Seasonal biting pattern of Simulium damnosum s.l and its implications on onchocerciasis treatment with ivermectin

The current control strategy adopted by African Programme for Onchocerciasis Control (APOC) is the mass distribution of ivermectin in all endemic areas for between 12-15 years. One of the major obstacles of the strategy, is the ability of the fly to migrate long distances and this brings about the possibility of flies migrating from uncontrolled areas to controlled areas.

To assess the impact of annual ivermectin treatment of onchocerciasis and to ascertain the seasonality of transmission and its impact on the current control strategy in the forest zone around a dam in Southern Nigeria.

A 36 x 11-hour entomological survey was carried out in the study area. Simulium damnosum s.l were caught using two human baits from 07.00 hours-18.00 hours, three days of every month for 12 months according to the method of Walsh et al. The head, thorax and abdomen of each insect were dissected separately. Catches were made from dawn to dusk.

Catches yielded a total of 2,807 female S. damnosum s.l. Higher mean biting rates (2,486 flies/man/year) occurred during the dry season. The mean monthly distribution of parous flies also showed distinct seasonality with preponderance of parous flies during the dry season (61.8%) as compared to the rainy season (38.6%). Flies harbouring developing and infective larvae were observed during the dry season in October and December when the monthly transmission potential recorded was 31 larvae/man/month. This dropped to 28 larvae/man/month and 25 larvae/man/month in January and February respectively. An annual transmission potential of 115 larvae/man/year was recorded.

In spite of the ivermectin treatment in the area at the time of study, the transmission of Onchocerca volvulus continued unabated in the area with a higher transmission in the dry season than the rainy season. The distribution of ivermectin should be intensified through improved coverage while other control measures may be necessary during the dry season which is the peak of transmission in the area.