Pinworm infection, a common problem in laboratory rodent colonies, is difficult to control because
anthelmintics like
ivermectin eliminate adult worms but have no effect on ova, which can survive ex vivo for prolonged periods. On the premise that repeated treatments with
ivermectin would keep rodents parasite-free until all ova matured into
ivermectin-susceptible worms in vivo or died in vivo or ex vivo, 80 rats and 25 mice heavily infected with pinworms (Syphacia obvelata and S. muris) were randomized to receive two to five courses of
ivermectin 3 days apart or no treatment. During each treatment,
ivermectin was given for 4 days in the
drinking water; based on water consumption, the mean
ivermectin dose was 2.9 and 4.0 mg/kg of
body weight per day in rats and mice respectively. Ova production was monitored by weekly
cellophane tape tests; 29 to 32 weeks
after treatment ended, all rodents were euthanized, and their evacuated large intestinal contents were examined for adult pinworms and ova. Despite intermittently negative
cellophane tape test results in untreated rodents (10 rats and 5 mice), all were infected with parasites at the end of the follow-up period. These findings underscore the limitations of the tape test for diagnosis of
pinworm infection. After two courses of
ivermectin, 1 of 10 rats and four of five mice were infected, whereas after three courses only 1 of 40 rats and one of five mice had parasites. In contrast, none of the 20 rats or 10 mice given either four or five courses of
ivermectin had parasites at 30 to 32 weeks of follow-up evaluation. This simple and well-tolerated
ivermectin regimen may help to treat and control
pinworm infection in laboratory rodent colonies.