BackgroundScrub typhus is a dominant cause of febrile illness in many parts of Asia.Immunity is limited by the great strain diversity of Orientia tsutsugamushi.It is unclear whether previous infection protects from severe infection or enhances the risk.Methods/principal findingsWe studied IgG antibody levels against O.
tsutsugamushi at presentation in 636 scrub typhus patients using Pod enzyme-linked immunosorbent assays (ELISA).The association between ELISA optical density (OD) and risk of severe infection was modelled using Poisson regression.OD was categorised as low (Conclusions/significanceIntermediate and high IgG antibody levels at the time of diagnosis are associated with a higher risk of severe scrub typhus infection.The findings may be explained by severe infection eliciting an accelerated IgG response CYPRESS OIL or by previous scrub typhus infection enhancing the severity of subsequent episodes.