A 13-year follow-up of treatment and snail control in an area endemic for Schistosoma mansoni in Brazil: incidence of infection and reinfection

Abstract

The incidences of Schistosoma mansoni infection and reinfection were investigated in an endemic area of Brazil (Peri-Peri, State of Minas Gerais) where chemotherapy and snail control had been used for 13 years (1974-87). Two cohorts were followed: the first consisted of 584 individuals with no evidence of infection at entry (infection cohort), and the second comprised 296 individuals who were treated and did not eliminate eggs 8-12 months afterwards (reinfection cohort). The incidence of infection (per 100 person-years) decreased from 7.5 in 1974-77 to 3.6 in 1986-87, and that of reinfection from 21.3 in 1974-77 to 3.7 in 1986-87. Calendar period, age at risk, and sex were independently associated with both infection and reinfection, while a heavy S. mansoni egg count prior to treatment (> or = 500 epg (eggs per gram of stools)) was independently associated with reinfection. The geometric mean number of eggs after treatment among those reinfected (47 epg) was approximately half that among those infected for the first time (81.5 epg). Age at risk had the greatest effect on both infection and reinfection. The rate ratios of infection and reinfection were 3 to 6 times higher among individuals younger than 20 years than among those aged > or = 25 years, even after adjusting for confounders. This suggests the existence of a strong protective effect with increased age (because of biological and/or environmental factors) for both infection and reinfection.

PIP: Between 1974 and 1987 in Peri-Peri, Capim Branco Municipality, Minas Gerais State in Brazil, the schistosomiasis control program conducted a snail surveillance every 2-6 months, collected stool samples from village members annually, and administered oxamniquine to every person with Schistosoma mansoni eggs in their stool. Incidence of S. mansoni infection and reinfection steadily fell (from 7.5 to 3.6/100 person-years and from 21.3 to 3.7/100 person-years, respectively; p .001). Males were more likely to be infected or become reinfected than females (9.3 vs. 4.8, p .001 and 12.3 vs. 8, p = .025, respectively). 5-9 and 10-14 year olds had the highest incidence of S. mansoni reinfection (24.2 and 21.2 vs. 15.3 for 15-19 year olds, p .001, respectively). Infection rates were highest in 10-14 and 15-19 year olds (15 and 13.8 vs. 3.1-10.1 for 0-4 and 5-9 year olds, p .001, respectively). Reduced water contact and/or increased resistance/immunity may have accounted for lower infection and reinfection rates in people older than 20 years old. Infected people whose stool had a least 500 S. mansoni eggs/gm (epg) of stools were more likely to become reinfected faster and more frequently than people who had less than 500 epg (rate ratio, 1.7; p .001). Mean number of eggs was lower in the reinfected cohort than in the infected cohort (47 vs. 81.5 [ranges, 12-1320 vs. 12-5544]; p .001), suggesting that treatment protected against heavy reinfections. People whose egg count was at least 500 epg before treatment and were at least 25 years old did not become reinfected at a greater rate, however, further supporting the protective effect of age. These results showed that calendar period, age at risk, and sex were each associated with infection and reinfection and that a heavy egg count before treatment was independently associated only with reinfection.