PROCEDURE OF DEFINITIVE CESSATION OF LARVICIDING IN THE ONCHOCERCIASIS CONTROL PROGRAM IN WEST-AFRICA - ENTOMOLOGICAL POST-CONTROL STUDIES

In 1989, the Onchoceriasis Control Programme in West Africa started the first larviciding cessation in basins in its original area where the entomological and epidemiological results were considered satisfactory. It was in this context that entomological post-control studies were initiated with a view to ensuring that there had been no resumption of transmission of the disease despite the cessation of larviciding and, therefore, a return of the blackflies to their original level. The 18 catching points studied have, overall, given satisfactory results : the pre-control infectivity rates, which ranged between 60 and 90 infective larvae per 1000 porous females, have decreased to less than one infective larva per 1000 parous, the threshold below which the risk of recrudescence of onchocercal infection is considered most unlikely. The only exception is one catching point where the relatively high blackfly infectivity rate (8.9 infective larvae per 1000 parous flies) has led to the resumption of larviciding. The values obtained at two other points, though acceptable (1.08 and 1.16), have made it possible to detect the presence of infected human migrants who have been treated with a microfilaricide, ivermectin, in order to avoid any resumption of onchocerciasis transmission. On the whole, the excellent results revealed by these entomological post-control studies have led to the definitive cessation of larviciding in more than 90 % of the river basins in the original Programme area.