DRASTIC REDUCTION OF POPULATIONS OF SIMULIUM-SIRBANUM (DIPTERA, SIMULIIDAE) IN CENTRAL SIERRA-LEONE AFTER 5 YEARS OF LARVICIDING OPERATIONSBY THE ONCHOCERCIASIS CONTROL PROGRAM

The major vectors of the blinding form of human onchocerciasis in West Africa are two blackfly species, Simulium sirbanum and Simulium damno sum s.s. (Diptera: Simuliidae), identified at the adult stage as the ' savanna group' of the Simulium damnosum complex. In 1988, in the centr al part of Sierra Leone, the average daily biting rate (females/man/da y) by savanna blackflies (mostly S. sirbanum) during the peak of the d ry season (April-May) was 59.9, making up 69.1% of total captures on a verage. There was evidence of a strong long-range immigration of adult females of S. sirbanum through eastern Guinea in the dry season, with a reverse movement towards Guinea in the rainy season. Therefore, in 1989, the World Health Organization's Onchocerciasis Control Programme (OCP) extended its vector control operations from central West Africa to rivers of central and northern Sierra Leone, and to rivers of east ern Guinea. Four years of efficient larviciding drastically reduced ad ult populations of S. sirbanum in Sierra Leone. In the peak of the dry seasons of 1993 and 1994, the average biting rate by savanna blackfli es in central Sierra Leone had dropped to 1.0, making up only 4.3% of total captures on average. Yearly biting rates by S. sirbanum in centr al Sierra Leone were therefore reduced to 2% of their pre-intervention levels. Based on larval samples, the S. sirbanum has been replaced by two forest species, S. leonense in the south and S. squamosum in the north. Since 1992, it has been possible to calculate accurate transmis sion rates for blinding onchocerciasis, based on DNA-probe identificat ions. From 1993, the risk of transmission has not only been reduced by vector control but also by mass distribution of ivermectin to rural c ommunities. In terms of control strategy, the authors conclude that la rviciding operations could be alleviated in central Sierra Leone witho ut increasing the risk of blinding onchocerciasis transmission, as lon g as the migration of S. sirbanum through eastern Guinea and northern Sierra Leone is prevented.