Onchocerciasis: its geographical distribution and vector species in the major endemic foci of the world, with special reference to east Africa and south Arabia

@ WORLD HEALTH ORGANIZATIONORGANISATION MONDIALE DE LA SANTE EXPERT COMMITTEE ON EPIDEMIO]OGY OF ONCHOCERC]ASIS Bahr el Ghazal, Equatoria ahd Upper Nile Provinces NorEhern Sudan oNc:Ho/wP/75.22 ORIG]NAL: ENGLISH Page 2 2 2 2 4 4 5 6 6 8 8 9 10 10 10 11 13 18 18 18 L9 20 Geneva 10-18 Novenrber 1975 Draft agenda item 2 L'C ONCHOCERCIASIS: ITS GEOGRAPHICAL DISTRIBUTION AND VECTOR SPECIES IN TI{E MAJOR ENDEMIC FOCI OF THE WORLD, WITH SPECIAL REFERENCE TO EAST AFRICA AND SOUTII ARABIA by M. H. Satri National Public Health Laboratories Khartor:m, Sudan CONTENTS o l, m -f r-l .-.rlr."'' DD'-o" f-r I 2 Introduction . Geographica 1 distribut,ion 2.L East Africa . Uganda . Kenya Tanzania Ethiopia Sudan 2.L.L 2.t.2 2.L.3 2.I.4 2.L.5 2.L.5.L 2.L.5.2 2.2 2.3 2.4 West Africa . America . . South Arabia Yemen Democratic Yemen . 3. Vector species . References . . Annex - Some notes on research proposals Vector biology and conErol . Insecticides and other methods Vector ecology and behaviour . Epidemiological surveys The issue of thrs document does not constitute formal publrcation. lt should not be reviewed, abstractod or quoted without the agreement of the World Health Organization. Authors alone are responsible for views expressed rn signed articles. Ce document ne constrtue pas une publrcatron. ll ne doit farre l'objet d'aucun compte rendu ou r6sumd ni d'aucune crtation sans l'autorrsation de l'Organisation Mondiale de la Sant6. Les oprnions exprimdes dans les articles sign6s n,engagent que leurs auteurs. 2.4.L 2.4.2 I 12 3 4 of control ..) t oNcRo/wP/75.22 Page 2 I. INTRODUCTION Onchocerciasis is widely distributed in Africa and Central America, and recently, a focus was described in southern Arabia. It is estirnated that about twenEy million people are affected by this parasitic di-sease, which may result in blindness and impaired vision and which causes onchodermatitis, sometimes with very troublesome itching, in a good proportion of those afflicted. This estimate of the affected population is considered very conservative indeed, as hitherto unknown infected areas are continuously being found. IE seems cerEain that if the problem were critically assessed, more areas would be included. In the Sudan,just recently in February 1975, we discovered a new focus along the Bahr eI Arab in southern Darfur, which is continuous with and lies to the north of the focus in Bahr el Ghazal Province, and there is reason to believe that this focus i.s linked with that of the Central African Republic. Moreover, it is probable that the Yabus river focus, described by Kirk et al.(1959), in fact, encroaches and joins with that in the Ethicpian plateau and also extends southwards into Upper Nile province. This is just one example and it is quite possible that similar undetected foci exlst in other areas. MeanEime, it is obvious that the population density in these areas must be very low because of the disease and the nuisance of the fly vector. Historically, Robles (1915, 1919) was the first to indicate the association of blindness with onchocerciasis in Guatemala. DLaz (1957) also gives him credit for having decisively incriminated Simuliidae as the vectors of this parasiEic disease. Brumpt & Muhlen (L927) considered that in Africa the disease was not associated with ocular manifestations. On the contrary, Hisserte (193I, L932, L937, 1938) working inZaire (former Belgian Congo) observed that onchocerciasis in that country was associaEed with a high incidence of blindness. In the Sudan, Bryant (1933, 1935) reported many cases of blindness amongst the Jur tribe of Bahr el Ghazal province and related this to Onghocerca volvulus infection. Ihe transmission of the parasite through the insect vector,-@*, was dernonstrated by Blacklock (L926, L927) in West Africa. A few years later, Strong et al. (1934) investigated in Central America the disease due to the filarial worm O. caecutiens, which in some areas of Guatemala was known to cause nodules on the head, frequenLly associated with visual defects and blindness. The specificname',@'|hadbeenconferredonthisparasitebyBrumPt(19r9)onthegrounds r.hat the papiltae and spicules of the male worm differed somewhat from those of the male O. volvulus in Africa, as well as on the geographical distribution and on what he believed to be differences in clinical and pathologica 1 manifestations betlreen the two t'species". 2. GEOGRAPHICAL DISTRIBUTION 2.L East Africa The disease in Africa is knor^m to extend from the southern edge of the Sahara desert down to Angola in the west and Tanzania in the east. To the north of the equator and up to its most northerly limit, Ehe disease covers the entire width of the continent, from Senegal in the west to Ethiopia in the east. It must be stressed that the distribution is patchy and interrupted, following rivers and streams in points where there are waterfalls or cascades. There are still many gaps in our knowledge as to the true distribution of the disease in this continent, especially in Ethiopia and the Sudan. In these two countries, there is presumptive evidence that the disease exists in certain localities, but neither the rnagnitude of the problem, nor the ext.ent of its distribution is known. At least in the Sudan, new foci come to light from time to time. 2.7.L Uganda Barnley (1968) stated: 'tThe distribution of onchocerciasis in Uganda is limited by two factors: the presence of free-flowing streams and an adequate density of human population in prolonged contact with the vector flies arising from such streams. Much of the drainage of Uganda is via Papyrus swamps rather than free-flowing rivers. The oxygen-deficient water of Papyrus slramps is unsuitable to the requirements of the early Simulium stages. Vectors of onchocerciasis are thus confined to the rapidly flowlng tracts of the Nile and iEs tributaries, and to regions where the velocity of rivers is high enough to keep them free of Papyrus, as I ar toNcHo/wP/ 75.22 Page 3 during their descent of mountain valleys and rift escarPments. The dense shade provided by the closed canopy forest inhibits the growth of Papyrus and well-aerated rivers producing S. neavei flow through most of the forests of Uganda. Onchocerciasis is a very common affliction of forestiy stafftt. He further maintains that I'since perennial streams of free-flowing waEer usually attract a high density of human population into their valleys, it can be stated that there are few areas in Uganda, under 22OO m through which free-flowing streams descend forested slopes, where onchocerciasis is not prevalent. In the Ruwenzori valleys and the valleys of the rivers descending the Western Rift escarPment in West Nile district, the incidence of the disease approaches 1OO%rr. Nelson (f958) carried out onchocerciasis surveys in West Nile district in Uganda, which is situated in the north-west corner of the country. r Th. Nile-Zaire watershed, Iying between 4OOO and 5OOO ft (1200 and 15OO m), forms the wesEern and southern borders, the frontier between Uganda and the Sudan the norEhern border, and the Albert Nite the eastern border which separates the district from the rest of Uganda. Numerous sma1l rivers and streams originate along the \^ratershed and cascade down an escarpment through well-wooded valleys. Nelson observes that this is an ideal habitat for @1 and finds that the conditions are similar ro rhose described by Buckley (1949) i., ferry". TEe area surveyed by Nelson (1958) lies in the Nyara valley, which he considers to be one of the most heavily infected valleys in Uganda. In the four villages studied, particularly in Lazebu, all persons over six years of age were infected, and in these villages the prevalence rate was about 1OO% in males belonging to the 11-20, 2]1-1fi, and 40 years plus age groups, and 88, 95 and IOO% respectively in females. The vector was S. neavei. Anderson & Fuglsang (1973), who made a study of the prevalence of onchocerciasis in certain parts of Uganda, state: rtln Uganda, it was known that there rdas a high prevalence of onchocerciasis on the foothills of Mount Elgon, but it was suspected that the disease ldas decreasing in severity as a result of a decrease in man-fly contact associated with the gradual disappearance of the forest; in the west of Uganda, on the contrary, the disease was thought to be severett. Two villages were selected by these authors, that of Bumadanda, at Mount Elgon, and of Bugoye, in the Ruwenzori. Bumadanda is in Bufumbo subcounEy and lies at an altitude of 1830 m, in the heavily cultivated foothills below the forest from which the Sisiwaki river ri.ses. Colbourne & Crosskey (1965) state thattrthe forest above Bufumbo is the most intense Simulium neavei bi ting area known in Uganda. S. neavei is the only known vector heretr. In the Ruwenzori focus, S. damnosum is the vector but the possible role, in transmission, of S. neavei should be investigated. Among the persons examined by Anderson & Fuglsang (1oc. cit.) the infection rate was 62% ac Mount Elgon, while in the Ruwenzori it was 56%. These authors conclude that the infection rate at Mount Elgon is under-estimated and severe and that control should be started as soon as possible. They also consider that in Uganda, onchocerciasis is, in fact, still prevalent in both the Mount Elgon and Ruwenzori foci, as weII as in the West Nile district where Nelson (1958) made his surveys. Barnley (1968) remarks that the conErol of Simuliidae is a simple operation, but the logistic problems involved may be very great. The difficulty is that if area-wide vector eradication is to be atternpted, the insecticide must be applied within Ehe same few days to all the tributaries of the river systerns in an infected area at points higher than the upper limit of the range in which the vector breeds. In mountain regions this upper linit of breeding is approximately 22OO m at which level the innumerable tributaries flowing Ehrough dense, trackless and inadequately-mapped forest are difficult to locate and almost impossible of access by teams carrying insecticide. The solution to this, in my opinion, is helicopter aerial spraying or lnsecticide bombs. Despite these problems, however, Barnley loc. cit. ) reports that: rrEffective control has been established on the 5O miles of rapids on the Nile between Owen Falls and Mbulumuti, vectors from which tract of breeding sites transmitted onchocerciasis at high intensity over 2O0O square miles of fertile land which was becoming progressively depopulated as a result of the fly nuisance. The control has reversed the I For the main geographical features, Lumsden & Buxton (f951) may be consulted oNcHo/wP/ 75.22 Page 4 trend; movement of population into the area continues and it now supplies a very high proPortion of foodstuffs which maintain the urban population of Jinja, Kampala and Entebbe. Vectors are also under control on rivers in thb Budongo Forest near I'Iasi-ndi and on the Ruimi River flowing from Ruwenzori into Lake George. Simulium control will be necessary on other rivers flowing from the Ruwenzori massif ln order to protect populations which will be attracted by irrigation schemes under current developmentrt. The S. neavei group includes S. woodi, S. hightoni, and S. nyasalandicum, the role of whictr, in@Elhas not been investigated (Barnley, loc. cit.). 2.L.2 Kenya Highton (L974) states: "ln Kenya onchocerciasis was endemic in six well-defined areas, all in Western Kenyarr. Kenya is one of the fehr countries where total elimination of an insect of medical import.ance has been achie'redr leading to the gradual disappearance of an incapacitating human dj-sease. In Kenya S. neavei was the only vector. Using DDT, Garnham & McMahon (1947) initiated the first control project in 1946 in the Kodera focus resulting in the complete eradication of S. neavei from that area. The discovery by McMahon (1951) that the immature forms of S. neavei live in phoretic association with freshwater crabs of the genus Potamonautes and a description of the pupa and the adult rrale fIy further facllitated the discovery of breeding places and, therefore, helped make possible the complete eradication of the vector in the country. Prior to eradication, there were five endemic areas, namely: Kodera, with a prevalence rate of 54.9% in males and 39.6% in females; Kisii, with 29.3% in males and 38.4% tn females; Riana, with 22.4% in males and 20.6% in females; Kaimosi, with 72.8% in males aod 69% in females; and Kalamega, with 41.3% in males atd 27.2% in fernales. These results were based on a survey in Kenya, carried out by Buckley (L949) and quoted by l{ighton loc. cit. ). In L967, Roberts et aI. re-evaluated the situat.ion in Kenya, 9, 11, and 18 years after the eradication of the vector. This follow-up survey was carried out in four foci in 1964. The areas surveyed were Kodera, Kaimosi, Ngoina and Kalamega. It is remarkable that these authors confirmed that none of those born after eradication in any of the four areas showed any microfilariae or nodules. Ihey stressed that there was no active transmission, despite the fact that there were a few foci infested with the vector. These were in an area on two river valleys at the foothills of MounE Elgon near the Uganda border. The results of their study showed that 11 years after the interruption of transmission live O. volvulus adults \{ere present in nodules and microfilariae were present in the skin; after 18 years, however, microfilariae were no longer found in the skin. It is presumed that in hyperendemic areas, parasites are acquired until shortly before interruption of transmissionl it can, thus, be po stulated that the 0. volvulus Tirorms lose their reproductive capacity in 16 years or perhaps a littIe earlier. 2.1.3 Tanzania Woodruff (1965) gives an excellent review of the distribution of onchocerciasis in Tanzania and some of the valuable information conEained in his rePort is abstracted here. Onchocerciasis has long been endemic in Tanzania in particular in the Amani region. Fisher (Lg32) reported a case of onchocerciasis on the north shore of Lake Malawi (Nyasa) and in Ubina. There is evidence, which in some cases has not yet been substantiated, that the disease is prevalent, in varying degrees, throughout most of the country along rivers and streams flowing down hills and escarpments, at altitudes varying from 600 ft (I83 m) to a litt1e over 3OOO ft (915 m). Earlier, the disease seemed to have an erratic distribution, either due to lack of vigilance or to true scarcity of cases, only 1I2 cases having been recorded between 1956 and L962. In northern and north-eastern Tanzania, a few sporadic cases were reported. One of these cases was seen by Professor Giaquinto, who was visiting Amani on behalf of WHO in 1957, and another was seen by Woodruff in 196I in a European who had been stationed for many years at Amani. Woodruff in L962 sahT more cases from Ehe same region. Dr A. B. Laing (quoted in Woodruff, 1965) indicates that a survey was carried out in 1963 in ) IoNc}JrolwP/75.22 Page 5 rhe eastern Usambaras in the Anani plateau at an altitude of 27OO ft (820 m) to 3200 ft (975 m), and in the nelghbouring areas with tea and sisal estates. The infection rate was 42% at Anani station and adjacent villages, ar.d 34% at both Sigi river valIey, and at Magorotto at an altitude of 2500 ft (760 m). The lowest infection rates were found amongst people living in the foothills (167") and on sisal estates (only 67.). Surveys carried out in the western Usambaras in 1964 revealed that in a sample of 376 persons examj.ned, 23 were infected of whom only 14 \{ere presumed to have acquired the disease local1y. Also in the western Usambarasrwhereabout nine cases had been descrlbed in 1963 by Laing & llegesa (f964), Sprengel (L967, 1968) carried out an extensive survey, which revealed a prevalence rate of up to 6L% in adults, the average infection rate being 37%. Subsequent to hospital reports of the existence of 29 cases of onchocerciasis in Luili, in the Ruvuma region, an organized survey was carried out by the East African Institute of Malaria and Vector-borne Diseases in August-September, 1967, and the findings were described by Wegesa (1968). This survey revealed the largest single focus described up to now in Tanzania (Wegesa, 1969). Its areq covers thro entire districts, Songea and Iulbinga, out of the three in the region, and infection rates ranging from 0.9% to 44.3% were recorded by a survey team from Amani. During the first half of 1969, two areas in Morogoro district were investigated and found to have onchocerciasis. One is centred in the Uluguru mountains i-mnediately south of Morogoro and the other in the Nguru mountains. Exgmination of a population sample of all ages in the Uluguru mountain area revealed prevalence rates ranging from 8%, on the periphery of the focus, to 66% in its centre on the southern slopes at Bwakira. No infection could be found at an altitude of 5OO0 ft (1500 m). In the Ngurus, out of 167 persons examined 14 were found infected. In the Anani and Tewe foci in Ehe Usambaras, the vector is S. woodi (Lewis, 1960; Raybould, 1967), in the focus in the Ngurus Ehe vector is sti11 unknown, and S damnosum dominates ln the threeotherareas. Hausermann (1966) found S. damnosurn to be the rnain vector in Mahenge with an anthropophi lic member of S. neavei occurring in small areas around Sali and near Mahenge township (Wegesa, 1969 In.the Uluguru s, S. damnosum has been shown to be the principa I vector at Brf,akira with a member of the S. neavei complex found in soa1l numbers in only two places, at Kinole and Bvrakira (Raybould, A Eeam from Amani collected man-biting Simuliidae from various places in the Ruvuma region near the north-eastern shore of Lake Malawi. In this, hitherto unrecognized focus of human onchocerciasis, anthropophi 1ic S. damnosum were wides pread; a species of the S. neavei complex was found biting man in one locality. In the report on the field and laboratory study of onchocerciasis vectors by I. J. N. Raybould and A. S. K. Yaguna, it is stated: rrlwo species of the Simulium neavei comP lex occur ln the eastern Usambara mountains; Sinulium yoe4i is the vector of human onchocerciasis in this area, whereas the Amani form of S. nyasalandicum feeds on cattle and only rarely on man. An ecological study of the i.rmpture stages of these two simuliids and of three species of crabs associated with them I4rEIs started in February 1966 and continued till June 1967. The results indicate that the breeding sites of the two species of Simulium are quite different; S. nyasalandicum favouring the larger rivers and S. woodi being most numerous in very small heavily wooded streams at altitudes ranging from 600 ft to 2870 ft. The breeding characteristic is likely to render control of S. woodi particularly difficultr'. 2.L.4 Ethiopia In 1961, at the second African Conference on Onchocerciasis sponsored by the World Health Organization, and held in BrazzaviLLe, Dr Diggs gave evidence which for Ehe first time definitely proved the existence of onchocerciasis in Ethiopia. Its distribution, however, still remains obscure. Diggs mentioned the presence of the disease in the Ethiopian plateau, on the Sudan border. This area had already been suspected of being infected and proof of the existence of onchocerciasis and its vector, S. damnosr:m , was referred ro by Kirk et al. (f959). In this corununication onchocerciasis was reported in the Yabus river area, which is on the oNCHo/I,JP/ 75.22 Page 6 Sudan side of the Ethiopian plateau. Moreover, unsubstantiated rePorts have recently been received from travellers who indicate that the western border of Ethiopia around Gambeila is infected with the disease as well as the adjoining Sudan border (Gassouma, L975), and who tell of the prevalence of the disease with its skin and eye manifestations. Oomen (L967), vrleo described the clinical manifestations in cases he saw in Jiruna Hospital, considers that the disease is very common in Kaffa province in south-west Ethiopia and that the whole border, in fact, is infected. Oomen has been investigating this and in a letter he sent to the late Professor Haseeb (personal coruounication) he writes: 'rAt present I am doing surveys in a number of places in this and neighbouring provinces. Although this work is not yet completed, there are strong indications that onchocerci.asis is presenE everywhere in south-T,rest Ethiopia, e.g. in the provinces of Gemu-Goffa, Kaffa, Illubabor and Wollega, with infection rates for men over 15 years of age between 20% and 70% (women always much lower), and I would not be surprised if it was found in other provinces as welltt. He Soes on to say: "Although I have seen several cases with onchocercal eye lesions, blindness does not seem to be a major complication of this infection here, although there is one report (fuller Torey) of a medical survey of the Say Say people in t.he BIue Nile gorge in the Ethiopian Medical Journal of two or Ehree years ago, which attributes Ehe high rate of blindness of these people to onchocerciasis. Tne vectors in this area have not been sEudied at all. Simulium damnosum is prevalent everywhere and I have found man-biting S. woodi in several places, but these have not been dissected up to nowrr. Oomen concludes: trlt would be very interesting to know if our onchocerciasis focus is continuous with thaE of the Sudan, which is really the reason of my writing this leEtertt. This is in fact the case and as far as we know the vector of the disease on Ehe Sudan side of the Ethiopian p lateau is S. damnosum. 2.L.5 Sudan Following the discovery and description by Bryant (f933) of an unusual form of blindness amongst the Jurs, Bellandas, Bongos, and to a lesser extent the Dinkas, of Bahr el Ghazal province and its association with onchocerciasis, further studies were carried out by various authors. The studies by Bryanr (1933, 1934), Cruickshank (L934), and McKelvie (1934, 1935) showed that the disEribution of the disease corresponded with that of O. volvulus. At the same time, no other etiological agent was discovered. Similar findings were reported from neighbouring countri.es and at a symposium convened by the East African Medical Journal (1939), information from Kenya, Uganda, Zaire and the Sudan was sumnarized. Some years later, studies on various aspects of the disease were made by Kirk (L949), who described the disease in the Kpaile and Raffile areas of Bahr el Ghazal, and by Woodman (L949), who noted its presence in Equatoria province in the area between the Bo and Sue ri.vers and on the Naarm river at Mvolo, as well as its sparse distribution in the area of Li Rangu. In L946, a progrartrne to study the different aspects of the disease in Bahr el Ghazal province was planned and was actually initiated by Morgan (L947). Kirk and later Satti were entrusted with the study of the epiderniological, clinical and chemotherapeutic aspecEs, while Lewis studied the biology of the vector, its distribution, its natural infection with 0. volvul-us and the feasibili ty of its control, and possible eradication, with the new insecticides. Consequently, as a result of a series of investigations carried out by various workers at different places and times, knowledge of the distribution of the disease has greaEly increased. 2.L.5.L Bahr el Ghazat, Equatoria and Upper Nile provinces Kirk (1947) described the disease round Kpaile and Raffile Mission School and station. Satti (1948) did a wider survey covering the Western district. (former Wau district) which includes the area from Wau to Raga and Boro, about 435 km south-west of the capital of Bahr el Gl;.azal- (Wau). Boro is about IO5-115 km from the eastern border of the Central African Republic and about the same distance from Kafa Kia Kinja on the Bahr el Arab river; this last point as well as other villages on this river have, of late, gained notice in view of the fact that onchocerciasis was found in them. Satti (f948) found the disease on both the Wau-Raga-Boro road and the Raga-Aweil-Wau road. On the former there are three ri-vers, t aoNcRo lwPl7s.22 Page 7 Kpangu, Kuru and Sopo; all the villages on these non-perennial rivers are infected. The heaviest infection was at Khor Shanrnam, where the rate was 55.4% and where 110 persons were blind out of a population of about 2OOO, giving a blindness rate of 5.5%. In Gossinga, which is only a few miles north on the Raga-Aweil road, the Prevalence is also high. This Dinka village lies on the river LoII. At the time, there l{ere no cases in Aweil or Nyam lyl. The iltoich areast' were free of the disease and of its vector as the water is sEagnant and the whole area swampy, and this situation also prevails in the areas north-r.lest of Wau like Gogrial andAkon. At Thiet, which lies north of Wau on the Wau-Meshraar road, there are occasional cases. In Raga Mission School 11 out of 59 boys under 10 years of age had nodules, giving an infection rate ot L8.64%. In the same year, Satti visited Meridi where he looked for onchocerciasis, but did not find any. The medical officer who had been living in the area for several years confirmed that during the time he had been residing in the districE he had never seen any. In 1958, Satti re-visited the area and encountered many cases of onchocerciasi-s. Ihis change luas due to the fact that a dam had been built on the river Meridi and the turbulence generated by the dam favoured the breeding of S. damnosum, which became a nuisance in the area (Haseeb et aL. , L962). In November 1959, Dr Torreolla Bueno, WHO Consultant on onchocerciasis, arrived in the southern provi.nces, accompanied by Dr Sherif, ophthalmologist. They jointly carried out a survey in both Bahr el GbazaL and Equatoria provinces. Generally the infection was more severe and widespread in Bahr el Ghazal than in Equatoria. The highest infection rate was at Wau (8L.2%), but as this included hospital patients, it was considered to be a biased sample. The next highest rate was 72.4% at Gossinga, followed by 68.4% at Raga Hospital and school. Two areas in the province, namely Rr:mbek and Bisailia, were found to be free from infection as they are situated away from riversl the latter locality lies on the Raga-Wau road, abouE 40 km from Wau. In Equatoria, the highest prevalence rate was in Mvolo, which had been known for being heavily infested with Simulium, and where Lerdis (1948, 1952, L956, 1958) did most of his work on the biology of S. damnosum. Meridi was found to have a prevalence rate of. L2%. Because of the prevalence of the disease as well as the nuisance of the fly, the CourE and other establishments have been moved to Simulium-free areas. In Equatoria (Woodman, 1949), Ehe disease is known to exist along th;-;;a Sue rivers, in the l,Ivolo region and in the Li Rangu area. The situation in eastern Equatoria is obscure, as no serious studies have included this area. The topography particularly in the south-east seems very favourable for the breeding of Simuliidae, and there are unconfirmed reports that the disease exists in these regions, in particular in the Imatong mountains, where on Ehe Uganda side, McCrae (L972) found S. neavei. In Upper Ni1e, extremely little is known of the distribution of the disease as no epidemiological studies have been carried out. The situation of the vector is also very obscure except in relatlon to the Yabus focus, which is partly in this province and partly in the BIue Nile province. The only other available information comes from Bloss (1949), wtro described the disease as existing in an area on the Sudan-Ethiopia border, from Pibor Post to a point opposite map reference Mel1ut. Pratt (quoted by Kirk et aI., 1959) reported that cases of onchocerciasis from the Boma plateau of Upper Nile were seen in Juba Hospital; this area is also situated on the eastern borders of Sudan. There are unsubstantiated reports that onchocerciasi-s is prevalent at the Wadessa-Dago and Kigli posts in Nasir district of Upper Nile; this area is continuous with the southern part of the Yabus focus. Refugees coming from Gambeila on the Akobo river speak of the prevalence of onchocerciasis in that areq on both sides of the border (Gassouma, L975). In 1963 a WIIO assisted onchocerciasis control project was established, but due to disturbances in the south of the country, all its objectives could not be achieved and the activities of the project were curtailed as far as the southern region was concerned. oNcr{o/wP/ 75.22 Page 8 2.L.5.2 Northern Sudan Onchocerciasis exists in northern and eastern Sudan, and recently, in February 1975, the presence of a focus in western Sudan was confirmed. However, this was not surprising since it was always suspected that the Bahr el Arab area must be infected as it is a geographical continuation of the Bahr el Ghazal focus northwards and is in constant contact with it due to population movements. This newly discovered focus, which is situated in southern Darfur province, is also continuous with the focus in the Central African Republic on whose eastern border it lies. In February L975, the writer visited the Radoom and Higairat area where he found cases of onchocerciasis with its evident manifestations of nodules, onchodermatitis, hydroceles, hanging groin and blindness. Onchocerciasis is particularly manifest in the village of Radoom, where part of the population comes originally from Wau and Raga and was moved to this area for political reasons during the Condominium Goverrunent. In April 1975, Dr Tahir Ismail Salim, the Medical Officer of Health for Darfur province, carried out a survey in the area, taking a random sample of 575 persons. These persons belonged to both sexes and all age-groups and came from several villages on the Bahr el Arab river. Results showed an infection rate of 2L% Ln the 1-5 year age-group and an overall infection rate of 55.6%. No one-year-old infants were found to be infected. The results are based on one skin snip from the same site; the percentage would of course be higher if more than one skin snip had been taken and from different sites. The results of his survey will soon be submitted for formal publication. In addition to those already mentioned several villages on the Bahr el Arab river are affected, as for example the villages of Kafa kiakinga and El fifi. In this connexion, a survey in the Jebel I'brra area is urgently needed, if we are to control the disease in the country effectively. There is no proof that the infection exists there, but a few Army people rdere found infected on their return from the south and it is feared that these may have started a focus of infection in the highlands of this mountainous region, which is rich in springs and rivers favourable for the breeding of Simuliidae. Although Lewis (1956) ruled out the presen ce of S. damnosum in Jebel Marra on the basis of a survey he made in L954, it is felt that it is possible that the vector exists or has established itself either in areas not covered by Lewisr survey or in streams down the slopes near Zalingi and other neighbouring areas. However Dr Osman Abdel Nur (1975) tells the writer that both Dr Ovazza and himself tracked some of these streams, particularly the main one down the mountain to Zalingi. He stresses that they did not cover all the streams coming down the mountain, but from Ehose examined, they confirm the finding s of Lewis thaE no S. damnosum is available. In the Nile province (former NorEhern province), a small focus has been described by Morgan (1958) in the Abu Hamad area where Lewis (L952) had already reported the prevalence of S. damnosum. The northern limit of the disease is not yet certain. A case was reported from Dongola, but this is not any further north latitude-wise than Abu Hamad, both being I9oN. However, it is further north along the Nile bend and S. darnnosum is said to be present north of this point; but Lake Nubia wi1l, of course, be a barrier after the building of the High Dam. In the Blue Nile province, a dam was built at Damazin, known as the Roseires dam. At this site Lewis (L952) reported the presence of the vector S. darnnosum and the disease is known to exist at Yabus, which is not far from here and from where labour is certainly going to be imported; this situation makes the extension of the disease to the dam site very probable. Kassal province. In Ehe forties, Robert Kirk and the present \,rriter saw cases of what appeared as t'craw crawrr or elephant skin in poachers operating along the Rahad and Atbara rivers. Onchocercal etiology was suspected, but skin snips were negative. Recently cases of this disease have been cropping up in the same areas (Sherif, L972). 2.2 West Africa Endemic zones of onchocerciasis exist in all the countries of West Africa: Upper Volta, Dahomey, Ghana, Gambia, Guinea, Ivory Coast, Liberia, Mali, Niger, Mauritania, Guinea Bissau, Nigeria, Si-erra Leone, Senegal and Togo. It is in West Africa that the most heavily and I 0NcH0/wP/75.22 Page 9 seriously affected foci are located. The infection as well as the blindness raEes vary from one localiEy to another, especially for infectlon of the savanna tyPe. According to Lewis & Duke (1966) Ehere are t\do varieties of vector: (i) a forest variety, which is dark in colour and associated with less ocular compllcations; (ii.) a Sudan savanna variety which is pale in colour and associated with various degrees of ocular involvement. In Nigeria the disease exists throughout the northern part of the country and the vector is S. damnosum (Crosskey, 1956). Crosskey loc. cit. ) states: rrln the southern Parts of the country, south of the Niger and Benue, many foci are found in the area of the Derived Savannah. Forest areas persist where surface water is abundant, and consequently several breeding rivers of the Derived Savannah Zone, especially in Kabba Province and also in Abuja area of Niger Province. Foci also exist on the Jos Plateau at over 4OOG ft where there is very little vegetationrt. In southern Nigeria where the dry and wet seasons are not so pronounced as in northern Nigeria , S. damnosum is abundant Ehroughout the year (Crosskey, f955). At the Kainji dam, reduction of the blackfly vector, S. damnosum , has been achieved by repeated application of insecticides over several years and this has been effective in controlling transmission. In Upper Volta, onchocerciasis is an i-mportant public health problem and the development of \^rater resources by daming and by starting the Volta river Project has aroused great interest in the reclamation of land for agricultural purposes, where the vector is prevalent and the disease rife. A voluminous literature exists for the French speaking African countries where the bulk of the work is carried out by the staff of the OCCGE and ORSTOM. In the English speaking countries a substanti.al amount of help has been rendered by the staff of the British Medical Research Council, especially in Cameroon where there is a research station at Kumba. In Zaire several places are infected in particular those areas neighbouring the Sudan and Uganda. Other areas in the ZaLre river basin are infected including both Kinshasa and Brazzaville. Infection also exists in the Central African Republic (CAR) and in Chad. The focus which \ilas recently discovered in the Bahr el Arab river region in Sudan links directly with that of the CAR, as the two feeder branches which form this river arise at the eastern border of the CAR and there is continuous population movernent between this country and the Sudan. Similar contact also exists between these two countries further south at the rrestern boundary of Bahr el Ghazal provincer. which is another endernic focus. The other part of the western border of the Sudan, which is opposite Chad, is not known to be infected; this may be due to the fact that no survey has been carried out there. 2.3 America Onchocerciasis is endemic in Guatenala, Mexico and Venezuela, while in Colorrbia there is one focus which was recently discovered by Assis-Masri & Little in 1965. The seriousness of onchocerciasis as a blinding disease was discovered for the first time in Guatemala, hence the name Onchocerca caecutiens , the blinding filaria. fhere are three foci of onchocerciasis in Guatenrala: a northerly focus which is contiguous with the one in Chiapas, Mexico, and two foci which 1ie close together on the southern slopes of the mountains, but which are not contiguous. In Mexico, onchocerciasis is endemic in the states of Oaxaca and Chiapas. In these Central American foci S. ochracer:m is the vector. It breeds close to or within the perimeter of villages and seems to find ideal conditions for biting the labour force working in the coffee plantations (Waddy, L967). In Colombia, as already mentioned, there is a single focus and in Venezuela there are E\^ro foci in each of which a mounEainous projection of about 13OO m forms the centre point. The vector in Venezuela is S. metallicum (Waddy, loc. cit.). oNc]Ho llv{P 175.22 page 10 2.4 South Arabia 2.4. L Yemen The skin disease known locally as rrsowdarr has been known to exist in the Yemen Arab Republic for a long time. The word trsowdarr in Arabic means black. Dr Merighi, Director of the Civil Hospital at Ta'Lzz, was the first to dernonstrate the existence of microfilariae in the skin of those affected with this condition. Although there are differences in the mode of presentation of the disease in Yemen and in Africa, it is believed that the parasite belongs to the genus Onchocerca. Nagaty (1963), while doing a parasitic diseases survey in Yeuren, suggested thaE the cases of onchocerciasis found there were imported from Ethiopia. This may be partly true, as there exists, in fact, a continuous contact. between the Yemen and Africa, especially East Africa. Yemenites are found throughout East Africa, and particularly in Ethiopia, Somalia, Sudan, TanzanLa, Kenya and Uganda, in which countries they work as traders, some of them residing there permanently. There is, therefore, a continuous traffic to and fro between these countries and Yemen. Anderson & Fuglsang (L973) have proved beyond doubt that the disease is endemic in the Yemen Arab Republic, thus confirming previous findings by others. The presence of the vector S. damnosum which was found by Merighi (L964), gives proof that onchocerciasis can be and probably is transmitted in the country regardless of whether the disease was first imported or originated locally. According to Anderson & Fuglsang (1974), onchocerciasis is mainly distributed in and around Tatizz which is situated in the southern part of the Yernen Arab Republic. It is found in such localities as A1 Barh which is about 80 km to t.he north-east of Tati,zz ar.d cases were also seen in the hospitals at Sanra and Hodeida. These aut-hors found nodules in a small number of cases and confirmed the existence of onchocercal ocular involvemenE, probably for the first time in this country. It is in this southern part of Yemen that heavy rains faII every year reaching up to 1OO0 nrn and it is from this region that neighbouring Democratic Yemen gets its Tirater, particularly for agriculture. Further investigation of onchocerciasis in this area, which is mountainous and in places difficult of access, has been recommended by Anderson & Fuglsang (1oc. cit.), who suggest thaL this focus probably extends throughout Yemen and possibly north into Saudi Arabia. 2.4.2 Democratic Yemen The first report of the existence of onchocerciasis in DemocraEic Yemen r,ras that of Fawdry (L957) who described about 50 cases of rrsowdarr that had come to Aden Hospital. He believed that all these cases were imported from the Yemen Arab Republic. In some of these cases nodules were removed and opened, and adult O. volvulus r^rere found. Taxonomic confirmation of the parasite species was provided by Professor Buckley of the London School of Hygiene and Tropical Medicine. Ln L972 the writer undertook two MlO assignments in the Peoplers Democratic Republic of Yemen (pOny). During his first visit to study the health aspects of the Wadi Tuban Soil and Water Conservation project, the writer (Satti, L972a) made a search for t.he presence of onchocerciasis in Democratic Yemen. Although the results of this search \rere negative, it was possible to collect useful informatlon indicative of the probable existence of the disease, characterized by an erratic distribution and a paucity of cases. However, it was not possible to visit certain areas like Beihan in the Fourth Governorate, where trachoma cases were described during an ophthalmological survey carried out a few years ago. It should be noted here that it can be difficult to differentiate between eye complications due to onchocerciasis and those due to trachoma and that the two infections may co-exist in the same patient. Moreover, certain lesions due to onchocerciasis may simulate those due to other paEhologies. While in Iahej, the writer saw two cases of what might be considered as a degenerative choroidoretinitis, and which looked like what Choyce ( ) sometimes called Bojengo fundus. These two patients were completely blind and both had resided in the Yemen Arab Republic for some years. The skin snips, however, were all negative and so was the Mazzotti test. oNcPiolwP/75.22 Page 1l During the writerts second visit, to Democratic Yemen in L972, a parasitic diseases survey was carried out and the following quoEation is reproduced from his assignment rePort (Satti, L972b): nNo cases of onchocerciasis were encountered in spite of a careful search carried out for them throughout the assignment. Of course, the entire Republic !'as not covered, aS for security reasons, the Fourth Governorate r^/as not visited by the writer at all. It is said that in this Governorate there were many cases of blindness which are considered to be due to trachoma. What is more confusing is that both trachona and onchocerciasis may co-exist' in the one patient. It, therefore, needs very careful scrutiny and investigation to establish which is which. 'tThe writer would have liked to find out whether onchocerciasis exists in Beihan, since from trustworthy sources there are reports that indicate that cases of onchocerciasis existed from time-to-time in PDRY. The writer was told by a health assistanE at Dha1a that in the Fifties he saw two cases of dermatitis, which according Eo his description simulated whaL is known as elephant skin over the lower part of trunk lncluding Ehe buttocks and both legs. This was the cause of considerable itching. Moreover there were nodules on the side of the chest. A British doctor, after consulting the textbooks, told him that these were symPtoms of onchocerciasis. The cases \,fere seen near the foot of the Jebel Gehaf near Dhala where there is a waterfall. ItMoreover, Dr Abdel Sa11am told the writer about a case of a dhobi in El llakhzan llospital who was suffering from a similar dermatitis telephanE skinr as was described in the two cases mentioned above for about four years. tle saw many skin specialists hrith no avail. Finally he came to him when he thought of filariasi.s as a cause and took a skin snip, which showed microfilariae which he admitted he could not idenEify, but Ehe condition was completely cured by giving Bayer 205 and Hetrazan. The writer saw this dhobi in March and he was in perfect health. ttTo all intents and purposes, PDRY seems to be a receptive area for onchocerciasis, as there are many favourable habitats for the breeding of the vector.tt The infection may be imported from the Yemen Arab Republic at any time. 3. VECTOR SPECIES This is not meant to be a comprehensive revi.ew, for there is a wealth of literature dealing with the various aspects of the subject in the various endemic foci in different areas of the world. Lewis (1948, 1950, 1956, L957, 1958, L959, 1960, 1968) and Lewis & Duke (1966) dealt with the distribuEion, biting habits and flight range as well as with meny aspects of the biology of Simuliidae, ur,ainly in the Sudan but also in other parts of the world. Barnley(L949, L952, L954, 1958) and Barnley & Prenrice (1958) studied the vector in Uganda, where both S. damnosum , in the Ruwenzori region, and .@!, in Mount Elgon, are the vecEors. Mcllahon (1940, 1958) and Garnham & Mcl,Iahon (1947) dealt \.rith control and other aspects of the vector S. neavei such as its association during its aquatic stage with the freshwater crabs. Garnham & McMahon (L947) succeeded in eradi-cating the S. neavei vector from Kenya, not only by insecticidal operations but also by Ehe destruction of the freshwater crabs of the Senus Potamenautes. In Tanzania, @!i and S. demnosum are the vectors of onchocerclasis. S. woodi is the principal vector in Amani and Tewe (Lewis, 1960; Raybould,, L967). In the Ngurus area, the vector has not been identified, but S. damnosum predominates. In the t0ahenge area, Hausermann (1966) found S. damnosum to be the main vector, with an anthropophilic menber of the .@i complex being found in small numbers in only two places, at Kinole and BwakiraIn Ethiopia the vector is believed to be S. damnosurn although no Ehorough entomological surveys have been carried out. oNcHo/wP/75.22 Page 12 rn the sudan, S. damnosum is the vector in all the known endemic foci , but it is believed that S. neavei may exist on its border with Uganda, in the Imatong mountains. In a letter to Dr Osrnan M. Abdel Nur, McCrae (f973) mentioned that he had been able to Erace S. neavei right to Ehe Sudan border, in an area which had not been previously surveyed. In the Americas, S. ochraceum is the main vector in Guatemala and Mexico, while S. metallicum is the vector i.n Venezuela. The Simuliidae in these foci have been studied by different workers including Lewis (L962). In Colombia, the only existing anthropophilic sPec ies is S. exiguum , and this country, like Guatemala and Mexico, has large areas of mountain coffee at heights of up to 2000 m on both sides of the central ridge of the Andes. The nature of Simuliun populatlons in these areas is not known; its investigation is worth undertaking, parEicularly in order Eo know if S. ochraceurD exists there (waddy, L967). S. metallicum and S callidum are vectors of minor importance Dunbar (f968) reports the existence of nine eytological types of S. damnosum based on t,he examination of the banding pattern of the giant chromosomes, and consequently we now speak of the Simulium damnosum complex. Of these nine siblings which were named after the sites of collection, rrNilerr is the most widespread and is associated primarily \{ith the Sudan. Lewis & Duke (1966) described two S. damnosum types: (i) a Sudan-savanna variety which is pale in colour, and (ii) a forest ""ffiffifEf, is dark in colour. rn the Sudan, in the Abu Hamedarea, there is also a desert variety which, like the forest variety, is dark in colour (Gassouma, L972). This appears to be a paradoxical situation in which two opposite types of habitat are producing the same colour in Simuliidae. oNcRo lwPl75.22 Page 13 REFERXNCES 1 Anderson, J. & Fuglsang, H. (1973) Clinical aspects of onchocerciasis in Uganda and Yemen Arab Republic compared \rith a rainforest and savanna focus in Cameroon (Unpublished document r{no/oNcHo/ 7 3.1o2) Anderson, J. & Fuglsang, H. (L974) Assis-Masri, G. & Little, M. D. ( Barnley, C. R. (1949) 1965) Trans. roy. Soc trop. Med. Hys. , 59, 7L7 Barnley, c. R. (1952) Barnley, G. R. (1954) Barnley, c. R. (1958) Barnley, c. R. (1968) Barnley, G. R. & Prentice, M. A. Blacklock (L926) (1958) Simulium neavei in Uganda, E. Afr, med. J., 35, 475-485 Blacklock (L927) Bloss (1949) Brumpt (1919) Brumpt & Muhlin (L927) Bryant (1933) Bryant (1934) Bryant (1935) Buckley, J. J. C. Kenya. I. (1949) Studies on human onchocerciasis and Simulium in Nyanza province, Distribution and incidence of O. volvulus I Most of these references need to be completed. , J. Helminth., 23 , L-24 oNc]Plo /wP/75,22 Page L4 Carlsson et al. (L974) Choyce (?) Colbourne, M. J. & CrosskeY, R. W document wHo/oNcllo/ 30.65 Rev (1965) onchocerciasis and its control in uganda (unpublished 1) Crosskey, R. W. ( 1955) Ann. trop. Med. Parasit., !2, t4z Crosskey, R Trans. (1956) rhe distribution of Simulium damnosum Theobald in Northern Nigeria, Soc. trop. Med. HYg , 50, I.I. roy. Cruickshank, A. (1934) Onchocerca vo lvulus 379-392 ( Leukart ) : its occurrence in the Southern Anglo-Egyp tian Sudan and its significance in the t! iology of endemic blindness (Thes is , Aberdeen. Dj,az (L957) Dunbar (1968) Abstracted in Helminth. Abstr., L934, 3' 19 1) East African Medical Journal (1939) Symposir:m on onchocerciasis in East Africa, J. , 15, 363 E. Afr. med Fawdry, A. L. (1957) Onchocerciasis in So'uth Arabia, 253-256 Trans. roy. Soc. trop. Med. Hyg. , 51, Fisher, 0. (1932) Areh. Schiffs-u Tropenhye 36 61 Garnham, P. C. C. & Mcl'lahon, J. P. (1947) The eradication of Simulium neavei Roubaud from an area of onchocerciasis in Kenya Colony, Bu11 enE. Res , 37, 6L9 Gassouma (L972) Gassouma (L975) Ilaseeb et al. (L962) Hausermann, W. (1966) Highton (L974) Hissette ( f931) HisserEe (L932) Hissette ( 1937) t oNcBo lwP/ 75.22 Page 15 t Hissette (1938) Kirk, n. (1947) gbservaEions on onchocerciasis in the Bahr-eL-GhazaL province of the Sudan, Ann. rrop. Med. Parasit. , 4L, 357-364 Kirk (1949) Kirk et a1. (1959) Laing & Wegesa (L964) Le Berre (L967) Le Berre (1968) Lewis, D. J. (1948) Lewis, D. J. (1950) Lewis, D. J. (L952) Lewis, D. J. (1956) Lewis, D. J. (L957) Lewis, D. J. (1958) Lewis, D. J. (1959) Lewis, D. J. (1960) Lewis, D. J. (L962) Lewis, D. J. (1968) Lewis, D. J. & Duke, B. O. L. (1966) Onchocerca Simulium complexes. II. Variation in WestAfrican fpmnle Simulium damnosum, Lumsden & Buxton (1951) McCrae, A. W. R. (L972) Ann. tro Med. Parasit. 60, 337-346 ,l oNcHo/wP/ 75. 22 Page 16 McCrae, A. W. R. (1973) McKelvie (1934) McKelvie (1935) McMahon (1951) Merighi (1964) Moghraby, A. (1975) Morgan (L947) Nagaty, H. F. (1963) A survey of malaria, bilharziasis, and other parasitic infections in the Yemen (Unpublished document wHO/EWEPID/9) Nelson, G. S. (1958) Onchocerciasis in the west NiIe district of Uganda, Trans. roy. Soc. trop Med. Hyg. , Z, 368-376 oomen (1967) ovazza (?) ovazza (196r) Raybould (?) Raybould (L967) Roberrs et al. (L967) Robles (1915) Robles ( 19 19) sarri, M. H. (1948) Satti, M. H. (L972a) Assignment report. SoiI and water utilization and conservation in the Wadi Tuban $/atershed (Unpublished WHO docrunent EWEPLD/22) ., M. H. (Lg72b) Assignment report. Parasitic diseases survey in the Peoplers Democratic Republic of yemen (Unpubtished WHO document EWPDI4 - EVI/EPID/Z4) I I, rriSa oNcHo/wP/ 75.22 Page 17 t t sherif (L972) Sprengel ( 1967) Sprengel ( 1968) Strong et aI. (1934) Waddy, B. B. (1967) Report on a tour of onchocerciasls foci in Mexico, Guatemala, Venezuela and Colombia, 2 November - 21 Deceurber 1966 (Unpublished docunent WHO/ONCHO/67,60) Wenk (1968) wegesa ( 1968) Wegesa (1969) Woodman (1949) Woodruff, A. W. (1965) The distribution of onchocerciasis in Tanzania (Unpublished document wHo/oNcHo/ 32.65) t I oNcRo /wP /7 5 .22 page 18 ANNEX SOME NOTES ON RESEARCH PROPOSALS Although this writer favours applied and operational research, he nevertheless feels that basic research may, at times, be urgently needed for the understanding of a Particular situation and so as to be able to devise ways and means of reversing certain situations that will assist in the control of the disease. Moreover, in order to be able to attack the vector efficiently on a sound scientific basis ecological and behaviouri.stic factors as welI as physiological characteristics ought to be studied. It would also be beneficial to study the rnetabolism and nuEritional requireroents of the parasite in the different endemic regions of the world to determine whether there is biochemical and physiological basis for the existence of different parasite strains in the different areas of the world. This could only be achieved by perfecting techniques with special reference to those required in attemPts to culture the parasite in vitro or in tissue culture. In a report to the National Council for Research, the writer indicated some areas where research is needed: 1. VECTOR BIOIOGY AND CONTROL In this area the ultirnate alm is to lower the cost of vector control. The perfection and simplification of the techniques used increases the efficacy of mass campaigns, and this becomes feasible by studying the various aspects of the biology and ecology of the vector under the existing local conditions and thereby devise ways and means of perfecting appropriate control methods. It is also important to study the various methods of insecticide application and compare the efficacy of the different control techniques such as larviciding, adulticiding and abolition of the habitats by engineering and other methods. Some specific vector studies are suggested below. Egg stage. The effect on this stage of insecticides and desiccation by the sun should be determined. Larval stagq. The distance of drift of larvae and differences between the larval and pupal sites should be investigated. Larval life under different conditions should be \ t studled. Le Berre (1968) reports that the larval cycle is about eight days but can be shorter under exceptional conditions. for S. darnnosum Adult flies There is lack of knowledge regarding 914llU9ql migration, dispersion andflight range under different environmental conditions. Study of these factors with special reference to females, will help attenpts to stop reinvasion of treated areas by flies froro untreaEed areas. The question of reinvasion needs thorough study: is it caused by reinvasion from permanently infested sites or is it a result of aestivation? Informatlon is required regarding the longevity of females at different seasons and in different ecological conditions, with particular reference to the northern linit of S. damnosr:m distribution. A better knowledge of resting places is also required as this rilfi.fEEult control feasible because we will know where to attack. The differences between the various types of Simulium populations should be studied, including their ecological differences as well as differences in relation to their distribution. 2. INSECTICIDES AND OfiIER TTETI{ODS OF CONTROL It is suggested that research in this area include: (a) testing of new larvicides and methods of application (e.g. aerial) as well as of new formulations and formulations Ehat are easily soluble in water like carbamates. (b) study of biological control of the Simulium vector, or "prey-predatorrr relationships (see "PredationGFsect.ion 3). t in other words possible I toNCHo/WP/ 75.22 Page 19 Annex 3. VECTOR ECOIOGY AND BEHAVIOUR The study of vector ecology and behaviour is considered essential as it will provide knowledge of the conditions that are favourable to the propagation of simuliids and will make efforts to control thern more effecEive. Such a study poses many quesElons that will need answers: (a) why are S. neavei limited to certai.n areas? Temperature is believed to be only one of several facEors; (b) why are there differences in colour If it is a question of oimicry, what are the Sudan, the desert variety is dark in type. Why is this when the differences between the various strains of S. damnosum? the physiological factors Uetrirrffi- colour (Gassouma, L972) like the rainforest between these two region are obvious? Eco logy A full study of the ecology of the onchocerciasis endernic areas is required. (a) Mecha4ical factors: (i) importance of the speed of the current and its swiftness("o*"@edins1owwaterandsomethriveinp1acidrmter);(ii) importance of turbulence and high oxygen content (some simuliids breed in water with high silt content where oxygen is reduced). (b) Food Pressure: this question should be investigated in the different ecological envj-ronments and analysed by comparative studies. (c) Phototaxic factors what is the role of light in the breeding of the different vector strains? (d) Thermotaxic effect the possibility of this effect needs to be evaluated i.n comparative studies (Ovazza, L96L; Le Berre, L967). (e) Limnologicql studies the various breeding sites the growth of the particula these should include all physical and cheoical factors inin the different geographical areas and their bearing on ,r strain. Behaviour Wenk (1968) studied the mating and swarmi.ng cit. ) made a similar study in the Abu Hamed area. behaviour of S. damrostrm and Gassouma ( loc I t Dry sea son survi.val where and how does the fIy spend the dry season and in what stage? Studies should be made in some non-Perennial rlvers and should include in Sudan nearly all the southern ri.versin Bahr el Ghazar province and some of those in Equatori-a province. The predator-Pr?ey relationship should be carefully studied with a view to possiblebiological control of Simuliidae. Parasites such as mermithids and fungi such asMicr-ospori9ia should receive full attention. carlsson et al. (L974) rnade a full investlgationof the ecology in upper Volta and this work should be extended to include aII the areas of9. damnosum and also S. neavei in the hope of throwing light on the etiological factorsdeterminj-ng tne aitteGffi vector behaviour, strains, etc. This may read to moreeffectlve methods of control. Preda t ion oNcHo/wP/ 75. 22 Page 20 Annex 4. EPIDEMIOIOGICAL SURVEYS proper prevalence surveys should be carried out in certain villages which should be chosen either by sampling or by selection on the basis of specific attributes. Health cards should be filled out for everybody in the villages. Reconnaissance surveys should be carried out in areas not previously covered like those in the Sudan provinces of Equatoria and Upper Nile where onchocerciasis is believed to exist but r,rhere there is no confirmation. This should include a search for the vector. A more detailed study can be carried out at a later time. I I I