Dermacentor and Amblyoma ticks pass the organism transstadially and transovarially, functioning as both vectors and reservoirs. Mosquitoes, fleas, horseflies, and lice can also transmit the disease, and there are numerous reports of zoonotic infections resulting from dog and cat bites. Carnivores may be infected by ingesting infected carcasses. The organism usually enters the host via inoculation by arthropods, skin abrasions, or mucous membranes of the eye or oronasopharynx. It is generally accepted that the organisms are taken up by local macrophages.
Intrahistiocytic replication occurs in local lymph nodes.
In acute septicemia, an initial neutrophilia is often followed by neutropenia. Disease susceptibility varies with species infected. Rodents and lagomorphs are most susceptible, and usually suffer fatal septicemia. Other herbivores, ruminants and birds are susceptible, but mortality is unusual. Carnivores are least susceptible, and require a large infective dose, rarely develop bacteremia, and only occasionally manifest overt disease.