Geographic distribution and epidemiological aspects of onchocerciasis in French-speaking West Africa

IWORLD HEALTH ORGANIZATION ORGANISATION MONDIALE DE LA SANTE oNcHo/wP/75.2 ORIGINAL: FRE{CH EXPERT COMI'IITTEE ON EPIDEI'fIOLOGY OF ONCHOCERCIASIS Geneva 10-18 November I 75 Item 2 of the provisional agenda GEOGRAPHIC DISTRIBUTION AI{D EPIDEMIOLOGICAL ASPECTS OF ONCHOCERCIASIS IN FRENCH-SPEAKING WEST AFRICA by J. J. Picq Servi.ce de Microbiologie Institut de M6decine tropicale du Service de Sant6 des Arm6es Marseille, France 1. INTRODUCTION This paper atteopEs to give a brief account of the geographical distribution and epidemio-logical aspects of onchocerciasis in French-speaking West Africa, in other words in an area bounded by longitudes 13'E and 17'W and latitudes 5o and 15'N and including Dahomey, Guinea, the Ivory Coast, Mali, Niger, Senegal, Togo and Upper Volta. Ihe subject matter is based on data from two sources: studies by the author of this PaPer and studies by other workers. The studles urade by the author of this paper were carried out from November 1968 to fuLy 1974 in the Parasitol-ogy Section of the Centre Wraz at Bobo-Dioulasso, Upper Volta, under the auspices of the Organisation de Coop6ration et de Coordination pour 1a lutte Contre les Grandes End6mies (OCCGE). During this time 13 surveys were conducted in 20 endemic foci of onchocerciasis in West Africa. This series of surveys had the advantage of being carried out by the srme team using the same methodology each time. With regard to the studies by other workers, information has been taken froro the surveys conducted by Puyelo & Holstein (1950), Jehl (1966), I(niittgen & Bijrrner (1968), Rolland &Balay (1969) and Lamontellerie (1972) and, of course, from the mass of facEs given in the imporE.ant I,IHo (1973) report on onchocerciasis control in the Volta River basin area (ocp/73.1). 2. METHODOLOGY OF THE SURVEYS 2.1 OCCGE surveys These surveys represent E.he joint work of the Parasitology Section of the Centre Muraz at Bobo-Dioulasso and an ophthalmological team from the Institut dtOphtalmologie tropicale enAfrique (I0TA) at Bamako, Mali. The methods employed have been described elsewhere (picq, 1971) and only the main points will be described here. The villages to be included in the surveys were chosen by a sampling system taking account of the hydrogeographic aspects of the region in question (with the assistance of the out- standing working tool represented by the L/2OO OOO maps prepared by the Institut G6ographique National). In general, five criteria or epidemiological signs were taken into consideration: I 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 rn signed articles Ce document ne constttue pas une publication. ll ne dort farre I'objet d'aucun compte rendu ou r6sumd ni d'aucune citation sans l'autorisation de I'Organisatron Mondiale de la Sant6. Les opinions exprim6es dans les articles sign6s n'engagent que leurs auteurs. oNcBo lwP /7 5.2 Page 2 Persons Persons Persons Persons Persons with wir.h wirh wirh with positive skin snips; onchocercal nodules; eye lesions caused by onchocerciasis; severe eye lesions caused by onchocerciasis; blindness caused by onchocerci.asis. I A quantitative skin snip technique was used (Picq, Coz & Jardel, 1971; Picq & Jardel, L974) to assess the severity of onchocerciasis in the individual and in the cormnunity. A number of skin snips from each focus were stained for morphological study of microfilariae and, in particular, to look for the possible presence of Dipetalonema perstans microfilariae. As a result of work done by Buck et al. (1971) tvro surveys also looked for Onchocerca volvulus microfilariae in urine samptes. A11 results were set out and analysed by age-group, sex, vi11age, focus and endemic zone. The age-groups taken were those proposed by a WHO expert comnittee (L966). In all, 13 surveys were carried out in four different bioclimatic zones: the Sahel zone, the Sudan savanna zone, the north Sudan savanna zone and the forest zone, in six OCCGE Member States (Dahomey, Ivory Coast, Mali, I'trauritania, Senegal and Togo). A total of 13 geographical regions, 20 d,ifferent foci and 208 villages were surveyed; 32 L24 subjects underwent clinical and parasitological examination; 14 768 positive subjects (with onchocerciasis) were also given anophthalmological examination; L2 48O skin snips were carried out. 2.2 Other surveys Puyelo & Hotstein (1950) carried out a mass survey in the Wtrite and Red Volta River basins in eastern Upper Volta, mainly in the Tenkodogo circle, where 102 188 persons were examined for onchocercal nodules, skin lesions, severe eye disorders and blindness. Ihe survey teams did not have an ophthalurologist with them, but in 1957 the ophthalmologists Vei11eux, Le Breton- Oliveau and Aubry (1960) examined 34 L7O persons from 150 villages in the same area. Jehl (1966) and his team examined over 30 0OO subjects in nearly 2OO villages in Upper Volta, in the basin of the Bougouri-Ba river, a tributary of the Black Volta. The methods used in this study, which gave useful results although also carried out without an ophEhalmo- logist, comprised a systernatic search for persons with nodules, the taking of skin snips from persons without nodules and an exami-nation for skin lesions and blindness. Rolland & Balay (L969), basing their work on preliminary surveys aimed at finding persons with nodules, carri.ed out in the Bisa focus, Upper Volta, on the banks of the White and Red Voltas, a c1inical, parasitological, ophthalmological and entomological survey of nine villages with a total population of L264. These authors also studied the socio-economic and human impact of onchocerciasis in this region. KnUttgen & Bijttner (f968) carried out an extensive survey in north-eastern Guinea in an area of the Niger Basin covering approximately 7O OOO km2; 18 634 people were exami-ned in 194 villages. A11 subjects were systernatically examined for onchocercal nodules and for the presence of microfilariae in two skin snips, one from the shoulder and the other from the buttock region. Visual acuity was also tested buE no assessment of eye lesions was made. The survey carried out by Lamontelleri-e (L972) studied four human filarial infections. It covered an area of approximately 10 7OO km2 in south-western Upper Volta and included the Como6 and L6raba rivers; 59 O82 people living ia 147 villages were examined for onchocerciasis. A search was made for onchocercal nodules and for microfilariae in skin snips, and cases of blindness were noted. The team did not have an ophthalmologist. Cj.CHO/wP 175.2 Page 3 3. EPIDEMIOIOGICALASPECTS The mass of data produced by these varlous surveys has yielded a consistent body of general features co[trDon t,o endemic onchocerciasis in West Africa. lhese general features will be considered first, and then detailed analysis will be rnade of individual features specific to a number of focl studied by the authors mentioned above. 3.1 Geographical dlstribution Although l^rithout. any intention of trespassing on the domain of the specialized entouologist, it ls necessary here to recaIl that transmission of O. volvulus frcm rDan to man is necessarily carried out through the intermediary of an insect vector, the b1ackf1y. In West Africa the only known vector species is Siruulium damrosum , which rnay be a species coroptex as cytotaxonomic studies will show. Itre growth cycle of the blackfly passes through four stages: egBr larva, nymph and adult insect. The first three stages are aquatlc. Blackfly larvae require four main conditions to be provided by their sites: r,rater flowing with a speed of between 0.5 and 2 m/second, natural or artificial supports, water rich in dissolved oxygen and water containing particles of organic material in suspension, on which the larvae feed. Experience shows that in tropical Africa these conditions are often found together not only in Permanent watercourses but also in temporary rdatercourses or rrmarigotsrr regardless of their6ize. As a result, larval sites of all slzes are widely distributed and rnay occur either as single sltes or, most frequently, in lines extending along kilometres of river. From theselarval sites the adult insects spread out over a radius ranging from several hundred metres to several kilometres. Occasionally they travel for some tens of kilometres and colonize other watercourses. Watercourses that form stagnant reaches or are even dry for part of the year are veryquickly recolonlzed at the start of the rainy season from breeding sites on permanent rrater- courses. At any rate, the epidemiologist studying endemic onchocerciasis in human populationsfinds that many foci of extremely variable size and importance are scattered throughout the whole of West. Africa in both the savanna and forest zones. In West Africa the northern limit of endemic onchocerciasis may be taken as latitude15oN. Ihis is in fact the line of demarcation between the Sahel zone and the norEh Sudan savanna zone. In places with less than 5OO to 75O rrn rainfall a year no onchocerciasis isfound aPart from rare exceptions involving very sma1l foci. It should be noted that outside West Africa the most northerly focus known in Africa occurs in Abu l{amed in the Sudan, atlatitude 18"N (Anderson er a]-., L974). Roughly speaking, south of latitude 15oN the density of endeoic onchocerciasis varies withthe density of the drainage network. wtrere Ehe drainage network opens out, onchocerciasisdensity is 1ess, the foci smaller in area and hyperendemic zones less frequent.l It is alsoto be expected that along shorelines where the speed of the current is unsuitable for blackflybreeding sites, enderric onchocerciasis will be less conrncrn or even absent. 3.2 Occurrence in foci (focal ization) Within watercourses such sites. this enormous area of distribution, onchocerciasis occurs in foci surrounding Ehelike sleeves. Each focus has at its centre a blackfly breeding site or line of 3.3 Balkanization of the endemic Foci vary greatty in size and in severity of the disease. They uray comprise a hundred orseveral thousand people and may be separate, contiguous or confluent depending on the hydro-geographical features of the region concerned. The whole of West Africa is covered by a 1 This shows up fairly clearly in Figs.43 and 4r+ in reporr ocpl73.r (wHo, 1973) I oNcHo/wP/7s.2 Page 4 chequerboard of foci and endeoic onchocerciasis is well and trulyttbalkanizedrr, to use a colourful term. Focalization and balkanization are characteristic features of the disease and explain why general figures for a region or country as a whole can only give a very approxi- mate or even a false idea of the endeuric onchocerciasis situation. 3.4 Stratification of degrees of endemicity The occurrence of strata (stratification) in the distribution of the various degrees of endemicity \rithin a focus further accentuates the balkanization of foci. The position of the various strata are governed by their disEance from the blackfly breeding sites, as this distance determines the frequency of contact between man and vector. In the case of large blackfly breeding sites, the strata along the edges of watercourses are highly endemic, or hyperendemic; hypoendemic strata with low endemicity mark the outer limits of the focus, and strata of medium endemicity or mesoendmricity lie in between. Hyperendemic strata vary considerably in width, ranging from 1 to 10 km depending on the size of the blackfly breeding site, and may even be missing. For this reason it may be preferable to use the terms first, second and third line villages to designate villages with endemic onchocerciasis in relation to their position with regard to breeding sites without makinB any prejudgenent as to their leveI of endemicity (Ro1land & Balay, 1969). 3.5 Cumulative aspect of onchocerciasis Ihis is an essential feature and makes onchocerciasis an rraccumulation helminthiasisrt. IE is demonstrated particularly cl-early in hyperendemic areas by a gradual increase in the prevalence of all epideuriological signs in going from the youngest to the oldest age-groups. Ttris phenomenon shows up clearly in a study of the tttypical epidemiological cross-sectionrr, or average epidemiological cross-section, of a hyperendecric village (Picq, Lor6a1 & Jardel, L972). Moreover, objective proof of this cumulative aspect is provided by the study of mean micro- filarial densities by means of the quantitative skin snip technique. Such densities gradually increase over the years in hyperendemic villages. Three main factors explain why onchocerciasis should be the typical example of an traccumulat ion he lmint.hias is rr : (i) Repeated infection (particutarly intense and frequent in a hyperendemic area); (ii) The long life of O. volvulus; (i.ii) The low degree of effectiveness of the inrnune resPonse. The severity of onchocerciasis, particularly in the savanna zone, seems to depend in each individual on the microfilarial density and on the length of time for which it has been supported. The higher the endemic Ievel the earlier onchocercj.asis aPPears and the greater the filarial density. Ihe cumulative nature of onchocerciasis in an area of high endemicity has been noted by nany authors, in particular by those who have tried to make a quantitative assessment of the infection such as Duke & Ibore (1968), Buck et al. (1969), Picq, Coz & Jardel (L97L), Picq, Ior6a1 & Jardel (L972), and Picg, RoIland & Richard-Lenoble (1974). 3.6 Desertion of the valleys In West Africa, particularly in the savanna zoie, valleys are often thinly populated and may even be uninhabited. This is evident from an examination of the ]-l2oo ooo maps PrePared by the Institut G6ographique National. IE must be adnitted, however, that the degree to which onchocerciasis is responsible for this phenomenon renains in dispute. I ol.tcHo/wP/75.2 Page 5 For example, as Remy (192o) has noted in the area upst.ream of the Red Volta, the villages nearesE to the rivers are being abandoned in places where onchocerciasis is only mesoendemic or even hypoendemic. In some foci of the forest type like that in the Aplahoue region(Picq et al., 1974) the area within a 7 km radius of the line of breeding sites is empty of villages. It should be noted irr passing thaE the blackfly menace is noE ruled out from having something to do with this situation, for there are no cases of blindness due to onchocerciasis. In the same way, onchocerciasis is found in Central Dahomey in the Bassila region (ficq C Jubin,1970),which has a very low population density. However this low densiry is probably explained by Ehe fact that its past history has made the area unsafe, since this corridor is a traditional zone of passage between North and South Dahomey and between Ghana in the west and Nigeria in Ehe east. The question of how far onchocerciasis has been responsible for the desertion of the valleys has been discussed by many authors, demographers, sociologists and pedologists as weIl as epidemiologists. Cu611ar (1972) quotes Hilton, who considered that six main factors affected the population density in northern Ghana: slave raids, wild animals, trypanosomiasis, onchocerciasis, other diseases, and soil erosion and exhaustion. The last factor, according to llilton, was perhaps the rnost important. Rolland & Balay (1969) in their study of oncho- cerciasis in the Bisa focus, Upper Volta, noted that before L946 forced recruitment of labourfor the building of the railway was an important derrcgraphic factor. The fact that oncho- cerciasis and uni.nhabited areas are found together, or that there is a negative correlation between the size of a village and the number of blind people in it, is not an unconditional argument for considering onchocerciasis as the main cause of desertion of the valleys (WHO, L973). Actually, the phenomenon must be viewed as a dynamic process and considered in all its asPects. As Remy (197O) has noted from observations made in Upper Volta: ttSeveral factors explain desertion of the valleys and stem from various aspects of the natural and human envj-ron- ment. The absolute or retative importance of each factor may have changed in the course of time. The combination of factors may have differed from one valley to another. The gradual desertion of the valleys and the emergence of vast uninhabited areas may have assisted the rapid development of new factors detrimental to human life or activity, either as a result of the increase in wild life or disease vectors, or as a result of a feeling of insecurity developing €rmong the riverine population. The state of equilibrium that existed between the hnman inhabitants and their environment was gradually destroyedtr. Bradley (L972) has views very close to this. He has written about a study carried out in Nigeria that rrdesertions are characterized by a number of interconnected factors . . . Onchocerciasis is part . . . of a dynamic desertion complex . . . It would appear that one of the critical ways in which onchocerciasis is contributory to village desertion is through its catalytic effect on factors already in operationtt. In short many contributory factors, among thenr onchocerciasis, have played and sti11 play a part in the desertion of African valleys. However, onchocerciasis may well be the main obstacle to repopulaEion of deserted riverine areas in Ehe savanna. AttemPts at repopulation are generally half-hearted and made by numerically small human groups, which will then be exposed to the full force of endemic onchocerciasis as a result of a favourable blackfly to man population ratio. It is noteworthy that the examples of villagesin decline always refer to very small groups of people. Ihus Rolland & Balay (L969) reported the geographical decline of the village of Yakala, which started out with 127 inhabitErnts. Rolland (L972) had described the fate of St. Pierre, a repopulated village of 5O inhabitants. To return to Bradleyrs analysis (L972), onchocerciasisrtdoes not appear to act exclusively in the form of a rpusht, driving away numerous victims of the disease, but it also deters the continuation or development of village lifert. Ihis explains why Hunt.er (L966) as quoted by Cu611ar (L972) has been justified in putting forward the hypothesis of a cyclic advance and withdrawal of villages, caused mainly by rtriver blindnessrr. Another important factor noted in Particular by Waddy (quotedby Rolland, L972) is the isolation of hyperendemic villages, which are nearly always at the end of a track or not on a track at all. oNcHo/wP/7s.2 Page 6 To sum up, although onchocerciasis in West Africa has been only one of several factors causing depopulation of the valleys it is probably the most i,nrportant obstacl-e to repopulation of hyperendemic areas in the savanna, Depopulation and repopuiation are two different phenomena with different dynamic Parameters. 3.7 other asp ects of onchocerciasis epidemiology 3.7.L Village size and level of endemicity In the savanna zone, village si.ze varies with the level of endemicity. This has been reported by authors who have a fairly extensive experience of onchocerciasis epidemiology. Hyperendemic villages are generally villages with less than 4OO, and most often less than 200, inhabitants. This is clearly illustrated in reporL OCP/73.1, where it is mentioned on page32 that the analysis, presented in Fig. 49, shows the association between village size and prevalence of O. volvulus infection in L757 conmunities from which basic data were available. Villages \.rere grouped into three categories, namely those having between 40 to I99 inhabitants, those with a population of 2OO to 699 persons and the remainder exceeding 7OO residents. The graph demonstrates that 22% of the smallest conrnunities had prevalence raEes in excess of 5O%, as compared to 1o7. of the medium sized villages and only 2% of the larger towns (I^IHO, L973). Lamontellerie (L972) has noted that in the Como6/Leraba focus as endemicity fa1ls the inhabitants are grouped into larger and larger villages. The hyperendemic cantons held 7.05% of the population and the hypoendemic cantons 47%, aLthough their surface areas were practically the same. Of the 147 villages surveyed only three out of 35 (87") in the hyperendemic area had more than 5OO inhabitants as against 39 out of 52 (75%) in the hypoendemic area. In particular, in a survey carried out in I,IaIi in the Yanfolila region (Picq & Aliou Ba, L97O), lying along the frontier with Guinea, where there were three foci of different degrees of severity lying along three different \n/atercourses, we found the largest villages to be in the focus with the lowest endemicity. 3.7.2 Cases of blindness Several figures may be put forward. Where t.he prevalence of bl-indness in a village exceeds 17" this represents an abnormal state of affairs with regard to eye disorders. In a hyperendemic savanna area where onchocerciasis is responsible for most cases of blindness, the term hyperendernicity may be used when the prevalence of blindness exceeds 3%. In villages wiEh very high endemicity the first cases of blindness appear €rmong subjects aged from 18 to 19 years and sometimes 1ess. In some villages with a very high level of endemicity, the prevalence of bl-indness may reach i2-13%, which means that in the over 30 age-groop 20% are blind. In a mesoendemic area the first cases of blindness appear towards the age of 40. These facts may warrant a further splitting of the age-groups proposed by WHO (1966). The age-groups to be added would be 20 to 29 years and 30 to 39 years. 3.7.3 Differences between the sexes Men are generally more affected by onchocerciasis than women. This difference between the sexes in the epidemiology of onchocerciasis has been noted by many authors. It sometimes appears in the figures for the prevalence of persons with positive skin snips and persons with nodules. This happens in the north Sudan savanna zone, for example, in the Bafoulab6 and Kayes regions lying along the Senegal river in Mali (Picq, Rolland & Richard-Lenoble, 1973). However, in most cases in the savanna zone, a clear-cut difference between the sexes appears only in the figures for the prevalence of persons with benign and severe ocular lesions and cases of blindness (Rolland & Balay, 1969; Picq, lor6al & Jardel, 1972). o\tcBo/wP /75.2 Page 7 Tiris difference also occurs in Ehe forest zone. ObJective proof is provided by the study of average microfilarial density in the savanna zone (Picq, LotlaL & Jardel, L972) ar.d in the forest zone (Picq, Rolland & Richard-Lenoble, L974). The difference appears not only in hyperendemic areas but also in mesoendemic areas (Picq, IarEaL & Jardel, L972). Sometimes, a difference between the sexes only appears after a given age; this has been dercnstrated €unong others by Plouvier & Lafaye (1975) in the Nikky focus in Dahomey where the flgures for the prevalence of persons with positive skin snips were the sErme up to 14 years of age and then diverged. Girls and boys have rcre or less the sasre occupations untll they are 14, when the girl begins her period of sexual activity. This is the age at which first pregnancies occur and when men and women take up different activities. The division of labour is generally recognized as the reason for the difference in infection rates between men and lromen. Such division of labour has been well documented in the Bisa region by Rolland & Balay (1969). When villages are very close to the breeding sites, differences in the prevalence of infection in t,he two sexes leve1 out, as happens in the Kandi region in northern Dahomey (Plouvier & Lafaye, L975). 3.7.4 llicrofilaruria The presence of O. volvulqs microfilaruria in onchocerciasis patients was shown by Buck et a1. (1969) i.r-iffiffir, ctaa. This is a phenomenon of general occurrence and has been found both in the savanna and forest zones (ficq C Roux, 1972, L973). Ttre percentage of onchocerciasis patienEs that spontaneously carry roicrofilariae ln their urine varies from region to region, with the case-finding techniques used and with the authors concerned. However, this percentage rises with the level of endemiclty and reflects the increase in micro- filarial density in the individual patient, since the most heavily infected patients have a Sreater chance of having microfilaruria (Buck et al., 1969). Recent work on this topic has been sunmarized in the WIIO monograph on onchocerciasis (Buck et a1. , L974). 3.7.5 Skin conditions The skin conditions caused by onchocerciasis are numerous: pruritis, prurigo lesions,ttcraw crawtr, lichenoid changes, senile atrophy of the skin and hypopigurentation. However, only depigmentat,ion, in the form of characteristic macular lesions appearing generally on the lower limbs, is of proved epidemiological significance. Such depigmentati-on indicates a high 1eve1 of endemicity in villages with many such cases (Lo to 25%), In the savanna zone, moreover, depigmentation is the rule among cases of blindness and severe ocular lesions caused by onchocerciasis. 3.7.6 Systemic effects In the savanna and in areas of high endemicity systemic effects and below norrnal heights and body weights are known to occur in severe cases infected at an early age, but their place and significance in the epideuriology of onchocerciasis stil1 ramains t,o be det,ermined. In the Bisa focus, Rolland & Balay (1969) used a very simple health indicator, the body weiSht, to aPProach the problem. They compared Ewo groups of adult males and femeles, oneborn in a hyperendemic and the other in a hypoendesric area. There were significant differencesin the body weights observed, the average weight being lower in the hyperendemic area. Buck et al. (1971) reported that the height/weight ratio of patients with microfilaruria was lower than that of patients r"Iithout it. Patients with microfilaruria were found to have higher microfilarial densities (determined by counting the microfilaria in skin snips). oNcHo/wP/75.2 Page 8 3.7.7 Location of fietds Although some small urban (or more properly suburban) foci do exist, such as the oncho- cerciasis focus described by Rolland (1972) in the Bamako area, onchocerciasis is primarily a rural and agricultural disease contracted in fields and plantations. I'he location of fields and plantations in relation to blackfly breeding sites is of considerable importance. Only two points relative to this problem will be made here. Some families in villages are more affected than others because the members of a given family work in the same fields or in neighbouring fields. In the same way, Buck et al. (L969) observed that the highest. frequency of microfilaruria was found among particular families. Again, some riverine areas are at times described as uninhabited because they are without villages although there are often fields of crops there or even hamlets occupied seasonally for agricultural purposes. Ihis explains the high 1evel of endemicity in some villages several kilometres distant from the breeding sites but whose inhabitants come to work near the breeding sites on a temporary basis. 3.7.8 Graphical and rrmathpmaticalrr representation of epidemiological data In many cases onchocerciasis follows strict quantitative laws. However, the population sample must be large enough for analysis of the daEa by age-group and sex, and then by family, village, endemic stratum and focus. Graphs, histograms and various curves seem then to be very significant. The graphs given in reporr OCp/73.1 (WHO, L973) taLl inro rhis class. The graph showing age specific prevalence of infection, blindness and two categories of impaired vision in a hyperendemic area (OCp/73.1, Fig. 22) confirms observations made by other authors. Good examples are also given by the graph of the percentage of blindness in relation to percentage of positive skin biopsies (OCP/73.1, Fig. 47) and the relationship between population size and prevalence of onchocerciasis (OCP/73.1, Fig. 49). As we have seen, Ehe results given by study of quantitative skin snips are particularly useful and provide object.ive proof of the cumulative nature of onchocerciasis in a hyperendemic area. In a recent study, Plouvier & Lafaye (L975) proposed a simpLified mathesratical model for the analysis of epidemiological daEa on onchocerciasis in a hyperendemic focus. They noted Ehat the prevalence by age of persons with positive skin snips and persons with nodules described a sigmoid curve in cartesian coordinates, indicating a double exponential function. This function may be used for detailed analysis of the epidemiological conditions prevailing in hyperendemic areas and in particular for characterizing the degree of homogeneity in the conditions of infection for the different age-groups. According to Ehe authors, the function may also be applied to the prevalence of eye lesions and the variations in averagemi.crofilarial dens ity. 4. BIOCLU'IATIC ZONES AI.ID EPIDE{IOLOGICAL ASPECTS A number of general features may thus be discerned in the epidemiology of onchocerciasis namely focalization, multiplicity of foci, stratificaEion of the degree of endemicity and the cumulative nature of the infection in hyperendemic areas. However, nothing is ever perfecEly simple in epideuriology and onchocerciasis shows clear epidemiological differences from one bioclimatic zone to another (see Table 1). There is an epidemiology typical of the savanna and an epidemiology typical of the forest. In West Africa, the different bioclimatic zones generally follow the parallels of latitude, and between latitudes 8oand 15oN onchocerciasi-s may be said to be of the savanna type and below 8oN of the forest type. Ttris contrast betv/een savanna onchocerciasis and forest onchocerciasis appears most clearly in a hyperendemic area. There are no differences between the prevalences of positive cases, persons with nodules or persons with benign ocular lesions (punctate keratitis), but such differences appear quite distinctly in the prevalences 0NcH0 /r,rP /75 . 2 Page 9 of cases of severe eye lesions and blindness. In the savanna zone, the proportion of severe eye lesions is very high and there are many cases of blindness; in the forest zone, the proportion of severe eye lesions is very low and there are practically no cases of blindness. These differences cannot be explained, as was logically expecEed at first, by differences in the rates of infection of persons living in the savanna or forest zone (Budden, 1963a). In savanna and forest regions with comparable endemicity levels, no differences are found in the average microfilarial densities of the people living there, sometimes indeed such densities are greaEer in the foresE zone (Picq, Ior6a1 & Jardel, 1972; Picq, Rolland & Richard-Lenoble, L974; Picq, Rolland & Roux, 1973). At our present stage of knowledge, the differences between savanna and forest. pose a real epidemiological pv,zzLe and we are reduced to the following \dorking hypotheses : different strains of parasite; differences associat,ed with nutritional factors; i.rrnunological factors, different timing of infection cycles (in the forest zone the transmission season lasts longer); the existence of an O. volvulus-Simulium complex, but this brings us back to the question of st.rains I differences in luminosity between savanna and forest. Duke (197 ), in animal transmissi.on experiments, tried to find whether a difference existed between strains. Incidentally, there appear to be no morphological differences between O. volvulus microfilaria from savanna or forest zones. Diet in savanna and forest zones is not the seme. Ilowever, study of the part played by nutrition seems to have been restricted to a search for a few vitamin deficiencies. A paper by lJoodruff et a1. (1963) shows that no significant differences were found in the vitamin A content of the sera of persons with or without eye lesions. Raoult (quoted by Rolland & Balay, 1969) at the end of a combined nutritional andophthalmologicalsurvey in the savanna and forest zones concluded that there ldas no evidence that any specific nutritional disorder had a major influence on the appearance of eye lesions in onchocerciasis. There was a connexion between vitamin 812 and niacin deficiencies and onchocercal eye lesions. In the savanna zone, vitaruin deficiencies of the pellagra and ariboflavinosis type were more frequent in valleys urith hyPerendemic onchocerci-asis and among onchocerciasis patients than other groups. In addition to onchocerciasis of the forest and savanna types, an onchocerciasis of the rrnorthern limitrr or rrnorthern Sudanrr type may also be distinguished (Picq, Rolland & Richard- Lenoble, L973). this epidemiological aspect may be regarded as a variant of the savanna type and occurs approximaEely between latitudes l3oand 15"N, i.e. between the 9OO and 600 nun isohyets. Here the hyperendemic strata are very narrow, forming bands 2 to 3 km wide along the edges of watercourses. Ihe highest 1evel of endemiclty found is at the lower limit of hyper- endemicity for savanna type onchocerciasis. Furthermore, the prevalence of severe eye lesions and blindness are lower than in the heart of the Sudan savanna. In addition, the differences between the sexes with regard to onchocerciasis show up quite clearly. These epidemiological asPects seem to be associated with environmental factors and with a changed behavi.our of the vector, which shows in particuLar some zoophilic tendencies. These are not purely speculative considerations and discovery of the reason for the differences between savanna and forest type onchocerciasis may have very important practical consequences. For the ismediate future, itr is necessary to know where to set a reliable limit to the regional onchocerciasis control prograrure. Who can say for certain whether exposure to different ecological factors will cause onchocerciasis of the forest type to take on the characteristics of savanna type onchocerciasis. 66 oNcHo/wP /7s.2 Page 10 5. TRAI{SITIONAL FEATURES Although only a rough outline of the epidemiology of onchocerci-asis in West Africa has been given, it represents quite an accurate picture of the actual sEate of affairs. Obviously, however, variations occur, aflrong them, such as transitional features. Thus in the Man region of the Ivory Coast on the banks of the Sassandra river, we found epidemiological features that could be interpreted as marking a transition between savanna and forest types(Picq, Rolland & Roux, 1973). In the Bamaf6 region along the banks of the Bafing river, features are found indicative of the transition from savanna zone to northern limit zone(Picq, Rolland & Roux, L974). However, these remarks apply to West Africa, where there is only one vector species and where geographical relief is fairly uniform and bioclimatic zones are quite distinct. As far as Africa as a whole is concerned, epidemiological variations from one region to another may be even more marked. Anderson & Fuglsangrs paper (1973), which compares several foci in differenE geographical regions, makes this clear. SOME FOCI IN WEST AFRICA (see map) .1 Foci in the savanna zone Bisa, Upper Volta one of the foci that have been known for longest is the White Volta focus in Upper Volta, known as the Bisa focus. Its centre lies on the intersection of latitude lIo3OrN and longitude O'30\,J, in the heart of the Sudan savanna zone. The waEercourses irrigating the area, the Red Volta and more parEicularly Ehe White Volta, are not permanent. Between t.he studies carried out by Richet in 1939 and Rolland & Balay in 1969 many successive surveys have been made in Lhis focus, among them thac of Puyelo & Holstein (1950), in which the principal features of the epidemiology of savanna type onchocerciasis were set out, namely: focalization, stratification of the degree of endemicity, and significance of the prevalence of blindness. Thus the situation in the cantons of the basins of the Volta rivers was contrasted with that in cantons eLsewhere. In the Volta basins, the prevalence of persons with nodules varied, depending on the canton, from 20.6%to 7O.3%, and the prevalence of blindness from 3.4 to 6.2%. In cantons outside the Volta basins, the prevalence of persons with nodules varied from O.6 t.o L7.3% and the prevalence of blindness from 0.1 to 1.97.. Rolland & Balayrs sEudy brought the epidemiology of thls long standing focus up to date. Ihese workers gave a more detailed description of the eye lesions seen and classified them; in addition, they analysed results by age-group, proposed the concept of first, second and third line villages, drew attention to the differences in prevalence between the sexes, to the influence of the division of labour and to the systemic effects occurring in hyperendemic zones, and finally attemPted to form an idea of the social and economic repercussions of onchocer- cias is . Bo i- Ba er Volta The focus on Ehe Bougouri-Ba river, Upper Volta, was surveyed by Jehl (1966) and appears to be a typical savanna type focus. This worker, however, found that some hyperendemic villages had a high prevalence of positive subjects in the one to three-year-o1d age-group. Ihis feature was also found in the Sansann6-Mango region in northern Togo (picq, Jubin & Plouvier , 1974). Comod/L6raba, Upper Volta Ihe cenEre of the focus studied by Lamontellerie (L972) lies on latitude lOo3O'N. This south-western Part of Upper Volta, bordering the Ivory Coast, receives an annual precipitation of the order of 1o0o-130o rmn. The Como6 and L6raba rivers are permanent watercourses. Ihis focus also shows the general features of a savanna type focus, such as an uninhabited zone oNcBo/wP /75.2 Page 11 along the edges of watercourses and stratification of the degree of endeoicity. Ilowever, this author notes that the northern part of the survey region, stretching from east to west and formed by the upper reaches of several rivers, raises the specific problem of how Simulium spread from one watercourse to another since the distribution patt.ern of the infection does not foIlow the rivers but the contour lines. We ourselves feel that the contraction of the drainage network in this area is enough by it.self to explain this particular distributionpattern. Lamontellerie considers that this focus has expanded recently and is not yet stabilized., Be that as it may, the overall prevalence arpng the 59 O82 subjects exnqined was 32.4%. onchocerciasis was found in 146 of the 147 vlllages visited; the prevalence in these 146 villages varied from 9 to 89.6%. A complex of foci in Guinea. 1 and Mali The region surveyed by Kniittgen & Biittner in Upper Guinea along the border with l'lali on the east (Bougouni Region) lies below latitude 11'N in the heart of the savanna zone. Ttrisfocus includes the upper reaches of the Niger river and its tributaries, one of which is the Tinkisso river. The whole of this region may be considered as an area of endenic onchocer- ciasis. The prevalence of persons with microfilariae fel1 below 5% Ln only one of the 197 villages surveyed. The mean prevalence of persons with microfilariae arpng the 18 634 persons examined was 45.8%. This area also shows marked focalization with Dany scattered foci of varying size, and stratification of the degree of endemicity occurs as a function of distancefrm a \^Tatercourse. In the vicinity of a junction of two rivers Ehere is mtrch hyperendemicity.For example, this survey found this to happen in the case of the junction between the Tinkisso and Bani6 rivers. Ttre prevalence of blindness was L.o77. in 55 hypoendeuic villages and roseto 4.56% in hyperendemic villages. Incidentally, these authors propose an index for evaluationof the prevalence of infection in a village. This is the A I 50 index, defined as the age-group in which at least 50% ot persons with microfilariae are found. Ttris region of endemic onchocerciasis is part of a larger complex and is bordered on the north-easE and east by foci in Mali, such as those in the yanfilila region, on the north-west.by the Haute-Gambie focus in Senegal, on the south by foci in Liberia and on the \ilest by otherfoci in Guinea belonging co the coastal drainage network. The region of eastern Senegal, lying on latitude 12oN, has two onchocerciasis foci, onewith its centre lying on a loop of the Ganrbia river and the other stretching along the Fa16meriver, a tribuEary of the Senegal river. Forty-nine villages in all were surveyed (picq,Ior6al & Jardel, L972) and onchocerciasis found everyvhere although at different degrees ofendemicity. Depending on the viIlage, the prevalence of persons with microfilariae variedfrom 16.6 to 86.1% and the prevalence of blindness froo o.2 to 11%. Ttre focus on the loop ofthe Ganbia river is an almost Perfect example of the epidemiology of savanna typeonchocerciasis.In their survey of this focus, the authors made a study of the iypical or Eean epideniologicalcross-section of a hyperendeoic village and also put forward a definition of mean microfilarialdensity. The Yanfolila region in southern l,Iali along the border with Guinea lies betweenlatitudes 1o'45r and 11'35'N. rhis region r{as surveyed by picq & Aliou Ba (1970) and is ofinterest because it is crossed by three different watercourses, the sankarani, ouassoulou Bal6and Baou16 rivers, which give rise to three adjacent foci of very different size. onchocer-ciasis was found in the 20 villages surveyed but the degree of endeoicity was very varied, theprevalence of persons with microfilariae varied from 6 to 837. and the prevalence of blindnessfrom O.2 to 8%. Togo: the Lama-IGra, pagouda and Nantougou triangle In central Togo the Lama-I(ara, Pagouda and Nantougou triangle (Roux, picq dl Aliou-Ba,1972) lies between latitudes 9'and lo'N. Although the main rilatercourse is the Kara river, theterrain is fairly uneven and ls crossed by a network of suall rilatercourses, sorne of which arePermanent. Endemic onchocerciasis is found everyuhere but is very difficult to classify. oNcHo/r{P /75. 2 page 12 In the 14 villages surveyed, the prevalence of persons with microfilariae varied between 24 and 84% and the prevalence of blindness between 0 and 6.17.. Ttre survey included the exami- nation of urine in six villages for the presence of O. volvulus microfilariae; the prevalence of microfilaruria varied from 0.83 to 6.6% and accorded with the degree of endeuricityobserved. Sansann6-Idango, Togo In northern Togo the Sansann6-Mango region (ficq C Jubin, 1973) is crossed by rhe basin of the Oti and Koumango rivers and lies below latitude 10"N. Onchocerciasis was found in the 17 villages surveyed (Picq, Jubin & plouvier, 1974). The degree of endemiciry was fairly uniform since the prevalence of persons with positive skin snips vari.ed in each village between 46 atd 76%. However, the prevalence of blindness ranged from 0.6 to 5%. Ihe prevalence of the disease seemed to be high among children aged from 1 to 9 years living in hyperendemic villages, but this might have been caused by the children becoming infected during the dry season when accompanying their mothers to the rrmarigotrr to fetch vrater. the T6hini-Bouna region in the northern Ivory Coast (picq et al., L974) forms a narrow strip bordered on the north by the frontier with Upper Volta and on the south by a huge, completely uninhabited forest and game reserve. A distinction lEust be made between the Bouna focus, which ls watered by the Black Volta and tsro of its tributaries, and the T6hini focus, watered by the Iringou river flowing through its centre and flanked on the west by the river Couo6. A notable feature of these two foci is the size of the uninhabited areas bordering the watercourses and the variable degree of onchocercal endemicity. Onchocerciasis was found ln the 12 villages surveyed in the Bouna focus, the prevalence of t.he disease varied from 17 to 84% arad the prevalence of blindness froro 12 to O.2%. The village most severely affected lay near the banks of the Black Volta. In Ehe T6hini focus, the prevalence of the disease varied from O.3 to 7% and the village rnost severely affected Iay on the banks of the Como6. Korho Coast The Korhogo region in the northern Ivory CoasE lies on latitude 9o3OtN and is the country of the Senoufo people. Ihe area surveyed by Picq, Jubin & Plouvier (L974) was enclosed in a loop of the White Bandama river which at this point turns through three-quarters of a circle. In the 1O villages surveyed the prevalence of the disease varied fron 17 to 76% and the prevalence of bllndness fron O.2 to 4.7%. I.Iajor development projects, incLuding the consEruction of many dams and the provision of piped irrigaEion, will radically alter the siEuation lilith regard to endemic onchocerciasis in this region and lead to a spontaneous regression of the disease. Bassila, Dahomey The Bassila region in central Dahomey forms a narrow corridor between the north and the south of the country and lies between latitudes 18'3Or and 9oN. It is crossed by a network of small tenporary liratercourses and has a very 1ow village and population density. Onchocer- ciasis was found in the seven viltages visited, the preval-ence of the disease varied from 35 to 62% and the prevalerrce of blindness from 2.L to O.2%. The area as a whole is mesoendemi-c but its epidemiological- type is difficult to classify. 6.2 Foci of the forest type Ihe centre of the focus in the Aplahou6 region lies along a short line of blackfly breeding sites on the river lbno, whi.ch at this point marks Ehe frontier between Togo and Dahomey. Ihis focus lies on latitude 7oN and provides a perfect example of the epidemiology of forest type onchocerciasis. In hyperendecric villages, a very 1ow prevalence of severe eye lesions and a total absence of blindness of onchocercal- origin contrasted with a very high prevalence of persons with microfilariae, nodules and punctate keratitis. On the other hand, T6hini-Bouna, Ivory Coast 0NcH0/wP/75.2 Page 13 a very significant feature was the exceptionally high mean microfilarial densities. Adetailed study of this focus was carrled out by Picq, RoIland & Richard-Lenoble (1974) and is discussed elsewhere. Ihe Danan6 region in the northern part of the south-western Ivory Coast lies betweenlatitudes 7olot and 7'35rN and is bordered on the west by Liberia and Guinea. Ttre drainage network is qulte dense. The main watercourses, the Cavally and the Nyon, generally flow in a nort,h to south direction. Onchocerciasis was found in the 12 villages surveyed, theprevalence of persons with microfilariae ranged from 36 to 43%. The prevalence of severe eyelesions hTas very 1ow and there were no cases of blindness of onchocercal origin in five of the six villages visited by the ophthalmologist, while the prevalence of blindness was 1.1% in the remaining village. Although mean microfilarial densit.ies were lower than in the Aplahou6 region, Ehey remained high in hyperendemic villages. Ttrese characteristics of savanna type and forest. type foci would need to be studied andtheir presence confirmed in other parts of Africa. Budden (1963a) acknowledged the differences betlileen onchocerciasis in savanna and forest zones and recalled Lhe work of a number of authors who have noted the same phenomenon, one of the first observations being made by Murray (1955)in Nigeria in a forest area. According to Budden, this difference in the severity of the two epidemiological tyPes is associated with a 1ow prevalence and a low density of microfilariaeln the cornea and the anterior chanber of the eye among patients in Ehe forest zone. He considered this low microfilarial density ln the eye to be itself associated with a low micro-filarial density in the skin. Anderson & Fuglsang (L973) compared the features of endemic onchocerciasis in two hyper- endemic villages, one in the forest zone and the other in the savanna zone of Cameroon. Differences between savanna and forest were found in the prevalence of sclerozing keratitis, which was distinctly higher (L2%) in the savanna zone than in the forest (less rhan 1%), andthe prevalence of microfilariae in the cornea, which was 73% in the savanna and 40% in theforest. Flrrthermore, the average number of microfilariae per milligram of skin (skin snip inthe buttock region) appeared to be higher in the savanna village, i.e. l8o microfilariae in Ehe sav€rnna zone as opposed to 55 in the forest zone. 6.3 Foci of the northern limit type From the epidemiological savanna epiderniological type. river and its tvro tributaries, regions of. I'la1i. standpoint, these foci must be consiCered as a variety of the This variety was primarily seen on the banks of the Senegal the Bakou6 and the Bafing, and in the Bafoulab6 and Kayes These foci 1ie betlnreen latitudes 13"and 14"N. Onchocerciasis was found in the 30 villages visited, the prevalence of the disease ranging from 18 to 67% and the prevalence ofblindness from 0 to 4.8%. However, the main features distirrguishing these foci were asfollows: hyperendemic areas were restricted to narrow strips bordering watercourses, and theleve1 of endemicity, even in those villages most affected by the disease, never exceeded thelowest Ievel of hyperendeuricity. Differences beEween males and females with regard to onchocerciasis were very clearly marked. Thus of all persons examined, 5.4% of males had severe eye lesions as againsE O.7% of females , ar.d 2,3% of males showed blindness of oncho- cercal origin as against O.L% of females. These foci were studied in detail by picq, Rolland & Richard-Lenoble (1973). It should also be nored rhat mean microfilarial densities were nor as high as in the heart of the Sudan savanna zone. MenEion should be made of the survey carried out by Lartigue & Gr6baut (1964) in the T6r6ko16 valley in the Y611man6 region, I'Iali, in a very small focus lying below latitude 15.N.Ttris focus showed to a marked degree the features of a northern limit focus. oNcHo /wP /7s . 2 Pate 14 In northern Sudan, Anderson et al. (L974) described a focus at and in the vicinity of Abu Hamed lying below latitude 19'N. This is the most northerly focus knor.rn in Africa. Ihe prevalence of the disease appears to be very 1ow and microbial dermatitis is the major sign of infection by O. volvulus. 6.4 Transitional aspects IE may be asked whether some epidemiological asPects may be considered as transi-tional ones and whether this is the case with those aspects observed along the Bafing river in the Bamafl6-Manentali region. This region lies between latitudes 12"and 13'N. and shows a change- over from the savanna to the nort.hern limit epidemioLogical types (Picq, Rolland & Roux, L974). This situation may also occur in the Man region il the sub-prefecture of Kouibly, Ivory Coast. In this region and along the Sassandra river the vegetation Pattern forms a mosaic of savanna and forest t)?es. In one of the villages surveyed, the rzillage of Kouibly, the epidemiological features appeared to be mid-way between those typical of the savanna and those typical of the forest. This vi11age, in which the prevalence of Persons with microfilariae was 8l% and microfilarial densities were fairly high, was found to have 8% severe eye lesions arrd 2% cases of blindness of onchocercal origin. 6.5 Onchocerciasis in the Sahel Ihere appears to be no onchocerciasis in the Sahel (apart from a very few small foci). A survey was carried out by Picq & Lor6al (1970) in southern lvhuritania, in the S6libaly region, below latitui,e 15oN. No endemic onchocerciasis was found. 6.6 Fo ci in Enelish-speaking West Africa Very large onchocerci-asis foci exist in English-speaking West Africa, but an analysis of these would take us out of the context of this study as defined in the Introduction. Mention should however be made of the foci in Ghana studied by Ridley (L945), Waddy (195I), and Rodger (1955), those in Nigeria studied by workers such as Woodruff & Choyce and Budden(1963a,b), Ehose in Sierra Leone studied by Conran & Conran (1956) and lastly those inLiberia studied by Burch (Liberia Research Unit, 1969). Ihe findings of these workers are very close to those of French-speaking authors. 7. SUMMARY This study attempts to bring together the epidemiological features of onchocerciasis in West Africa (longitude: 13"E to 17'w; latitude: 5o to l5"N). IE js based on work carried out by the writer of this paper and also on the findings of many other research workers. In West Africa, the northern limit of endemic onchocerciasis seems to 1ie along latitude 15oN. The general epidemiological features of onchocerciasj.s are as follows: focalization, chequerboard pattern of foci, stratification of degrees of endemicity in terms of distance, and the cumulative nature of onchocerciasi-s in a hyperendemic area. Other topics considered are the desertion of the valleys, the significance of village population size wiEh respect to degree of endemicity, differences in prevalence between the sexes and transitional epidemio- logical features. The author suggests that three main epidemiologicaL tyPes should be distinguished in West Africa, namely: a forest epidemiological type below latitude 8oN, a savanna epidemiological type between 1-atitudes 8oand 13oN and a north Sudan or northern limit epidemiological" type bet\"reen latitude l_3'and 15'N (this l-ast should be considered as a variety of the savanna epidemiological type). Ihe reasons for the differences between savanna and forest epidemiological types are not well understood are and in dispute. As a result, the difference between savanna and forest types remains an epidemiologi-ca1 pozz\e" cNc]clolvJrP 17s.2 Page 15 In a final section the author mentions a number of features and figures found for the prevaLence of disease in about 20 different foci throughout West Afrlca. Thls review has been deliberately brief, since it refers readers to documentation of a much more complete nature. REFERENCES Anderson, J. & ftrglsang, H. (1973) Clinical aspects of onchocerciasis in Uganda and Yemen Arab Republic compared with a rain forest and savanna forns in Cameroon (Document I'IHO/ oNcHo/73. 102 ) Anderson, J. et al. (L974) Some aspects of onchocerciasis in northern and southern Sudan (Document wHO/ONCHo/74. 108 ) Bradley, A. K. (L972) The effec A case study of onchocerci Buck, A. A. et a1. (1969) Oncho Amer. J. trop. lGd. Hyg. Buck, A. A. et al. (1971) Microfilaruria in onchocerciasis, a clinical and epidemiological follow-up study in the Republic of Chad, Bu11 Wld Hlth Or . , 45, 353-359 Buck, A. A. et a1. (L974) Onchocerciasis. Symptomatology, pathology, diagnosis, Geneva, World Health Organization Budden, F. H. (1963a) Comparative study of ocular onchocerciasis in savannah and rain forest, Trans. roy Soc. t.rop. lIed. I{yg ., 57, 64-70 Budden, F. H. (1963b) The incidence of microfilariae in the eye and of ocular lesions in relatlon to the age and sex of persons living in comunities where onchocerciasis is endemic, Trans. roy. Soc. trop. Med. Hyg ., 57 (L), 7L-75 Conran, O. F. & Conran, A. (1956) ltedical survey of Tonkolili and adjacent valleys, Sierra Leone, J. Erop. Med. Hyg., 2, 285-294 Cu61lar, C. B. (L972) Considerations on the transmission and epidemiology of onchocerciasis in Wesr Africa (Documenr WIIO/ONCIIO 172.94) Duke, B. 0. L. & Moore, p. J. (1968) Ihe conrribution of different age groups to rhe r,rans- mission o 62, 22-29 f onchocerciasis in a Cameroon forest village, Trans. roy. Soc trop. Med. Hyg. Jehl, R. (1966) Lronchocercose humaine dans le foyer de la Bougouri-Ba, R6publique de Haute- volta (document mim6ographi6 occcE, Bobo-Dioulasso, 42 pp. 6 cartes) Ihiittgen, H. J. & BUttner, D. W. (1968) Untersuchungen zur epidemiologic und Bedentung der Onchozerkose in oberguinea, Z. Tropenned. Parasit ., 19, L-42 Lamontellerie, M. (L972) R6sultats dtenqu3te sur 1es filarioses dans ltouest de la llaute-Volta (Cercle de Banfora), 44n. parasit. hum. comp. , g, 783-838 Lartigue, J. J. & Grdbaut, S. (1964) Enqu6te s6roclinique polyvalenEe en R6publique du l,lali. R6gions de Yeliman6 et Nioro du Sahel (Document roneotyp6, Centre de Documentation et deStatistique, OCCGE, Bobo-Dioulasso, Haute-Volta) Liberia Research unit (1969) In: Fourth semi-annual report, Institute of tropical Medicine, Ilamburg WHO Technical Report Series No. 53O (Second report of the MIO Expert Comittee on Onchocer_ ciasis ) wtto (1973) onchocerciasis control in the volta river basin area (Document ocp/73.1) Picq, J. J. (1971) Etude 6pid6miologique du r6servoir de virus dans lronchocercose humaine: m6thodologie actuelle des enquBtes de OCCGE, Ann. Soc. belge Med. trop ., 51, 5gL-597 Picq' J. J. & Aliouba (1970) Lrend6mie onchocerquienne dans le cercle de yanfolila, R6publiquedu l'tali (documenE oCcGE No. 24/PARA 70, Centre Mrraz, Bobo-Dioulasso, Haute-Volta) .t of disease on rural economy social structure and settlement. .asis in the Hawal Valley, Nigeria (D,ocument WII0/0NCHO/72.93) cerciasis: some new epidemiologic and clinical findings, L8, 2, 2L7-23o oNcHo /r,[P /75 . 2 Page 16 Picq, J. J. & Jubin, R. (1970) Prospection sur Itonchocercose dans la r6gion de Bassila R6publique du Dahomey (document OCCGE dactylographi6, Centre Mrraz, Bobo-Dioulasso, Haute-Vo1ta) picq, J. J. & Ior6al, E. (1970) EnquOte sur lronchocercose dans la r6gion de Selibaby en I'lauritanie du Sud (document OCCGE No. 1/PARA 70, Centre Mrraz, Bobo-Dioulasso, HauEe- Vo 1ta ) picq, J. J., Coz, J. & Jardel, J. P. (1971) Une m6thode dr6valuation des densit6s micro- filariennes drOnchocerca volvulus Leuckart, 1893 chez des onchocerquiens. 1. Technique an6es, Bull. Wld Hlth Or ., 45, 5L7-52Oet ternps de lecture des biopsies cuE, Picq, J. J., Ior6a1, E. & Jardel, J. P. ( K6dougou au S6n6ga1 oriental (n6publ de Savane (document wHo/oNcHo/72.92 Picq, J. J. & Roux, J. (L972) Preliminary Onchocerca volvulus in the urine of , L972) L' end6mie onchocerquienne dans Ia r6gion de ique du S6n6gal). Un foyer typique dronchocercose French only) studies on the presence of microfilariae of onchocerciasis paEients (Docurnent. WHO/ONCHO /72.96) picq, J. J. & Jubin, R. (1973) Ltend6mie onchocerquienne dans Ia r6gion de Sansann6-Mango(n6publique du Togo) (document technique OCCGE No. 5815, Bobo-Dioulasso, Haute-Volta) picq, J. J., Rolland, A. & Richard-Lenoble, D. (L973) Lrend6mie onchocerquienne dans les r6gions de Bafoulab6 et de Kayes, R6publique du I'lali: un foyer de savane de type nord- soudanien (document OCCGE No. 128/PARA 73, Centre Muraz, Bobo-Dioulasso, Haute-VoIta) picq, J. J., Rolland, A. & Roux, J. 0973) Prospection sur ltend6mie onchocerquienne dans les r6gions de Danan6 et de ltan (R6publique de COte dtlvoire) (document OCCGE No. I29/pane 7:, Centre l$traz, Bobo-Dioutasso, Haute-Volta) picq, J. J. & Roux, J. (1973) Faits nouveaux dans Ironchocercose: la microfilarurie, sa r6partition g6ographique, ses rapports avec les densit6s microfilariennes cutan6es, lralbuminurie et 1a chimioEh6rapie. Premiers r6sultats, !g!1!112q., 33, 45L-46L picq, J. J. & Jardel, J. P. (1974) Une m6thode dr6valuation des densit6s microfilariennes dtOnchocerca volvulus Leuckart, 1893 chez des onchocerquiens. 2. R6partiEion des densit6s microfilariennes suivant les sites et niveaux de p r6LEvements des bioPsies cutan6es: variations des densit6s microfilariennes au cours des 24 heures, Bull. Wld Hlth Org., 51, 145-153 picq, J. J., Rolland, A. & Richard-Lenoble, D. (L974) Ltend6mie onchocerquienne dans la r6gion drAplahou6 au Dahomey: un important foyer dronchocercose de type forOt (document WIIO/ONCHO/7 4.LO7, French onlY) picq, J. J., Rolland, A. & Roux, J. Og74) Lrend6mie onchocerquienne dans la r6gion de Bamafl6(R6publique du l"Iali) picq, J. J. et al. (Lg74) Prospection sur lrend6mie onchocerquienne dans 1es r6gions de T6hini et de Bouna, R6publique de COte drlvoire (document OCCGE, Centre Muraz, Bobo-Dioulasso, Haut e-Vo lta) picq, J. J., Jubin, R. & Plouvier, S. (L974) Prospection sur lrend6mie onchocerquienne dans la r6gion de lbrhogo, R6publique de COte dtlvoire) (document techn. OCCGE No. 5798, Bobo- Dioulasso, Haute-Vo lta) plouvier, s. & Lafaye, A. (1975) Interpr6tation de donnees 6pidemiologiques concernant - lronchocercose en foyer nyperend6rnique d 1?aide drun modEle math6matique siurplifi5. In: Rapport final XVEme conf6rence technique de ItOCCGE, Bobo-Dioulasso, (Haute-Volta) puyelo, R. & I{olstein, 1,1. }1. (1950) Lronchocercose humaine en Afrique noire frangaise - maladie sociale, Med. troP., No. 3 R6my, G. (1970) Mouvements de population eE facteurs sanitaires en llaute-Volta. In: Rapport final lOEme conf6rence techn. OCCGE 2O9, Bobo-Diou1asso, Haute-Volta Ridley, H. Brit. Rodger, F. C. (f955) Onchocerciasis in the nort.hern Gold CoasE, 161- 165 (1945) Ocular onchocerciasis, including an invesEigation in the Gold Coast, J. Ophthal. , Ilcnograph supplement X The Ni erian Field oNcrro/rrP/75.2 page 17 20, RoIland, A. (L972) Onchocerciasis in the village of Saint-Pierre: an unhappy experience of repopulation in an uncontrolled endemic area, Trans. roY Soc. trop. Med. Hyg. , 66, 913-915 Rolland, A. & Balay, G. (f969) Lronchocercose dans le foyer Bisa (document No. lll/oNCHo du 30.5. 1969, OCCGE, Centre lturaz) Roux, J., Picq, J. J. & Aliou-Ba (L972) Ltend6mie onchocerquienne dans les circonscriptionsde Lama-Kara, Pagouda et Niamtougou au Nord-Togo (document OCCGE No. 36/PARA 72, Centre Mrraz, Bobo-Dioulasso, Haute-Volta) Veilleux M., Le Breton-Oliveau, G. & Aubry, M. (1960) Document OCCGE, Bobo-Dioulasso, Haute- Vo lta I.Iaddy, B. B. (1951) Onchocerciasis and bl s in the Northern Territories of the Gold Coast(mimeographed document J.E.M. 12 51, p. 3 maps Woodruff, A. W. et al. (1963) Onchocerciasis and rhe eye i.n western Uganda, Trans. roy. Soc.trop. I,Ied. Hyg. , 57, 5o-63 oNcrHo/wP 175.2 Page 18 d .ri Ji(/) 0)>(,) '.{ 0.|JJJOJ .r{)Q(U o'c,o"r OO,F{trGtr(,)(d OJ -C .d J ..1JJ!O(H ..r.HcoJoB B o xo l{ (/) .,t 0.t (a (J 0,@r,(/)o.rt..{At0JoCErJ o o oJ kro ..{(, Q.U) o tr d (atr...{lfO>...{(dd occJxoJo0, a.(a orq, (,J-o E! sl '.{ (.)(! .r't !(d - 'r{q{ ol{(,Joo. .F{ .rl dF oocitrI 'r{r^ llJO 6{r) I D{|r1NH\t llro NoN I LN No\o ! d(dN oo-f11t otr c.l 0.) 0) B! 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