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

Abstract

This paper describes, for the human onchocerciasis focus of southern Venezuela, the age profiles of Onchocerca volvulus microfilarial (mf) and nodule prevalence, mf intensity, and mf aggregation for the whole examined population (836 Yanomami people) living in 20 villages, and for these communities classified according to endemicity levels (hypoendemic: < or = 20 %; mesoendemic: 21-59 %; hyperendemic: < or = 60 % infected). Mf prevalence and intensity increased with age, particularly in the hyperendemic areas, and there were no marked differences between the sexes. 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 the k value of the negative binomial distribution and by the variance to mean 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 age. Since infection intensity and host age were themselves positively related, it was not possible to draw definite conclusions about age-specific changes of parasite aggregation. There was not a significant decrease of mf intensity 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. These results are discussed in relation to detection of density dependence in the human host, selection of an indicator age-group for rapid epidemiological assessment (REA) methods, and strategies of ivermectin distribution in the Amazonian focus. It is recommended that, for the Amazonian onchocerciasis focus, the indicator group for REA consists of all those aged 15 years and over.

Human infection patterns and heterogeneous exposure in river blindness

“…Most models focus on onchocerciasis in West Africa (13)(14)(15)(16)(17), with some models focusing on Latin America (19), but few attempt to bring epidemiological commonalities and differences into a single and coherent mathematical framework (29). Age-specific exposure to vector bites (13)(14)(15)(16), operation of parasite-related human mortality (13,15), and parasite-induced immunosuppression (12,17,26) are among the mechanisms put forward to explain observed departures Observations of ABR and L3 load are independent from the data to which the model has been fitted. of O. volvulus patterns from predictions generated by simple immigration-death models (7).…”

“…Cross-sectional surveys of skin mf load (mf per mg of skin) were conducted before introduction of mass ivermectin treatment in 37 Sudan savannah (Mboum and Dourou) villages of the Vina du Nord valley in northern Cameroon (23), Mayan populations living in nine coffee fincas of central Guatemala (24), and 21 Yanomami communities of the Venezuelan Upper Orinoco basin (12,25,26). The number of people (and percentage of the total population) examined was, respectively, 5,040 (50%), 872 (76-99%), and 995 (70%).…”

“…In Guatemala, onchocerciasis is transmitted mainly in coffee growing, mountainous areas, with men generally more exposed to vectors while working in such areas (11,19,24). Among the Yanomami, use of clothing is minimal, traditional houses lack enclosing walls, and men and women engage in activities around and outside villages (26,30). However, we cannot dismiss the possible influence of host genetics, because the study populations belong to distinct ethnic groups.…”

Neglected tropical diseases in Brazil

“…In the Americas, foci of the disease were found in six countries (Mexico, Guatemala, Colombia, Ecuador, Venezuela, and Brazil). There were an estimated thirteen foci of the disease on the American continent according to the WHO 48 . In Brazil, the disease occurs in the states of Roraima and Amazonas.…”

Onchocerciasis Amongst Children Of An Endemic Community In Edo State, Nigeria

“…Consequently, in hyper-endemic communities such as Ekpan, where transmission is already intense at young age, the process of early infection may predispose most individuals to acquiring increasingly higher worm burden as age progresses (12). Differences observed in the prevalence of nodules between male and female children tend to suggest that male children are more exposed to infective bites.…”