Factors affecting infection or reinfection with Schistosoma haematobium in coastal Kenya: survival analysis during a nine-year, school-based treatment program

Abstract

Urinary schistosomiasis remains a significant burden for Africa and the Middle East. Success of regional control strategies will depend, in part, on what influence local environmental and behavioral factors have on individual risk for primary infection and/or reinfection. Based on experience in a multi-year (1984-1992), school-based Schistosoma haematobium control program in Coast Province, Kenya, we examined risk for infection outcomes as a function of age, sex, pretreatment morbidity, treatment regimen, water contact, and residence location, with the use of life tables and Cox proportional-hazards analysis. After adjustment, location of residence, age less than 12 years, pretreatment hematuria, and incomplete treatment were the significant independent predictors of infection, whereas sex and frequency of water contact were not. We conclude that local physical features and age-related factors play a predominant role in S. haematobium transmission in this setting. In large population-based control programs, treatment allocation strategies may need to be tailored to local conditions on a village-by-village basis.

Figure 1

Map of the Msambweni study area on the southern coast of Kenya. Shown are the nine participating villages with their respective numbers of included subjects. To the right of each village label is a pie chart that indicates by internal shading the village prevalence of infection at the outset of the control project, and by relative size the local rate of infection/reinfection over the eight-year follow-up period.

Figure 2

Prevalence of schistosomiasis infection in the Msambweni study area, by study year. After baseline testing in 1984, a program of annual drug treatment was implemented and continued until 1992. a, yearly prevalence for the total population. b, yearly prevalence among age-stratified subgroups. c, yearly prevalence among sex-stratified subgroups.

Figure 3

Incidence of schistosomiasis infection and reinfection in the Msambweni study area, by year. Annual rate of infections per 100 population at risk is shown as a percentage for each year, as indicated by the numbers above each bar. a, combined incidence of new infection and reinfection between years 2 and 9 of the study (1985–1992). b, incidence of new infection among previously uninfected subjects, c, incidence of reinfection among treated subjects in the years after a documented parasitologic cure.

Figure 4

Kaplan-Meier plot of time to infection or reinfection in the nine Msambweni study villages over an eight-year follow-up period. The hazard of infection was consistently different between villages. Low risk was observed in Mwambe, Vingujini, Bowmani and Sawa Sawa, intermediate risk in Kisimachande, and high risk in the inland villages of Milalani, Marigiza, Vindungeni, and Nganja.