Seroconversion and seroreversion rates of anti-Strongyloides IgG in rural areas of the Amazon: a population-based panel study

Abstract

Using a panel study design, we aimed to estimate the seroconversion and seroreversion rates of anti-Strongyloides IgG antibodies from surveys carried out 11 months apart in a rural community in the Amazon Basin in Brazil. We used enzyme immunoassays to measure anti-Strongyloides IgG antibodies in 325 baseline plasma samples and 224 others that were collected 11 months later from residents in the agricultural settlement of Granada, Acre State. We observed anti-Strongyloides IgG antibodies in 21.8% of the baseline samples (which showed that 3.4% of participants had larvae in their stool) and in 23.7% of the follow-up samples. The seroconversion rate estimated at 9.7 episodes/100 person-years at risk agrees with ongoing transmission. Specific antibodies were relatively short-lived and nine (25.0%) of 36 seropositive participants at baseline were seronegative when retested 11 months later. Fecal surveys can severely underestimate the prevalence of S. stercoralis infection in rural Amazonians. Serology provides a field-deployable diagnostic tool to find high-prevalence populations, identify associated risk factors, and monitor intervention programs.

Figure 1. Anti-Strongyloides IgG (450nm) of plasma samples from Amazonians tested at baseline (n=325) and follow-up surveys (n=224). The dashed lines represent the positivity threshold (cut-off 0.286). Horizontal bar indicates the median.

Figure 2. Distribution of rural Amazonians tested both at baseline and 11 months later (n = 158) according to Strongyloides IgG status in each serosurvey.