Expert Advisory Committee: fourteenth session: Ouagadougou, 7-11 June 1993: briefing session, 5 June 1993: VCU activities in the western zone, April 1992 - March 1993

ONCHOCERCIASIS CONTROL PROGRAMME IN WEST AFRICA Expert Advisory Committee Fourteenth session Ouaeadoueou. 7-ll June 1993 Briefing session, 5 June, 1993 OCP/EACl4/Briefing paper no5 ORIGINAL : FRENCH VCU ACTIVITIES IN THE WESTERN ZONE, APRIL I992-MARCH 1993 Lanicides Compared with the previous period (April l99l-March 1992), the only noteworthy larviciding coverage difference was in Sierra Leone where local conflicts forced us, for security reasons, to suspend larviciding. That was why the Kolente, the Mongo, the Kaba and the Rokel were not treated from l4 December 1992 to 27 March 1993 and larviciding has been suspended on the Sewa, the Bagbe, the Bafi, the Pampana and the Teye since I November 1992. 84,500 additional litres of B.t. H-14 were used during this period to make up for the bad quality of the batches received in 1992 by doubling the operational dose. The greater part of these defective batches has been replaced by the company concerned and a last batch will reach us soon. Insecticide rotation, aimed at preventing possible blackfly resistance phenomena and limiting the effects of larviciding on the non-target fauna, has once again proved its efficacy because no decrease in susceptibility, particularly to Abate, has been reported and the impact of the chemical insecticides on the environment has been considered satisfactory by the Ecological Group. If, in addition to the cost of the insecticides, the parameters related to their application and performance in river (dose, carry, flight hours, transport of the insecticide and fuel to the catches, etc.) are taken into account, although the purchase price of pyraclofos is high, it has proved to have a good cost-effectiveness. On the other hand, the application of carbosulfan is costly and it can be used only at very high discharges. 2. Aerial operations The aerial operation activities of the Odienne base have been carried out without any major problem. The new aerial spraying contract between OCP and the aerial contractor, signed for a three-year period with effect from I January 1993, mentions a significant decrease in the cost of helicopter flight hours and the cancellation of the larviciding aircraft (Thrush) and the liaison aircraft (Cessna). In the Western Zone, because of the doubling of the B.t. H-14 dose resulting from the existence of defective batches, in May, June and July 1992 the number of helicopters exceeded the forecast for the year, raising the number of months/aircraft to 58 instead of the 55 initially planned for 1992. The insecticide spraying system developed in l99l and improved in 1992 with the help of a small computer on board now works satisfactorily. This improvement does not, however, solve the difficulties encountered by the pilots at the end of the rainy season when they have to shift from long-carry chemical insecticides to B.t. H.l4 which necessitates the larviciding of one breeding site after the other. This leads to a poor application of the biological insecticide the consequence of which, as we observed this year, notably in Mali in the Baoule and the Bagoe, is an increase in the number of flies, fortunately without any effect on transmission. 3. Entomological evaluation Since November 1992, the entomological evaluation in the Western Zone has been benefitting from the generalization of morphological identifications of females caught and from the DNA-probe identification of infective larvae of Onchocerca volvulus. This has resulted in the refinement of the transmission parameters which will now distinguish between the OCP/EACI 4/Briefing paper no 5 Page 2 transmission potentials of savanna females and those of forest females. Furthermore, within each cateS,ory, a distinction will be made between the transmission of savanna-strain O. volvulus, the transmission of forest-strain O. volvulus, and the transmission of animal filariae (O. ochenqi) or, still, undifferentiated filariae (non-volvulus). There was also an intensification of ivermectin distribution during this period since almost all the basins in the Western Zone are now under therapeutic coverage. In the Guinea part that has been under ivermectin since 1998 and under larviciding since 1989, onchocerciasis transmission has been reduced by 77Vo, on average, after only four years of intervention. Compared with the original area where only larvicides have been used, the decrease in transmission, after four to six years of insecticide spraying, is only 2l% even though sometimes it could, under very favourable conditions reach almost 58%. This beneficial effect related to the combined use of two control methods makes it possible to tolerate a greater number of flies without there being an increase in transmission. The primary consequince is a change in the la-rval control strategy which aims at reducing the larviciding coverage and, consequently, the cost of the operations. The resumption of larviciding, in April 1993, on the Kolente, the Mongo, the Kaba and the Rokel will make it possible to prevent possible movements of savanna flies which will have settled in Sierra Leone at the end of the dry season. Although it has not been possible to resume larviciding on the Sewa, the Bagbe, the Pampana and the Teye, these rivers do not seem to present a real danger to the rest of the Programme in terms of reinvasion. In the whole Western Zone, seven catching points have, for the savanna species, an Annual Transmission potential greater than 100 but in any case less than 200. These results are very satisfactory insofar as the first infective larvae identification results showed that four of these catching points had a great proportion of animal or undifferentiated filariae, thus raising to three the number of catching points where the transmission by savanna females is more than 100 larvae per man per year. In the original area, post-larviciding catches have been made where larvicide applications have been suspended, following the good epidemiological results indicating the almost complete disappearance of adult worms in man. On the White Bandama, post-control studies coviring 20,335 parous females have enabled only 22 infective females to be isolated, i.e., quite an acceptable infectivity level of 1.08 females with L3/head/1000 parous. On the Baniiing IV,infectivity level is only 0.39 females with L3lhead/1000 parous, but because of the ,ery lo* numbers (7 670 parous females caught), an additional study will be carried out this year.