Onchocerciasis Control Programme Area Research Articles

Recrudescence of onchocerciasis in the Comoé valley in Southwest Burkina Faso

Onchocerciasis control by vector control was instigated in southwest Burkina Faso in January 1969 by ORSTOM/OCCGE, and continued until operations were taken over by the WHO Onchocerciasis Control Programme (OCP) in February 1975, which itself ceased operations in the area in 1989 when onchocerciasis was judged to have been reduced to insignificant levels. Initially (1969–1975) vector immigration maintained unacceptably high levels of transmission, but OCP was much larger than the preceding campaign and in 1975 the Annual Transmission Potential (ATP) dropped below 100 at all sites in the Comoé river valley except Folonzo, which continued to be subject to reinvasion, along with the whole of the Léraba river valley. However, after the southern extension of the OCP in 1979, ATPs dropped below 100 everywhere in the Comoé basin (including the Léraba valley), and further dropped to insignificant levels after the western extension of the OCP in 1985. Thus transmission dropped more quickly in the Comoé river valley than the Léraba river valley (which had been subject to vector reinvasion), and this was also reflected in prevalence of microfilaraemia in the human population. After 1986 prevalence was less than 5% in all villages in the Comoé river valley (except for two, which subsequently dropped to 0% and 3.7% by 1999). However, in 2001 (12 years after the cessation of vector control) the prevalence in one village in the Comoé river valley had increased to 39.6%, and two more had increased above 5% by 2007. New epidemiological surveys in 2011 and 2012 showed that in 13 out of 30 villages in the Comoé river valley prevalence of microfilaraemia was above 5%, although this was not observed in the Léraba river valley where prevalence remained low. This is the first documented case of recrudescence of onchocerciasis in the old OCP area, and the reasons are not clear. It is possible that there has been immigration of parasites with humans or vectors from areas where there has been a shorter period of control, or that control has been less effective. It is possible that in spite of very low levels of transmission the local parasite population was never reduced to a level below the transmission breakpoint, or that there has been a local recrudescence due to stochastic population effects. In any case it is clear that the distribution of ivermectin against lymphatic filariasis in the area since 2004 has failed to prevent the recrudescence of onchocerciasis, and the Burkina Faso Programme National de Lutte contre l’Onchocercose (PNLO − Ministere de la Santé) has instigated a programme of Community Directed Treatment with Ivermectin specifically aimed at onchocerciasis in accordance with the strategy developed by APOC and recommended to governments by OCP when it was dissolved in 2002.

Morphological differentiation of vectors of onchocerciasis, <i>Simulium damnosum</i> complex, in south-east Nigeria

The reliable identification of biting adult females of the sibling species of the Simulium damnosum complex transmitting the different species and strains of Onchocerca volvulus is very essential in studies of transmission and epidemiology of human onchocerciasis. A novel technique for morphological differentiation has already been developed for the Onchocerciasis Control Programme area of West Africa and this method was tested on specimens obtained both from human baits and pupae rearing in the south-east of Nigeria. A total of 126 adult female flies, belonging to the Simulium squamosum sub-complex and the Simulium damnosum sub-complex were identified from 8 sites. The morphological identifications were compared with the cytotaxonomic identifications of larvae collected within the same period at the same site. The method was found to be reasonably reliable. Keywords: Simulium damnosum complex, onchocerciasis, differentiation; Onchocerca volvulus.Nigerian Journal of Parasitology Vol. 29 (1) 2008 pp. 61-66

Sibling species distributions of the Simulium damnosum complex in the west African Onchocerciasis Control Programme area during the decade 1984-93, following intensive larviciding since 1974.

During the decade from 1984 to 1993, nine species of the Simulium damnosum complex of blackflies (Diptera: Simuliidae) were identified from the area covered by the Onchocerciasis Control Programme. These were S. damnosum s.s., S. dieguerense, S. konkourense, S. leonense, S. sanctipauli, S. sirbanum, S. soubrense, S. squamosum, and S. yahense. Some of these species were found to consist of two chromosomal variant populations. These were S. konkourense 'Konkouré' and 'Menankaya' forms, S. sanctipauli sensu stricto and 'Djodji' form, S. soubrense 'Chute Milo' and 'Beffa' forms. The distribution of these twelve cytological taxa was assessed in relation to the two main vegetation zones of West Africa (forest and savanna), topography, river size and other factors. The range of each species was influenced by seasonal climatic changes in wind movement and river water level. The most widely distributed species were S. sirbanum and S. damnosum s.s., associated with savanna areas, recorded from all river basins. Simulium dieguerense was restricted mainly to Western Mali on the Rivers Bafing and Bakoye in the Senegal River basin. Simulium squamosum was identified from rivers draining mountainous areas in both the forest and savanna zones. Simulium yahense was found in small permanent rivers along a wide forested band parallel to the coast and was absent from the plains of Togo and Benin. Members of the S. sanctipauli subcomplex had restricted distributions except for S. sanctipauli s.s., which was widespread in large rivers of the forest zone from Sierra Leone to the Volta Lake in Ghana. Simulium soubrense 'Beffa' form occurred in Togo and Benin, S. soubrense 'Chutes Milo' form in Guinea, both 'Konkouré' and 'Menankaya' forms of S. konkourense occurred predominantly in Guinea and S. leonense in Sierra Leone. The relevance of the distribution maps and the importance of the data bank to vector control larvicidal operations are discussed.

Cytotaxonomic revision of the Simulium sanctipauli subcomplex (Diptera: Simuliidae) in Guinea and the adjacent countries including descriptions of two new species

AbstractThe Simulium sanctipauli Vajime & Dunbar subcomplex of the West African S. damnosum Theobald complex is cytotaxonomically revised for the western part of the Onchocerciasis Control Programme area. The subcomplex is defined and a chromosomal key provided for the identification of the sibling species and forms recognized. Two sibling species are newly described, S. leonense Boakye, Post & Mosha (Sierra Leone) and S. konkourense Boakye, Post, Mosha & Quilleévéré (Guinea and Sierra Leone). Detailed chromosomal data are provided as warranty for the conclusions about the specific or infraspecific status of the taxa recognized.

Laboratory toxicity of potential blackfly larvicides on some african fish species in the onchocerciasis control programme area

The Onchocerciasis Control Programme of the World Health Organization uses larvicides to fight against the aquatic stages of the vector Simulium damnosum s.l., and thereby interrupt transmission of the disease. Since the appearance of resistance to Abate and chlorphoxim in certain cytotypes of the vector, the efficacy of many possible replacement insecticides has been tested and the impact of the best of them (permethrin, cyphenothrin, pyraclofos, and carbosulfan) on the aquatic fauna evaluated.

Measures aimed at controlling the invasion ofSimulium damnosumTheobald s.l. (Diptera: Simuliidae) into the Onchocerciasis Control Programme Area. III. Searches in the Upper Niger Basin of Guinea for additional sources of flies invading south‐eastern Mali

Abstract Very large breeding sites of the onchocerciasis vector, Simulium sirbanum were discovered along the Rivers Niger, Mafou, Kouya, Niandan and Milo in the Upper Niger Basin of Guinea. Flies reared from these breeding sites and caught biting man nearby were found to be in the same size range as those collected in the south‐eastern Mali invasion zone of the Onchocerciasis Control Programme Area. The periodicity of the Mali invasion appeared to be related to the hydrological conditions in the Upper Niger Basin during the early monsoon. During this period, rising water levels greatly increase vector breeding in the principal rapids of the Niger and its tributaries which lie upwind and to the south‐west of the invasion sites in south‐eastern Mali. The Mali invasion ceases towards the height of the rainy season when many of these rapids are flooded. It was concluded that the very large breeding sites associated with the principal rapids in the Upper Niger Basin were the main sources of the S. sirbanum invading south‐eastern Mali.

Reinfestations of the southeastern flank of the Onchocerciasis Control Programme area by windborne vectors

During the rainy season of 1980 the southeastern flank of the W.H.O. Onchocerciasis Control Programme was extended to include potential sources of Simulium squamosum , S. damnosum s.s. and S. sirbanum , vectors of onchocerciasis that were reinfesting the controlled zone in Togo. The experimental extension with a radius of 100 km from the reinvaded sites only partly reduced the scale of the reinfestations. To examine whether some S. squamosum , the principal species involved in Togo, were travelling more than 100 km, a larger experimental extension was treated in 1981, which further reduced the numbers reinvading. The results of these trials and data on the ages and species compositions of the reinfesting flies, their likely sources and the meteorological conditions in the area are discussed. It was concluded that S. squamosum may travel for 150 km or more in Togo and that S. damnosum and S. sirbanum travel similar distances in both Togo and Benin.

Development of a Model for Simulium damnosum S.L. Recolonization Dynamics at a Breeding Site in the Onchocerciasis Control Programme Area When Control is Interrupted

(1) Problems associated with Simulium damnosum s.i. population dynamics and control are discussed in relation to the Onchocerciasis Control Programme in the Volta River Basin. (2) Nulliparous and parous biting rates were observed during the recolonization of Grechan rapids when larvicide applications were interrupted for 3 months during the dry season. (3) A new method of data analysis is proposed and used to estimate population parameters. (4) A new mathematical model of recolonization is proposed which mimics the key features of the process. The model is used to simulate the effect of larvicide and could form the basis for an investigation of alternative management strategies.

Species composition and cyclical changes in numbers of savanna blackflies (Diptera: Simuliidae) caught by suction traps in the Onchocerciasis Control Programme area of West Africa

AbstractTen large suction traps were operated continuously, day and night, throughout the wet season of 1977 at four widely separated places within the area of the Onchocerciasis Control Programme in the Volta River Basin (OCP) in West Africa. At least ten species ofSimuliums.l. were trapped and the daily catches were analysed qualitatively and quantitatively. In the total catch of 9189 flies there were 187 males and 9002 females; among the latter only six were gravid and only five contained a full blood-meal. Cyclical changes in numbers, with periodicities ranging from 10 to 20 days, were analysed in series of overlapping Gaussian distributions, each possibly reflecting the rise and fall of a distinct population. Similar patterns in the sequence of cycles were shown byS. adersiPomeroy,S. ruficorneMacq.,S. evillenseFain, Hallot & Bafort and other species at a particular trapping site. Patterns differed between catching sites, except in the general tendency for population maxima to increase during the season. In some species, notablyS. adersi, populations decreased in late July and early August before increasing greatly in September (end of the rains) prior to the October onset of the dry season. Among the species caught,S. evillensewas present at all four trapping sites and particularly abundant at two of them, although the species has never been recorded before in West Africa and its early stages remain undiscovered there despite special searches for them since the capture of adult flies. By contrast, only four specimens ofS. hargreavesiGibbins were trapped although this species is abundant in local breeding sites. Diel flight periodicity was recorded over a one-week period at the extreme end of the wet season and showed that most flies were caught in the daytime;S. ruficorneshowed a greater tendency to a unimodal periodicity than other species. The behaviour of flies at the time of capture is discussed, including a consideration of the local wind speeds and the air speed ofSimuliums.l.

Effect of an organophosphorus insecticide, abate, used in the control of Simulium damnosum on non‐target benthic fauna

The effect of Abate used in the control of Simulium damnosum on non‐target benthic invertebrate organisms was studied in river Oti (West Africa) in the Onchocerciasis Control Programme area. Abate was administered by aerial spraying and pre‐dosing and post‐dosing Surber samples were compared. Baetidae and Leptophlebiidae, two of the three predominating organisms were seriously affected by the Abate. Other non‐target organisms that were killed were Gomphidae, Libellulidae and Dytiscidae. Though the larvicide was toxic to other non‐target organisms, particularly Neoperla and Orthocladinae, the effect did not seem to be significant.