The microfilarial periodicity of Wuchereria bancrofti in North-eastern Brazil

The periodicity of Wuchereria bancrofti microfilariae (mff) in peripheral b lood was analysed in 42 microfilaraemics living in Maceio, in Alagoas state , north-eastern Brazil. Nine blood samples were collected from each subject , over a 24-h period, and two quantitative (60-mu l) thick smears were prep ared from each sample. Although all the subjects had detectable microfilara emias 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/mu l than those collected at 01.00 hours, when microfilaraemias were generally most intense. For diagnosis of bancroftian filariasis in Maceio, blood sam ples 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 an d microfilarial density. Mathematical analysis, using a simple harmonic-wav e model, indicated a periodicity index of 127.6 and that maximum microfilar ial densities occurred at 00.51 hours, confirming the nocturnal pattern of microfilaraemia in the study area.