When enterobiasis cases were treated with anthelmintics only for one time, the interval to recurrence was variable by different drugs used. And this phenomenon is supposedly connected with biological or developmental cycle of the worm and the consequent efficacy of the different anthelmintics. This study was undertaken to confirm this fact by studying the expelled worms morphologically to correlate the anthelmintics efficacy and stage of worm development in Enterobius vermicularis. A total of 131 children in 3 orphanages was examined by 4 anal swabs (mean positive rate, 80%). They were randomized into 5 experimental groups. Each group was treated with placebo, mebendazole, pyrantel, pyrvinium , and piperazine (70 mg/kg, single dose) respectively. After treatment, all stool were collected for 3 days to get the expelled Enterobius. A total of 6,165 pinworms was studied under the microscope. The sex was discriminated and the length was individually measured. A number of male pinworms was collected in all groups. Females of 2-11 mm in length were also collected in 5 groups. However, significantly larger number of short females was observed in mebendazole group compared with other groups. Twenty-one days after the first treatment, all children were again treated with mebendazole. Once more stool were examined. A total of 1,853 worms was collected. In mebendazole group, there were no females longer than 8.74 mm in the second treatment. In pyrvinium group, 8.31mm in lenght was the longest for female. However in control, pyrantel and piperazine groups, females of 2-11 mm in length were collected. From above results, one could conclude that the removing ability of mebendazole and pyrvinium was satisfactory for the worms in the early stage of development in Enterobius. Pyrantel and single dose of piperazine showed less effective in worm reduction ability especially on those at the early stages.
