Declining Seroprevalence of Strongyloidiasis in Okinawa, Japan: A Cross-Sectional Study

Strongyloidiasis remains endemic in Okinawa, Japan; however, epidemiological data over the past 30 years are limited. This study aimed to assess the current seroprevalence of strongyloidiasis in Okinawa. We conducted a cross-sectional study at Okinawa Chubu Hospital in 2022. Serum samples from 1,485 participants were screened using an ELISA-based antibody test with the recombinant Strongyloides stercoralis antigen NIE. Seropositive participants underwent three stool examinations: the formalin-ether concentration method, agar plate culture, and real-time polymerase chain reaction. The crude seroprevalence was 3.2% (n = 47/1,485, 95% CI: 2.3-4.2%), and the age-adjusted seroprevalence was 2.4% (95% CI: 1.3-4.2%). Age 80 years old or older was a significant risk factor (odds ratio: 7.84, 95% CI: 3.59-17.2; P <0.001). This age group had 8.5% seropositivity (95% CI: 5.5-12.4%) and comprised 51.1% of all positive participants. However, five seropositive participants were ages younger than 40 years old. Of the seropositive individuals who underwent stool testing, 30.8% (n = 8/26) had positive results, and all were 70 years old or older. Despite the absence of a strongyloidiasis control program over the past 30 years, the seroprevalence has markedly declined. The shift toward older age among seropositive participants suggests that environmental transmission is now rare in Okinawa, with most infections likely acquired decades ago. Nonetheless, the persistent high endemicity among the elderly calls for continued clinical awareness of strongyloidiasis.