Dynamics of anti-Strongyloides IgG antibody responses and implications for strongyloidiasis surveillance in rural Amazonians: A population-based panel data analysis.

Abstract

Background: Human strongyloidiasis was recently incorporated into the World Health Organization roadmap for neglected tropical diseases targeted for control in 2021-2030. However, the prevalence, incidence, and clinical burden of Strongyloides stercoralis infection remain understudied in remote communities across the Amazon due to its chronic nature, usually with absent or unspecific clinical manifestations, and the lack of practical and sensitive diagnostics for large-scale use. Here, we apply repeated antibody testing to estimate the prevalence of anti-Strongyloides IgG responses and identify incident infections in five farming settlements in the Amazonas State of Brazil.

Methodology/principal findings: We used an in-house enzyme immunoassay, with a S. venezuelensis larval extract as the solid-phase antigen, to detect specific IgG antibodies in 898 plasma samples collected during consecutive cross-sectional surveys over 4 years from 426 study participants aged >3 months, with an average of 35.9 years. Overall, 465 (51.8%) samples tested positive. However, only two infections that had been detected by fecal microscopy at survey 1 (March-May 2010) were treated with ivermectin. Antibody prevalence rose from 45.9% in 2010 to 61.1% in 2013, consistent with an increased (re)exposure to infective larvae over time. On average, there were 24.5 seroconversion events (a proxy of recent exposure to infection) per 100 person-years of follow-up, with 18.1 seroreversion events per 100 person-years. Nearly all participants with high antibody levels (i.e., above the median absorbance of seropositive tests) remained seropositive over the next years, with a single instance of high-to-nil antibody transition. Long-lasting high-level IgG responses were most likely due to frequent re-exposure to infective S. stercoralis larvae, chronic carriage of adult worms in the absence of treatment, or both. Conversely, over one-third of participants with low anti-Strongyloides antibody levels had transient IgG responses and seroreversed within 12 months.

Conclusions/significance: The results support the use of repeated antibody testing for monitoring temporal changes in S. stercoralis transmission in remote populations.