Study 9 — The Okinawa Natural Experiment: Strongyloides Eradication, HTLV-1 Suppression, and ATL Decline

Study 9 of the Hidden Infections research series. Evidence tier: STRONG. Strongyloides stercoralis co-infection amplifies HTLV-1 viral load through gut-barrier disruption and mucosal HTLV-1 facilitation, accelerating progression to adult T-cell leukaemia/lymphoma (ATL). Okinawa, Japan, was the world's most concentrated region for both S. stercoralis and HTLV-1/ATL. In 1992, Okinawa launched a community-wide ivermectin mass drug administration (MDA) — the only such programme globally. This natural experiment documents: ATL/NHL proportion fell from approximately 15% to approximately 8% of lymphomas over 16 years post-MDA, while HTLV-1 seroprevalence fell 50-60% in the MDA-exposed birth cohort. Kyushu (comparator, no MDA) showed no equivalent ATL decline. The 10-15-year lag between ivermectin rollout and ATL decline is consistent with viral load suppression latency. Group B: Causal natural experiments. CC0 public domain. AI-assisted draft pre-print.