Strongyloides stercoralis, but Not Hookworm, First-Stage Larvae Require Stool-Soluble Components for Growth and Development

In regions with prolonged rainy seasons where hookworm and Strongyloides stercoralis (S. stercoralis) are co-endemic, the prevalence of hookworm infection frequently exceeds that of strongyloidiasis, despite both parasites sharing a similar route of infection via skin penetration by third-stage larvae (L3). Climate factors may contribute because rainwater can dilute stool, reducing bacterial abundance and stool-soluble component concentrations that may affect first-stage larvae (L1) development. In this study, we aimed to compare the stool-soluble component requirements of S. stercoralis and hookworm L1 in water-diluted stool and soil. Four sets of stool suspensions at 1:20, 1:480, and 1:30,720 dilutions, each containing 30-40 hookworm or S. stercoralis L1, were prepared. Two sets were adjusted to a minimal volume for soil addition, with and without bacteria, while the other two remained unadjusted. All suspensions were incubated at room temperature for 6 days, and the L3 were counted. In 1:20 suspensions, both hookworm and S. stercoralis L1 developed into or produced L3. However, in 1:480 suspensions, only hookworm L1 progressed to L3, regardless of bacterial addition. Neither species developed into L3 in 1:30,720 suspensions without bacteria, although hookworm L1 successfully developed into L3 when bacteria were added. Additionally, hookworm L1 developed into L3 in soil, whereas S. stercoralis L1 did not. These findings indicate that hookworm L1 require only bacteria, whereas S. stercoralis L1 depend on both bacteria and sufficient stool-soluble components. Water-diluting stool suppresses S. stercoralis L1 growth and development but not hookworm L1, which explains the higher prevalence of hookworm infection in regions with long rainy seasons.