Parasitology (1987), 95, 339-354 339 With 9 figures in the text The distribution and abundance of Enterobius vermicularis in a South Indian fishing community M. R. HASWELL-ELKINS 1 , D. B. ELKINS 1 , K. MANJULA 2 , E. MICHAEL 1 and R. M. ANDERSON 1 1 Parasite Epidemiology Research Group, Department of Pure and Applied Biology, Imperial College of Science and Technology, University of London, Prince Consort Road, London SW1 2BB 2 Department of Zoology, Madras Christian College, Tambaram, Madras 600 059, Tamil Nadu, India (Accepted 17 March 1987) SUMMARY The distribution and abundance of Enterobius vermicularis in a fishing community in South Tndia, as determined by counting worms expelled following mass anthelmintic chemo- therapy, was examined in samples of patients stratified by age, sex and family grouping. The results of a worm expulsion study in January 1984 are compared with those of a second expulsion programme in November 1984, following an 11-month period of reinfection. The prevalence of Enterobius infection was consistently high in all age groups of both males and females. A comparison of the overall prevalence and intensity of infection in the January and November surveys revealed significant increases in both the percent infected and the mean number of worms harboured in November relative to the initial level. No significant trends in the intensity of Enterobius according to host age or sex were detected in either survey. The frequency distribution of Enterobius was found to be highly aggregated in the population as a whole and within age groups. At both sampling dates, the most heavily infected 25 % of the community harboured over 90% of the total pinworms recovered. There was a significant pattern within most age groups for individuals to re-acquire worm burdens of a similar magnitude to their initial pre-treatment burdens. Heavy infections were found to be as- sociated with household. A selective or targetted approach to treatment of heavily infected individuals or households is likely to be an efficient means of reducing parasite abundance on a community-wide basis. It must be stressed, however, that treatment must be applied at frequent intervals in order to achieve a long-term reduction in pinworm abundance and to avoid the possibility of increasing the intensity of infection, as was observed in this community. INTRODUCTION The human pinworm, Enterobius vermicularis, is a parasitic helminth of the human large intestine (Faust, Russell & Jung, 1970; Akagi, 1973; WHO, 1981; Pawlowski, 1984). It is a cosmopolitan species in its global distribution and is common in both developed and developing countries, and in tropical and temperate regions of the world. Much of the published epidemiological research on Enterobius concerns its geographical distribution and occurrence in various age and social groups of infected communities (Cram & Reardon, 1939; Hayashi, Sato, Takada, Shirasaka, Fukui, Sasa, Sukigara & Hiraki, 1959; Vajrasthira & Harinasuta, 1962; Seo, Rim, Loh & Lee, 1969; Chobanov & Salekhov, 1979; Kan-Chua & Sinniah, 1980; Wagner & Eby, 1983). However, pro- blems have arisen in the interpretation of the reported patterns of infection as a consequence of practical difficulties in accurate diagnosis and the quantification of worm burdens (Cram & Reardon, 1939; Akagi, 1973; Pawlowski, 1984). Assessments