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The epidemiological parameters of bancroftian filariasis were investigated in three suburbs of Matara, within the south-western coastal belt of Sri Lanka where the disease is endemic. The overall prevalence of microfilaraemia and the geometric mean density of the microfilaraemias observed were 4.4% and 20.6 microfilariae/60 microl fingerprick blood, respectively. Prevalence was significantly lower in the female subjects than in the male, and in males aged < 20 years than in older males. Overall, 9.5% of the subjects had the clinical manifestations of bancroftian filariasis (6.4% had filarial fever, 3.0% had elephantiasis and/or oedema, and 6.2% had hydrocele). The prevalence of elephantiasis/oedema was generally higher among the female subjects (4.2%) than among the male (1.4%), and an age-prevalence plot for this manifestation showed a linear increase in prevalence after the age of 40 years. Hydrocele also became commoner with increasing age, but this increase in prevalence began at the lower age of 20 years. More than 60% of the cases of elephantiasis/oedema but only 26.3% of the subjects found to have hydroceles experienced filarial fever attacks. The cases of fever and elephantiasis/oedema (but not those of microfilaraemia or hydrocele) were aggregated within households. However, the children whose mothers were microfilaraemic were much more likely to be microfilaraemic themselves (8.7%) than the children who had amicrofilaraemic mothers (2.8%), microfilaraemic fathers (0.0%) or amicrofilaraemic fathers (2.7%). The results of entomological surveys indicated that transmission of Wuchereria bancrofti occurred throughout the year in the study community.

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