Vector control results in eastern zone: June 1991- May 1992

ONCHOCERCIASIS CONTROL PROGRAMME IN WEST AFRICA OCP/EACl3/Briefing paper no. 7Expert Advisory Committee Thirteenth session Ouaeadoueou. E-12 June 1992 Briefing session, 6 June 1992 l. VECTOR CONTROL RESULTS IN EASTERN ZONE (June l99l - May 1992) RAINFALL AND HYDROLOGY Compared to the usual case observed in the eastern zone, the l99l rainfall had three characteristics, viz., it was abundant, came early and lasted for a longer time. The rains recorded were l5 to 60% greater than those of 1990. The river flows also began early and in certain cases (Niger and Oueme basins) they started seven weeks earlier than the previous years. The l99l discharges were far higher than those of 1990, with spates never equalled in the past 30 years. Many secondary and tertiary tributaries presented breeding sites very favourable for the development of the onchocerciasis vectors, whereas in previous years their flow had been intermittent. Consequently, it become necessary to extend the larviciding coverage to some of these tributaries. The l99ll92 low-water period (dry season) lasted longer than that of 1990/92 in that the drying-up of the principal watercourses was slower. A quicker resumption of river flow is therefore to be expected. 2. INSECTICIDE USE: COVERAGE AND EFFICACY The six operational larvicides were used in rotation and according to the constraints imposed by the discharges and susceptibility of the vector strains. The insecticide coverage was the same as in 1990, with its extension, however, to some tributaries of the Oueme and Kulpawn/Sissili basins as well as to the Mole river system. Abate was greatly used with success on all the tributaries of the Oti as well as in all the basins situated to the east of the Oti. Its reintroduction in the Kulpawn and the Sissili (White Volta basin) in October 1991, after an interruption of some years because of resistance, g,ave results considered to be 95% successful after five larviciding cycles. Likewise, good resuls were not obtained with this product in the Asukawkaw basin to the ea:it of the Volta Lake (August/September) as well as on the lower Mono below the Nangbeto Lake at the beginning of t992. The use of B.t. H-14 on the Oti encountered coefficient of flow problems. Permethrin was used on the lower Mono with a view to making savings because of the high discharges noted at the end of September and at the beginning of October 1991. The results obtained on the onchocerciasis vectors were not very satisfactory after five weeks of larviciding. It should be recalled that permethrin has practically never been used in the Mono basin where cotton growing calls for the use of other pesticides in the pyrethroid family. The other insecticides gave satisfactory results where they were used. lt should be pointed out that the use of permethrin and carbosulfan was extended to some tributaries of the Oti and of the Niger because of the high discharges recorded and that pyraclofos was used on a larger scale than in 1990 because of its availability. -2- Aerial larviciding and prospection activities consumed about 3,450 flight hours for the two types of aircraft used and for the whote of the new zone. About 730 flight hours were used for the treatment of the new watercourses in Cote d'Ivoire. From the viewpoint of cost of control activities (flight hours and insecticides), the expenditures made solely for the former eastern zone show an increase of only 890 in l99l compared to 1990, despite the sudden high discharges and the early flows recorded. This situation indicates a satisfactory managment of aerial operations in 1991. 3. OVERALL ENTOMOLOGICAL RESULTS The rainy-season entomological results were, on the whole, satisfactory in the eastern zone as regards the transmission rates, particularly in the initial area. [n the southern extension zones, the results were comparable to those of the previous years. At certain places, the biting rates were higher than in 1990, notably in the border areas of the treated zones either within the prog.amme area (larviciding-stoppage zones) or at its edge (Nigeria, southern Ghana/Cote d'Ivoire). A breakdown of the hydrological data reception station from mid-May to the end of September l99l might have caused an inappropriate treatment of the local blackfly breeding sources. 4. RESULTS OF SPECIFIC STUDIES 4.1. Contamination of the centre-east by blackfties: case of Bui and Nakong Bui - Because of the successful larviciding of the last reach of the lower Comoe, from the Abengourou bridge to Koutoukro (l7th to 30th weeks), there is a proof that the lower Comoe could not, at least this year, be a source of contamination of the Bui. Despite this success, blackflies, all savanna-dwelling type, were caught in the Bui (in relatively small quantities). Their origin could be the immediate tributaries of the Black Volta, to the east of the Bui, which were not treated at that time. Following the start of the treatment of these tributaries, particularly the Kohodio and the Fanko, a reduction of the blackfly densities in the Bui was noted. Nakong - Here also, it has been proved that according to the extent of the rainy season, as in 1991, there can be blackfly breeding on some tributaries of the Sissili (around Nakong), of the Kulpawn and of the Mole. There is reason to believe that the origin of the Nakong blackflies could be these watercourses which were treated occasionally in the past but have now been included in the regular treatement circuits. 4.2. studies on the contamination of the oueme by s.soubrense (beffa) The objective of these studies was to determine the spatial extent of rhe contamination of the Programme area by this forest species whose vectorial capability is great. The impact of rhis contamination on transmission was therefore to be quantified. The results obtained in l99l have confirmed: (a) the low dispersal of this species because of the insecticide pressure which prevents its development in the watercourses in the programme area; and (b) the high transmission it generates because of its great vectorial capability. 4.3. Vector control strategies in the Soth focus During the l99l/92 dry season, activities were carried out from the ground to treat as many breeding sites as possible on the Sota in order to neutralize the blackfly populations there. The initial results are convincing since about ten of the most important breeding sites were treated weekly resulting in a significant decrease in blackfly numbers, notably at Gbasse. It is suggested that this ground larviciding be continued this year during the dry season concurrentty wiitr ttre mectizan treatment for which the modalities for distribution in this focus are yet to be defined. 29.04.92