Inthawong et al 2024 Scrub typhus NHP model intradermal inoculation dose evaluation

Scrub typhus, endemic to the Asia-Pacific region is caused by Orientia tsutsugamushi and transmitted to humans through bites of infected larval Leptotrombidium mites (chiggers). Currently, no protective vaccines exist. Animal models have substantially advanced our understanding of immune response dynamics, pathophysiology and pathogenesis of scrub typhus. Our group previously validated a non-human primate (NHP) model using an intradermal (ID) challenge approach. To further refine this model, we evaluated the effects of progressively increasing doses of human pathogenic O. tsutsugamushi on disease progression, clinical outcomes, bacterial kinetics and immune responses in rhesus macaques (Macaca mulatta). Macaques were inoculated ID with O. tsutsugamushi strain Karp at doses of 106, 107 and 107.8 based on murine median Lethal Dose (muLD50). All NHPs demonstrated characteristic signs of human scrub typhus, including eschar formation, regional lymphadenopathy, elevated body temperature, and bacteremia. Dose-dependent responses were observed, with higher doses (107 and 107.8 muLD50), showing more severe clinical signs and pronounced adaptive immune responses. Increased production of Th1 and Th2 cytokines, with a significant rise surrogate markers, were observed in the 107 muLD50 group. This study demonstrates the critical role of differential serum IFN- levels in shaping immunity, and suggests that the 107 muLD50 dose is optimal for reliably inducing surrogate markers and immune responses resembling human infection dynamics. Our study highlights the importance of thorough inoculum dose characterisation in animal models to ensure accurate and reproducible outcome measures. The fundamental findings will serve emerging research on scrub typhus, particularly in establishing experimental studies.