuliid populaEions largely depend on Ehe hydro- logical system and consequently on rainfall. Ilowever, even this concePt glves a sufficient approximation for comparative studies. This intensity depends on many factors, Ermong which the vector potential of the populations, governed in particular by entomological factors such as longevity, Iength of gonotrophic cycIe, dispersal, zoophily of females, and PotenEial larvae which they can harbour. load of infective O. volvulus On considering the life tables of Le Berre et al. (1964) it appears that Ehe ratio of potential infective females in the Sudan savanna to the forest ones is Sreater than 1OO which - all other factors being equal, moreover, and allowing for the ratio between the average parasite loads of infective females in the two zones (of the order of one-third) - ca1ls for biting female densities more than 30 times higher in the forest to obtain comparable transmission intensities in both savanna and forest. This would seem to explain the difference in seriousness between savanna and forest, but it is now known that enormous densities of biting females may be found in the forest zone, that the average longevity of females in that region may Iocally be considerably greaLer than that indicated by the data of Le Berre et a1. (1964), and thaE the infectivity rate of the forest females may reach very high figures. In fact, transmission intensities of several thousands and even several tens of thousands of Larvae/man/year may be encountered in the forest (Duke, 1968a; Philippon et al L97I; Garms, 1973), associ.ated with clinical feaEures which are nevertheless typical forest ones (rarity of serious eye lesions, almost complete absence of onchocercal blindness), whereas in the savanna region, where transmission intensity is related to the seriousness of the disease (fhilippon et al., L969c; Duke et a1., 1975), transmission intensities of the order of a few thousand larvae/man/year correspond to a serious clinical picture of onchocerciasis (high rates of grave eye lesions, blindness rate between 5 and 1O7.), and the threshold beyond which the desertion of the most affected onchocercal valleys cormence, may be fixed, under natural conditions, at 25OO-3OOO larvae/man/year. In the savanna region there are very strong annual varj.ations in the transmission intensity: in general it is maximal during the rainy season and minimal during the cold dry season; in temporary foci the rainy season is the sole transmissi.on season, and it may vary from four to seven months in length according to the latitude and the size of the rrater- course; when transmj.ssion intensities are equal the influence of this duration sti11 reroains to be determined. Consequently there is agreeuent betrileen the calculated transmission intensity and the observed gravity of the illness only within each of the two large bloclimatic groups: savanna on the one hand (sub-Sahelian zone, Sudan savanna and Guinea savanna) and forest on the other (pre-forest sector and forest). It is impossible to eroploy the sane scale of intensity for the West African bioclimatic zones as a whole. 7. CONCLUSIONS As regards entomological characterisEics, a special feature of savanna onchocerciasis is the relative homogeneity of the vector species complex since, apart from isolated andlimited foci and the overflows of forest forms towards the south, the associated Nile and Sirba cytotypes are practically the only vectors everywhere. oNcHo/wP/75.20 Page 20 Alrhough it is not yet possible to distinguish clearly between entomological characteristics of genetic and of ecological origin, respectively, these vectors show a longevity higher than that encountered arnong forest vectors, a mean longevity which gradually increases, moreover, from the south towards the north within the savanna belt. Dispersal is primarily seasonal and always much more limited than in the forest. These two characteri- stics result in the concentration of females with a very high survival rate near water- courses and they completely explain the focalization of the savanna foci as \"Iell as the clinical difference observed between riverine villages and villages some distance from vrater- courses, between the first line villages and second and third line ones' Ihe intrinsic vector Potential of the savanna females is the forest fernales. The latter seem much more itpermeableil to 1ow as compared with that of O. volvulus parasitism, while they withstand very well the complete developrnent of numbers of larvae significantly much higher than Ehose encountered in the savanna. As in many situations, the higher longevity factor in the savanna is insufficient to compensate both for higher vector potency and for the much greater abundance of forest females, so that transmission intensity is sometimes higher in certain forest sectors than in any savanna focus. fhis does not correspond to the difference in the seriousness of the disease in the t\^ro zones, where the savanna always predominates, since it is the only zone where catastrophic onchocercal blindness rates are combined with the desertion of the foci most affected by such blindness. While entomological criteria can explain many facts of local epidemiology they are inadequate by themselves to elucidate the o1d problem of the difference in clinical features(ophthalmological in particular) reported by many authors between savanna and forest onchocerciasis. To do this it is necessary to take into account the differential Patho- genicity of the parasite (or parasites) for man (in Particular for the eye) in the savanna ". "o*p.."d with the forest (Ouke et al. , L966; Duke & Anderson, 1972; Garner et al-, L973; Anderson et al., 1974a, b), or the man-parasite adaptation, which differs in the two zones. 0NcH0/I^tPl7 5 .20 Page 21 RXFERENCES Anderson, J., Fuglsang, H., Hsmilton, P. J. S. & Marshall, T. F. de C. (L974a) Studies on onchocerciasts in the United Cameroon Republlc. I. Coroparison of populations with and wiEhout Onchocerca volvulus, Trans. roy. Soc. trop. Med. Hyg., 68, 190-208 Anderson, J., Fuglsang, H., Haoilton, P. J. S. & Marshall, T. F. de C. (1974b) Studies on onchocerciasls in the United Cameroon Republic. II. Comparison of onchocerclasis in rain-forest and Sudan-savanna, Trans. roy. Soc. trop. Med. Hyg., 68, 209-222 Bain, O. 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(L972) Ecologle et dynanique dee populatlons de Slouliuo damnosutr Ttreobald l la 'limite nord de son aire de dietrlbutlon en Afrlque oc"iaerrtaffirrseq.r-ences 6pid6ulologiques, Proc. 13th lnt. congr. Entomology, Moecow 195g, J, 223-224 Philippon, 8., S6chan, Y., Chauvin, M. & Bernadou, J. (1968) Etude drune population deSimuliuu daonosum dans une zone lnhablt6e drun foyer dronchocercose de savane guindenne en saison sEche, Doc. OCCGE ron6o. No 70 , 10 pages Phillppon, B., S6chan, y. & Bernadou, J. (1969a) Map showing damnosum in l,Iest Africa (situation in October 196g) Phlllppon, B., Le Berre, R., Balay, entmologlques, drun mod}le ma 1 ronchocercose. Relatlons avec 9e Conf. techn. OCCGE, l, 232 the distribution of Simulium , documenr wHO/oNCHO /69.73 Phlllppon, 8., S6chan, y. & Dtallo, I. (1969b) Etude drune population de Slmuliuo damnosuur dans une zone d6sert6e par lrhome, Rapport final 9e conf. techn. oc.@--- G. & RoIland, A. (1969c) Mise au point, Aparrirdecrirares th6rnatlque traduisant lrintensit6 de la transmission de les manifestatlons cliniques. Applications, Rapport f inal Phillppon, B., Le Berre, R., Mawasa-Klratta, R. & Bantwanga, D. (1970) Lutte contre simulitm damnosuo eur Ie slte drrnga. Convention EDF-oRsTcD{. Rapport annuel I968-19G9,5iGieo. epFToRSio{, 40 pages Phllippon, 8., S6chan, Y. & Pendrlez, B. (1971a) EnquGt Cavally (COte drlvoire), Doc ron6o. OCCGE No6 e entomoOncho/71 Philippon, B., S6chan, Y., Pendrlez, B. & pangalet, p. (1971b) Co dronchocercose du baeeln du fleuve S6n6ga1 en R6publique du enqu3tes entomologiques, Doc. ron6o. OCCGE N' 138/0ncho 7L logique dans le bassin de , 13 pages ,ntribution b lr6tude du foyer Mali. R6sultats de quatre 18 pages Qu1116v6r6, D. (L974) Etude cytotaxonomique du cmplexe Slmuliuo dmnosum en Afrique occiden-tale. 3. cartes chromogomiques et r6partitloo geogrEllTlFiffir)rpea, Doc. ron6o. occ@, N" 79loncho/74, 41 pages Qu1116v6r6, D. (fS dr€tude. Iden(A parattre) 7. 5) Etude du cooplexe Simuliuo damnosun en Afri que de lrouest. I. Techniques 13tlflcatlon dee cytotypes, Cahiers ORSTCEI, 56r. Ent m6d. Parasit.. 0NcH0/wP/7 5.20 Page 26 Qu1116v6r6, D. & Pendrlez, B. (1975a) Recherches cytotaxononlques 8ur le cmplexe Sioulir:ro damnosun, Doc. ron6o. 0CCGE/C80. N" 16/oncho/75, 9 pages Qut116v6r6, D. & Pendrlez, B. (1975b) Etude du eomp texe Slmuliuu dannosuo en Afri que de 1 rOuest.II. R6partitlon gEographlque des cytotyPes en C0te drlvoire, Cahiers ORSI0}I S6r. EnE. m6d. Parastt. 13 (a parattre) ftrompson, B. H. (1974) The tlning of consecutive blood-meals ln Simuli"m dannosun, Proc. 3rd int. Congr. Parasitology, Yqqigh, 2, 929 Vajime, C. G. & Dunbar, R. W. (1975) Chrmoemal identification of eight species of che subgenus Edwardsellun near and lncluding liun Edwardselluo damnosum Theobald(Diptera, Simuliidae), Z. Tropenmed. Parasit. ,26,111-138 Walsh, F. J. 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(I, o cn >a a .r{ ql & h Ho ht{A h a .J q, c h E! .r.l ql d h tr .r{ q, & hl{o htr>a .r{ lrdad >| tr .r{ rtl& >' tr .r{ rt& h c .F{ dc E.gaoE>,io o, a o N o .'.{ +Jqld .r.l F{ o o .r{ Fq Na U) a U)z (h(A z U) U)z IA TA U) U) U)() u)p{ tq fr{ h k (,l o&h +J o!h(J a + 'r{z G. s. .r{ U) + .r.l z .r{ a o u) + .F{(h + ..{ z .F{ z + .0 Fq 6 q, )r + (u Eq + .F{ rA + 2t oa + q, Fq G. .r{ o oh o oh(u Y r0, E\(u F{qt Er{ d .r{ ,.o Ho t{ ooAp rd ',{oa ql(, o t{ !d .r{ E .F{ a!0, €td E u) ot{! o M d tt, o ro, l{(, o +.1 H FI oN ao F{A o '{) oH tll 'r{F o H ..{ rtlN (u @ aH \o trl Flla4H t oNcHo/wP/7s.20 Page 32 TABLE 7. REDUCTION IN NIMBER OF PA.RASITES INGESTED (saMRNosNr, DRY SEASoN) a Average number of mf ingested by engorged female Proport.ion of females parasitized bY Lhe 7th day Average parasite load of infective fernales Onchocerciasis case 1 232 L8/4L = 43.9% 2.7 Onchocerciasis case 2 L7 33/83 = 39.7% 3.4 It
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Biology, ecology, and vector characteristics of onchocerciasis vectors of the west African savannas
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