Amazonian onchocerciasis: parasitological profiles by host-age, sex, and endemicity in southern Venezuela

This paper describes, for the human onchocerciasis focus of southern Venezu ela, the age profiles of Onchocerca volvulus microfilarial (mf) and nodule prevalence, mf intensity, and mf aggregation for the whole examined populat ion (836 Yanomami people) living in 20 villages, and for these communities classified according to endemicity levels (hypoendemic: less than or equal to 20 %; mesoendemic: 21-59 %; hyperendemic: greater than or equal to 60 % infected). Mf prevalence and intensity increased with age, particularly in the hyperendemic areas, and there were no marked differences between the se xes. The prevalence of nodules followed the same age pattern. Fifty percent mf prevalence was reached in the 15-19 year age-class when the population was taken as a whole; nearly in the 10 to 14-year-olds for the hyperendemic level, in those aged 20-29 years in mesoendemic areas, and not reached at all in hypoendemic villages. The degree of mf aggregation was measured by t he k value of the negative binomial distribution and by the variance to mea n ratio (VMR). The relationship between the standard deviation (S.D.) of mf counts and the mean mf density was also explored. These 3 indices (k, VMR, and S.D.) showed a tendency to increase with both mean mf load and host ag e. Since infection intensity and host age were themselves positively relate d, it was not possible to draw definite conclusions about age-specific chan ges of parasite aggregation. There was not a significant decrease of mf int ensity after an earlier peak neither was there a shift towards younger ages of the maximum no, of mf/mg reached as the endemicity level increased. The se results are discussed in relation to detection of density dependence in the human host, selection of an indicator age-group for rapid epidemiologic al assessment (REA) methods, and strategies of ivermectin distribution in t he Amazonian focus. It is recommended that, for the Amazonian onchocerciasi s focus, the indicator group for REA consists of all those aged 15 years an d over.