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

Abstract

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.