The impact of Mectizan on the transmission of onchocerciasis.

For many years there was no suitable drug available for the control of onchocerciasis. The advent of Mectizan (ivermectin, MSD; an effective microfilaricide), its registration in October 1987 for the treatment of human onchocerciasis, and its suitability for large-scale application were major break-throughs in the control of human onchocerciasis via chemotherapy. Several studies, both fly-feeding experiments and community trials, have established that Mectizan treatment causes a significant reduction in the transmission of infection. Although long-term treatment in some isolated foci (such as occur in the New World and in some hypo- and meso-endemic areas elsewhere) appears to interrupt transmission, more prolonged treatment is required to prove if transmission can be stopped. Advantage could be taken of the significant impact of Mectizan on transmission by giving treatment while or just before transmission by blackflies is most intense.

Impact of combined large-scale ivermectin distribution and vector control on transmission of Onchocerca volvulus in the Niger basin, Guinea.

As part of the WHO Onchocerciasis Control Programme in West Africa (OCP), the attack phase of operations in the Niger basin in Guinea began in 1989 with the simultaneous use of ivermectin and vector control. Larvicide applications coupled with annual large-scale ivermectin distribution have greatly reduced blackfly infectivity (by 78.8% for the number of infective larvae per 1000 parous flies). The combination of vector control and ivermectin has permitted excellent control of transmission. In the original OCP area, it took 6-8 years of vector control alone to obtain an equivalent decrease in blackfly infectivity. For the same number of flies caught, transmission was much higher in areas where ivermectin had not been distributed. The combined use of ivermectin and vector control has opened up new prospects for carrying out OCP operations with, notably, the possibility of reducing larviciding operations.

PIP: As part of the World Health Organization Onchocerciasis Control Programme in West Africa (OCP), the attack phase of operations in the Niger basin in Guinea began in 1989 with the simultaneous use of ivermectin and vector control. All the 16 catching points were in holoendemic foci: 8 in the Niger basin in Guinea and 8 in the original OCP area (Mali, Ivory Coast, Ghana, and Burkina Faso). The data were analyzed according to prevalence of microfilariae in the skin and the mean community microfilarial load (CMFL). Between 1990 and 1992 the number of people in the villages treated increased by a factor of 6. In 1992 a total of 91,840 persons were treated in 550 villages. The study covered 10 years, during which 34,492 blackflies were caught at the 8 sites, 87.8% of which were parous. Larvicide applications coupled with annual large-scale ivermectin distribution had greatly reduced blackfly infectivity (by 78.8% for the number of infective larvae per 1000 parous flies; the number infective larvae in the head fell by 75.7% compared with the 1986-87 data before treatment began). After 2 years of large-scale ivermectin treatment, the reduction was 64.6%. In February and March of 1992 a defective larvicide worsened the situation. The average transmission potential during this period in Guinea was 7.3 compared with 93.7 for the original area. For the same number of blackflies caught, transmission in the original area was 5.6 times higher. The combination of vector control and ivermectin permitted excellent control of transmission. In the original OCP area, it took 6-8 years of vector control alone to obtain an equivalent decrease in blackfly infectivity. For the same number of flies caught, transmission was much higher in areas where ivermectin had not been distributed. The combined use of ivermectin and vector control has opened up new prospects for carrying out OCP operations with the possibility of reducing larviciding operations.

Criteria for the selection of larvicides by the Onchocerciasis Control Programme in west Africa.

One of the weekly decisions the Onchocerciasis Control Programme has to make, in its operations in 11 west African countries, is the selection of one insecticide out of the six used that is most appropriate to the river stretches to be treated. This decision depends on several criteria, linked not only to the compounds themselves but also the hydrological conditions and blackfly populations involved. Given the great number of breeding sites (gites) to be treated, in 23,000 km of rivers at the height of the rainy season, this paper identifies the parameters needed to facilitate the choice of insecticide and to optimize the larviciding in terms of cost effectiveness, management of resistance and minimizing the environmental impact.