Studies on the dynamics of transmission of onchocerciasis in a Sudan-savanna area of North Cameroon III: Infection rates of the Simulium vectors and Onchocerca volvulus transmission potentials

Abstract

Populations of Simulium damnosum s.str. and S. sirbanum were examined for infections with filarial parasites during three years in the areas of Tcholliré and Touboro, at sites at different distances from Simulium breeding rivers, and in relation to villages with different endemicities of onchocerciasis. A total of 60 353 flies from 23 fly-catching sites were dissected.

The overall infection rate was low, 11·8% of 35 357 parous flies dissected. 1681 flies (4·8% of the total parous) contained 3557 infective larvae, 68·8% of which were morphologically indistinguishable from Onchocerca volvulus and 31·2% were infective ‘Type D’ larvae of non-human origin, indicating a high degree of zoophily of the fly populations. It was estimated that only 20–40% of all bloodmeals were taken from man.

The majority (54%) of all infective O. volvulus larvae were found in the heads of the flies, the remainder being in the thorax (34%) and abdomen (12%). Only 54% of the O. volvulus infective larvae left their vectors during a bloodmeal which, however, was not completed in most cases.

During the rainy season infection rates with O. volvulus infective larvae were 3·5% of the total parous flies, as compared with 1·8% during the dry season. The average number of infective larvae of O. volvulus per infective fly was 2·6 and 2·2 during the rainy and dry seasons respectively.

These variations in the vectorial efficiency of the fly populations, as well as variations from one site to another, could be explained by different survival rates and man-biting habits of the various vector populations during the dry and rainy seasons and in different regions, rather than by different endemic profiles of onchocerciasis in the human population.

The intensity of transmission varied seasonally and was highest (1609 to 3076 infective larvae/man/year) near the main breeding sites, where transmission was almost perennial. At distances of more than 3 km from the river transmission was mainly restricted to the rainy season and the Annual Transmission Potential was below 200, whereas low to zero levels of transmission were measured inside villages more than 3 km distant from the river. The coefficient of variation of the Annual Transmission Potentials over the three years of studies was from 31% to 192% of the mean, being higher that the variations in the corresponding levels of the biting rates, due to the low numbers of infective flies dissected at sites with low transmission.

The frequency distribution of infective larvae O. volvulus in infective flies could be adequately described by the truncated form of the negative binominal distribution.