Resistance to reinfection after treatment of urinary schistosomiasis.

The process of reinfection after treatment was studied in a cohort of subjects in a focus of intense Schistosoma haematobium infection. Detailed observations were made at water contact sites of cercarial densities and of water contact by members of the cohort. Individual values of a cumulative index of exposure to infection were calculated using these observations and assumptions which were made about the effect of different water contact activities on the entry of cercariae into the skin. Among groups of subjects with an apparently similar intensity of exposure to infection, reinfection tended to be much heavier in children under 10 years of age than in 10 to 14-year-olds, while only light infections were found in the few adults who became reinfected. This trend for reinfection to decrease with increasing age, after an allowance for variation in exposure, was highly significant (p less than 0.001). These observations suggest that subjects in this area slowly acquire an increasing degree of immunity to the acquisition of S. haematobium infection which is effective in the absence of a mature egg laying infection.