A followup of Loa loa and A4ansonella perstans microfilaremia was camed out in an adult population living in a highly endemic area of the Congo. Infection rates and parasite loads were found to be stable in the general population, both in the short- term (two months) and long-term (3-4 years) followup. The microfilarial status of most of the subjects examined did not change between tests. At the individual level, the microfilarial densities of L. loa and M. perstans also remained remarkably constant over time. This results in a qualitative and quantitative stability of the parasitic material available for the vectors. Epidemiologic studies on filariasis cases with microfilaremia are most often cross-sectional, and the microfilarial status of an individual is deter- mined from a single parasitologic examination camed out during the period of maximum dis- tribution of microfilariae in the peripheral blood. However, to make a diagnosis of filarial infec- tion, repeated blood sampling is recommended, given the variable degree of detection of micro- 'filariae in low-density camers. Indeed, the de- tection of microfilariae is inconsistent from one day to another in approximately I 5% of the pop- ulation infected with Mansonella perstans and Loa loa.' In regions endemic for these parasites, all subjects may be camers of M. perstans mi- crofilariae, whereas the prevalence rate of L. loa camers does not exceed 40%.2-4 It has been sug- gested that the absence of L. loa microfilariae in a high proportion of the population may be due to genetically controlled re~istance.~ However, it has not been possible to clearly demonstrate this supposition since the dynamics of production of microfilariae in human hosts harboring several female worms is unkn~wn.~ To confirm the short- and long-term durability of microfilarial status and to study the variability of the parasitic load in humans over time, we undertook a qualitative and quantitative followup of microfilaremias in a stable population living in a region highly en- demic for L. loa and M. perstans. MATERIALS AND METHODS The survey was conducted on an adult Bantu
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