Risk of reinfection with Schistosoma haematobium following chemotherapy in West Africa
Description
Reinfection with S. haematobium after treatment was studied to identify factors associated with reinfection and to confirm the concept of an age-related resistance to reinfection. The investigations were carried out in two different settings at two different periods. The first study was undertaken between 1986 and 1988 in 2 villages of Southwestern Mauritania characterized by semi-permanent water surfaces and an overall low endemicity. The second study, carried out between 1989 and 1992, involved 14 communities of a highly endemic area for schistosomiasis within a large irrigation scheme in central Mali. The study population in Mauritania comprised 368 6- to 20-year-old subjects and a random sample of 3559 individuals 6-year-old and over in Mali. Subjects were treated with praziquantel (single dose of 40 mg/kg) according to different strategies (mass or selective chemotherapy) and then followed for one or two years. Infection status at start and reinfection status 12 and 24 months after treatment were ascertained by urine filtration. Detailed water contact observations were conducted in 3 Malian villages where malacological data were available and 4 types of cumulative index of exposure to infection were constructed. In both settings, univariate survival analysis and multiple logistic regression controlling for gender and pre-treatment level of infection showed that the reinfection risk decreased with age. Gender and pre-treatment level of infection were also significant predictors of reinfection in Mauritania but less consistently in Mali. In the villages where water contact activities were recorded, exposure to infection decreased with age and the risk of reinfection increased linearly with exposure. A stratified analysis on exposure to infection and a logistic regression analysis, allowing for exposure confirmed the hypothesis of an age-related resistance to reinfection which can be distinguished from the age-related exposure. No age by exposure interaction was present and exposure was not a confounder of the age-reinfection association. The risk of reinfection was 9 times lower among the 15-year-old children and over than those 6- to 14-year-old after allowing for exposure (RR = 0.11). These findings confirm the previous research done in The Gambia and supports the concept of an age-acquired immunity