A Hematogenously Disseminated Orientia tsutsugamsushi-Infected Murine Model of Scrub Typhus

Abstract

Orientia tsutsugamushi, the etiologic agent of scrub typhus, is a mite-borne rickettsia transmitted by the parasitic larval stage of trombiculid mites. Approximately one-third of the world's population is at risk of infection with Orientia tsutsugamushi, emphasizing its importance in global health. In order to study scrub typhus, Orientia tsutsugamushi Karp strain has been used extensively in mouse studies with various inoculation strategies and little success in inducing disease progression similar to that of human scrub typhus. The objective of this project was to develop a disease model with pathology and target cells similar to those of severe human scrub typhus. This study reports an intravenous infection model of scrub typhus in C57BL/6 mice. This mouse strain was susceptible to intravenous challenge, and lethal infection occurred after intravenous inoculation of 1.25×10 ⁶ focus (FFU) forming units. Signs of illness in lethally infected mice appeared on day 6 with death occurring ∼6 days later. Immunohistochemical staining for Orientia antigens demonstrated extensive endothelial infection, most notably in the lungs and brain. Histopathological analysis revealed cerebral perivascular, lymphohistiocytic infiltrates, focal hemorrhages, meningoencephalitis, and interstitial pneumonia. Disseminated infection of endothelial cells with Orientia in C57BL/6 mice resulted in pathology resembling that of human scrub typhus. The use of this model will allow detailed characterization of the mechanisms of immunity to and pathogenesis of O. tsutsugamushi infection.

Author Summary

Scrub typhus is a disease found in Southeast Asia that infects over 1 million people each year. This disease is caused by the intracellular pathogen Orientia tsutsugamushi transmitted by the bite of chigger mites. Scrub typhus is characterized by pulmonary disease and in severe cases, multiorgan system failure. The current research model utilizes an intraperitoneal route of inoculation of mice to study the host response to Orientia infection. Infection via this route results in severe peritonitis that does not occur in human scrub typhus. The development of animal models that accurately portray human disease is an important step toward understanding and managing disease. In this manuscript we describe a new mouse model that results in scrub typhus-like pathology following intravenous inoculation of mice. This model presents dose-dependent mortality with scrub typhus-like pathology that parallels human disease. Utilization of this model will provide a valuable research tool for characterizing the immune response and pathogenesis induced by O. tsutsugamushi allowing development of better treatment and an effective vaccine.