Onchocerciasis control strategies

Abstract

Sir—Frank Richards and colleagues (May 13, p 1663) raise strategic issues relating to the onchocerciasis control programmes. The strategy of the African Programme for Onchocerciasis Control (APOC) is based on annual mass treatment with ivermectin. Richards and colleagues state that APOC accepts that this strategy “will not stop transmission” and that “treatment may need to be continued indefinitely”. However, community trials have shown that mass treatment with ivermectin results in a major reduction in onchocerciasis transmission and that repeated ivermectin treatment reduces the productivity of adult O volvulus. Computer simulations indicate that interruption could be achieved after 15–25 years of annual treatment. The objective of APOC is to establish sustainable, communitydirected treatment with ivermectin by 2007, when the programme will come to an end, with the goal of eliminating onchoceriasis as a public health and socioeconomic problem throughout Africa. APOC does envisage an endpoint for yearly ivermectin treatment but the required duration is being determined by the establishment of a monitoring system to assess the decline in infection and transmission levels. In most of the area covered by the Onchocerciasis Control Programme in West Africa (OCP) that has been under effective control, the parasite has been virtually eliminated. In these areas, vector control ceased in 1989 and although the vector returned to pre-control densities within weeks, transmission has remained interrupted and no further intervention has been needed. OCP established a surveillance system for early detection of possible recrudescence and had developed an intervention strategy that aims to quickly stop transmission by ivermectin treatment. Thus, for much of the OCP area, surveillance by national teams will be the only intervention when OCP comes to an end in 2002. Contrary to Richards and Colleagues’ assumptions, only the OCP areas that have not fully benefited from vector control, will switch to the APOC strategy. In an isolated focus in Senegal, ivermectin treatment was given biannually in an attempt to interrupt transmission. Preliminary results indicate that this aim has been achieved after 9 years of intervention. If confirmed, these results might warrant further experimentation with 6-monthly treatment in large hyperendemic areas in Africa. Richards and colleagues also refer to 6-monthly treatment in Guatemala and Ecuador. However, experiences in Africa and South America are difficult to compare because of differences in vectors and numbers of people affected. Doubling the number of treatments would have major implications, and Richards and colleagues do not recognise the logistics and costs of additional treatment. These include geopolitical realities of distribution in complex emergencies; the need to continue expansion of APOC into the remaining hyperendemic and mesoendemic areas; and the uncertainties about onchocerciasis transmission in hypoendemic areas. Whatever the frequency of treatment, a definite solution will be difficult with ivermectin alone. Research on alternative drugs that would kill or sterilise the adult worms is going ahead. Promising results have been obtained with moxidectin. Work on the effect of albendazole 400 on onchocerciasis, whether alone or in combination with ivermectin (the same dose as recommended for lymphatic filariasis elimination), shows that albendazole 400 had no effect on the viability or reproductive capacity of adult O volvulus. We agree the lymphatic filariasis programme offers synergy and reinforcement of control. But annual treatment with albendazole and ivermectin is unlikely to have more effect on onchocerciasis than ivermectin alone.