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 re infection with Schistosoma haematobium after treatment was studied in a cohort of 468 subjects six years of age and over living in an irriga tion scheme area in Mall. Prevalence and intensity of S. haematobium i nfection were measured each year between 1989 and 1991, but the reinfe ction study period was restricted to the last year of the follow-up. O bservations were made at the principal water contact sites where the n umber of Bulinus truncatus shedding furcocercous cercariae was recorde d. A cumulative index of exposure taking into account time, duration a nd type of contact, and malacologic data was calculated for each subje ct. Univariate analysis showed that the reinfection risk decreased wit h age and increased with exposure and pretreatment intensity. These re sults 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 t o infection and pretreatment intensity were significant. This study su pports 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. haematob ium infection.