Ehrlichia chaffeensis, the etiologic agent of human monocytic ehrlichiosis, is a tick-borne rickettsial pathogen that is infectious to a wide range of mammals, including dogs and people. Amblyomma americanum, the lone star tick, is considered the primary vector of E. chaffeensis, but this pathogen has been detected in other tick species, including the brown dog tick, Rhipicephalus sanguineus. We hypothesized that the Arkansas strain of E. chaffeensis is infective to R. sanguineus, and used a novel PCR assay to test for acquisition of this pathogen by R. sanguineus and A. americanum ticks that were simultaneously fed on experimentally infected dogs. Although E. chaffeensis was not frequently detected in peripheral blood of these dogs, the pathogen was detected in both tick species and in canine lung, kidney, lymph node, bone marrow and frontal lobe samples. One dog (AFL) was maintained for several years, and ticks again acquired E. chaffeensis from this dog 566 days after intradermal inoculation with E. chaffeensis, but the pathogen was not detected in ticks fed on the same dog at 764 or 1,086 days after the intradermal inoculation.

Introduction: Ehrlichia chaffeensis is an emerging zoonotic tick-borne rickettsial pathogen that has been detected in a wide range of vertebrate hosts, including ruminants, canids and primates. Although white-tailed deer (Odocoileus virginianus) are considered the primary reservoir of E. chaffeensis, this pathogen was also reported in other naturally infected cervids, including Korean spotted or sika deer (Cervus nippon) and Brazilian marsh deer (Blastocerus dichotomus).Case presentation: A captive adult bull elk (Cervus elaphus) from east-central Missouri was submitted for post-mortem analysis. The elk was in poor body condition with easily palpable ribs and vertebral spinal processes. Excoriations were noted on the occipital region of the head and on the left scapula, which had moderate amounts of maggots within the lesions. Large numbers of ticks were scattered over the body. Novel and established PCR assays were used to detect E. chaffeensis in blood and spleen samples from this elk, but the pathogen was not detected in Dermacentor albipictus ticks collected at necropsy. Portions of several gene sequences were analysed from the infecting agent.Conclusion To the best of our knowledge this is the first report of E. chaffeensis infection in an elk. It was not determined whether the pathogen contributed to cause of death. Notably, the pathogen was not detected in D. albipictus ticks collected from the elk.