Seasonal variations in human onchocerciasis transmission by black flies (Simulium damnosum s. l.) in a forest area of Cross River State, Nigeria

Observations on seasonal variations in onchocerciasis transmission by Simulium damnosum sensu lato were made in a study carried out in Kwa Falls, Aningeje in Akamkpa LGA (a typical forest area) of Cross River State, Nigeria. The study was carried out for three months of the rainy season (August, September and October, 1999) and three months in the dry season (November, December, 1999 and January, 2000). Black flies were caught on five consecutive days of each month by two flycatchers working alternate hours for a total of eleven hours in a day (07 hr. – 1800 hr.). The black flies caught were dissected fresh in the field to determine the physiological age (parity). Parous flies were stained with Mayer\'s Haemalum and dissected to determine the infection\\infectivity rates (percentages) due to Onchocerca volvulus. During 30 days of black flies catching, a total of 7,403 S. damnosum s. l. vectors were attracted to man. Out of this figure, 230 (3.1%) black flies were parous. More black flies were caught in the rainy season than in the dry season. This was also true of the parous black flies identified. The infection\\infectivity rates (percentages) averaged 9.4\\5.7. The problem of onchocerciasis in the study area is more of a biting nuisance by the vector.