Age-acquired resistance and predisposition to reinfection with Schistosoma haematobium after treatment with praziquantel in Mali.

The effect of age, previous intensity of infection, and exposure on reinfection with Schistosoma haematobium after treatment was studied in a cohort of 468 subjects six years of age and over living in an irrigation scheme area in Mali. Prevalence and intensity of S. haematobium infection were measured each year between 1989 and 1991, but the reinfection study period was restricted to the last year of the follow-up. Observations were made at the principal water contact sites where the number of Bulinus truncatus shedding furcocercous cercariae was recorded. A cumulative index of exposure taking into account time, duration and type of contact, and malacologic data was calculated for each subject. Univariate analysis showed that the reinfection risk decreased with age and increased with exposure and pretreatment intensity. These results were confirmed by fitting a logistic model that showed that this risk was seven times lower among those 15 years of age and older than among the 6-14-year-old children, while linear trends with exposure to infection and pretreatment intensity were significant. This study supports the concept of an age-acquired resistance to reinfection and is in favor of a predisposition to infection that raises the question of a genetic factor controlling susceptibility/resistance to S. haematobium infection.