Onchocerciasis in Venezuela: prevalence of microfilaraemia in Amerindians and morphological characteristics of the microfilariae from the Upper Orinoco Focus.

Data are presented on microfilaraemia in 191 Yanomami Amerindians from two areas of Venezuela's Upper Orinoco Basin, where an endemic focus of onchocerciasis has been recently detected. Onchocerca microfilariae were found in the blood of 12.6% of the persons examined. In the village with the higher number of examined individuals (N = 162), the prevalence of microfilaraemia in the age groups covaried with the prevalence of microfilariae in the skin and with the parasite load (mf/mg of skin). A positive correlation was found between age and these three variables. The prevalence of Onchocerca microfilaraemia in the Upper Orinoco focus is high, and may be related to the ability of the strain to invade the blood stream. Morphobiometric characteristics of the Onchocerca microfilariae isolated from human blood are similar to those of O. volvulus fixed in formalin from skin and from the uterus of female O. volvulus worms. All have a very short cephalic space. Comparisons with the entity from the Upper Caura river designated as Microfilaria bolivarensis were also made. It was concluded that there are no valid biometric differences in thick blood smears between microfilaria bolivarensis and microfilaria of O. volvulus from the Upper Orinoco deme. A more striking difference may be in the concentration of microfilariae observed in the blood, which in one case of mf. bolivarensis was more than forty times the highest number recorded for microfilaria O. volvulus in Parima.