Comparative densities of Wuchereria bancrofti microfilaria in paired samples of capillary and venous blood.

Concurrent finger-prick and venous blood samples were obtained from 43 patients infected with Wuchereria bancrofti. Microfilariae were counted in blood smears and on nuclepore filters. Based on the numbers of microfilariae in 20-microliters finger-prick samples, an expected (theoretical) number of microfilariae in venous samples was calculated. This expected value was compared to the actual number of microfilariae present in venous blood. The actual number of microfilariae present ranged from 0.7 to 30 times (median 3) less than expected. The exponential function y = 16.74e.17x, r2 = .69, where y = venous counts and x = capillary counts was found to accurately reflect the relationship between venous and capillary counts. Each unit (microfilaria) change in capillary blood would result in an expected 19% unit increase/decrease in venous blood. The results support strongly the theory that microfilariae are unevenly distributed in the blood system. The epidemiological implication is that persons with low or ultralow microfilaremia levels in venous blood may have a much greater pool of microfilaria available in capillaries. We believe that the present study explains, at least partially, why mosquitoes feeding on these kinds of microfilaria carriers (frequently) have a greater uptake fo microfilariae than expected.