Characterization of the rhesus macaque (<i>Macaca mulatta</i>) scrub typhus model: Susceptibility to intradermal challenge with the human pathogen <i>Orientia tsutsugamushi</i> Karp

<div><p>Background</p><p>Scrub typhus is an important endemic disease in tropical Asia caused by <i>Orientia tsutsugamushi</i> for which no effective broadly protective vaccine is available. The successful evaluation of vaccine candidates requires well-characterized animal models and a better understanding of the immune response against <i>O</i>. <i>tsutsugamushi</i>. While many animal species have been used to study host immunity and vaccine responses in scrub typhus, only limited data exists in non-human primate (NHP) models.</p><p>Methodology/Principle findings</p><p>In this study we evaluated a NHP scrub typhus disease model based on intradermal inoculation of <i>O</i>. <i>tsutsugamushi</i> Karp strain in rhesus macaques (n = 7). After an intradermal inoculation with 10⁶ murine LD₅₀ of <i>O</i>. <i>tsutsugamushi</i> at the anterior thigh (n = 4) or mock inoculum (n = 3), a series of time course investigations involving hematological, biochemical, molecular and immunological assays were performed, until day 28, when tissues were collected for pathology and immunohistochemistry. In all NHPs with <i>O</i>. <i>tsutsugamushi</i> inoculation, but not with mock inoculation, the development of a classic eschar with central necrosis, regional lymphadenopathy, and elevation of body temperature was observed on days 7–21 post inoculation (pi); bacteremia was detected by qPCR on days 6–18 pi; and alteration of liver enzyme function and increase of white blood cells on day 14 pi. Immune assays demonstrated raised serum levels of soluble cell adhesion molecules, anti-<i>O</i>. <i>tsutsugamushi</i>-specific antibody responses (IgM and IgG) and pathogen-specific cell-mediated immune responses in inoculated macaques. The qPCR assays detected <i>O</i>. <i>tsutsugamushi</i> in eschar, spleen, draining and non-draining lymph nodes, and immuno-double staining demonstrated intracellular <i>O</i>. <i>tsutsugamushi</i> in antigen presenting cells of eschars and lymph nodes.</p><p>Conclusions/Significance</p><p>These data show the potential of using rhesus macaques as a scrub typhus model, for evaluation of correlates of protection in both natural and vaccine induced immunity, and support the evaluation of future vaccine candidates against scrub typhus.</p>