Transmission potentials of Onchocerca volvulus and the associated intensity of onchocerciasis in a Sudan-savanna area
Thylefors, B.; Philippon, B.; Prost, A.
Tropenmedizin und Parasitologie 29(3): 346-354
1978
ISSN/ISBN: 0303-4208 PMID: 214908 Document Number: 136354
The relation between the amount of transmission of Onchocerca volvulus and the pattern of onchocerciasis in man is described from investigations in 4 villages in West Africa. Observations on simuliid vectors (especially Simulium sirbanum Vajime & Dunbar and S. damnosum Theo. in Upper Volta and the Ivory Coast, S. squamosum (End.) in Upper Volta, and S. soubrense Vajime & Dunbar and S. sanctipauli Vajime & Dunbar in the Ivory Coast) biting man were made for varying periods in 1961-76, and villagers were examined medically in 1975-77. From data on the simuliids, the annual transmission potential was calculated from the product of the number of female simuliids taken per month and the number of days per month, divided by the number of catching days and multiplied by the proportion of female simuliids that were found on dissection to contain infective larvae of the pathogen. For the 4 villages, the transmission potentials were 220, 1200, 1600 and 2000 infective larvae/man per year, the corresponding total prevalence of onchocerciasis 54.8, 70.3, 73.0 and 76.0%, of ocular onchocerciasis 14.8, 33.3, 55.0 and 59.0%, and of blindness 0.2, 2.1, 4.5 and 7.6%. The possibility is discussed of establishing a threshold level of transmission, where onchocerciasis exists without giving rise to serious ocular complications. Investigations in 4 Sudan-savanna villages in West Africa indicated annual transmission potentials (AnTP) of 220, 1200, 1600 and 2000 infective Onchocerca volvulus larvae/man/year. The total prevalence of onchocerciasis was of limited value for correlation with the level of transmission; it was mainly unchanged around 70% with AnTP levels of 1200 to 2000. The prevalence of ocular onchocerciasis appeared to be of more value, increasing from 14.8 to 59% with increasing AnTP levels. An increasing intensity of infection was found in the villages with higher transmission potentials, and this was also associated with an increasing proportion of blindness and severe ocular lesions due to onchocerciasis. No case of sclerosing keratitis or complicated anterior uveitis was found in the village with the lowest AnTP level, but several cases of posterior segment lesions were diagnosed.