Exposure variables in bancroftian filariasis in the Nile Delta.

To demonstrate focality of filariasis within endemic rural areas and to define exposure variables which may influence this phenomenon, the population of an agrarian endemic village, of 12,500 individuals, in the Nile Delta of Egypt was censused. A sequential sample of individuals residing in every fifth house was tested for microfilaremia (239 households with 8.6 +/- 3.5 individuals per household (HHD). Three areas of the village were tested simultaneously and a questionnaire was filled out for each sampled HHD with special emphasis given to the entomological and environmental factors that might affect filarial infection. One area (area A) had a higher intensity of larvae and biting adults of the main filarial vector, Culex pipiens, than the other two areas (areas B and C). Of the 1488 persons who agreed to be tested in the three areas 181 (12.2%) were microfilaremic. Microfilaremia prevalences were the same in males and females and microfilariae were present in all age groups. Filarial infection was most prevalent in area "A" (1.16 +/- 0.14 infected people per HHD) than in area "B" (0.44 +/- 0.11) or "C" (0.72 +/- 0.10) (ANOVA; p = 0.0003). several possible predictor variables were analyzed by logistic regression with the presence of infection as the response variable. Among individuals residing around the main Cx. pipiens development sites, those living in houses facing vacant land are exposed to more mosquito bites and had a greater chance of having filarial infection (relative risk [RR] = 1.5; logistic regression, P = 0.0089). People residing in large households had a reduced chance of having filarial infection (RR = 0.87; logistic regression, p = 0.0015). These data show that the distribution of microfilaremic individuals is uneven within the study village and suggest that small HHD and houses that bordered open areas containing mosquito development sites are potential risk factors for acquiring filarial infection.