Factors affecting the intensity of reinfection with Schistosoma haematobium following treatment with praziquantel.

Infection with Schistosoma haematobium was studied in a rural community of approximately 500 persons in eastern Zimbabwe. The overall prevalence of infection, as determined by urine egg counts, was 40.1%, and of heavy infections (greater than or equal to 50 eggs/10 ml urine) was 11.0%. The prevalence of both heavy and all infections was highest in the 8 to 10-year-old age class. During 1987-88 data were obtained from 102 individuals on intensity of reinfection 14 weeks after treatment with praziquantel, the efficacy of treatment having been determined after 4 weeks. The water contact made by these individuals during 2-week periods immediately following treatment was recorded. The relative abundance of patent infected intermediate host snails, Bulinus globosus, was also monitored. An index of exposure was developed which weighted each water contact by its duration, the type of activity, the time of day, and the abundance of infected snails at the site used. The relationships between rates of reinfection, rates of exposure, and age were examined. Although only 13 individuals showed positive rates of reinfection, there were statistically significant and independent effects of both exposure and age on reinfection rate. Quantitative estimates of reinfection rates suggested that individuals aged 12 years or less acquired substantially more infection (measured as egg output) than individuals more than 12 years old.