Onchocerciasis in the Amazonian focus of southern Venezuela: altitude and blackfly species composition as predictors of endemicity to select communities for ivermectin control programmes

In preparation for an ivermectin distribution programme, the prevalence and intensity of infection due to Onchocerca volvulus as well as the species c omposition and abundance of Simulium vectors were investigated in 22 Yanoma mi communities situated along 2 altitudinal transects in the southern Venez uelan onchocerciasis focus. These transects corresponded to the Ocamo-Putac o and Orinoco-Orinoquito river systems, covering a range of elevation betwe en 50 m and 740 m above sea level (as1). A total of 831 people underwent pa rasitological examination in this survey and an additional 196 patients fro m a previous study, at an altitude of 950 m, were included in the analysis. A total of 92659 man-biting blackflies were collected and identified to mo rphospecies. S. oyapockense s.l. was the predominant simuliid up to 150 m a s1, whereas S. guianense s.l. and S. incrustatum s.l. prevailed above 150 m . Communities located below 150 m were found to range from hypo- to mesoend emic; all villages above 150 m proved to be hyperendemic (>60% microfilaria l prevalence) and mass ivermectin treatment should be implemented. Age abov e 10-14 years, altitude of the village and biting rate of S. guianense s.l. up to 200 m asl were found to be statistically significant independent pre dictors of infection by multivariate logistic regression using a spline mod el. There were no differences in infection status according to sex. Above 2 00 m, microfilarial rate and density remained approximately constant, preva lence averaging 79% regardless of blackfly abundance. For the implementatio n of ivermectin-based onchocerciasis control programmes in the Amazonian fo cus, altitude and species composition of the blackfly population might be a dopted as useful indicators aiding selection of the most affected communiti es. However, below 200 m additional parasitological indicators may also be necessary. As a direct result of this study, regular mass-ivermectin delive ry to meso- and hyperendemic communities is now in progress.