The microfilarial periodicity of Wuchereria bancrofti in north-eastern Brazil

Abstract

The periodicity of Wuchereria bancrofti microfilariae (mff) in peripheral blood was analysed in 42 microfilaraemics living in Maceió, in Alagoas state, north-eastern Brazil. Nine blood samples were collected from each subject, over a 24-h period, and two quantitative (60-microliter) thick smears were prepared from each sample. Although all the subjects had detectable microfilaraemias from 23.00 hours to 06.00 hours, no mff could be detected in most (71.4%) of the smears prepared from samples collected at 15.00 hours. Samples collected during the day, at 15.00 hours, contained 170 times fewer mff/microliter than those collected at 01.00 hours, when microfilaraemias were generally most intense. For diagnosis of bancroftian filariasis in Maceió, blood samples should be collected between 22.00 and 03.00 hours, when microfilarial counts will be at least 90% of the peak counts. The circadian variation observed was independent of the subject's gender and microfilarial density. Mathematical analysis, using a simple harmonic-wave model, indicated a periodicity index of 127.6 and that maximum microfilarial densities occurred at 00.51 hours, confirming the nocturnal pattern of microfilaraemia in the study area.