Spotted Fever Rickettsioses, Flea-Borne Rickettsioses, and Typhus
• First Described: Rickettsia rickettsii was first described in Montana, United States, 19091 (Ricketts). • Cause: Rickettsia rickettsii, a gram-negative, obligately intracellular bacteria. Other Spotted Fever group (SFG) Rickettsia spp., typhus group (TG) Rickettsia spp., and Rickettsia felis (a separate lineage designated ancestral group) are important causes of emerging infectious disease in people. Rickettsia conorii, a SFG Rickettsia spp., causes Mediterranean spotted fever (MSF) in people and a doxycycline-responsive disease in dogs that is similar to Rocky Mountain Spotted Fever (RMSF). A novel Rickettsia species causing a febrile illness similar to R. rickettsii and R. conorii has been more recently reported in dogs in the United States. • Affected Hosts: Humans and dogs (R. rickettsii). • Geographic Distribution: North, Central, and South America. • Primary Mode of Transmission: Ticks (Dermacentor spp., Rhipicephalus sanguineus, Amblyomma spp.). Most affected dogs lack a history of a tick bite. Rickettsia conorii is transmitted by Rh. sanguineus in the Mediterranean basin. • Major Clinical Signs: RMSF is generally an acute, febrile illness. Clinical signs are consistent with a vasculitis because R. rickettsii infects endothelial cells. Major clinical signs include fever, vomiting, ocular signs (retinal hemorrhage, uveitis, episcleral injection), lymphadenomegaly, splenomegaly, peripheral edema, cutaneous hyperemia and necrosis, polyarthritis, and neurologic signs. Signs may be complicated by prior or simultaneous infection with other tick-borne pathogens. The degree to which other SFG Rickettsia, TG Rickettsia, and R. felis cause illness in dogs and cats is being investigated. • Differential Diagnoses: Infection with other tick-borne agents such as Anaplasma phagocytophilum, Ehrlichia spp., Bartonella spp., Babesia spp., and Borrelia burgdorferi should be considered. Other causes of severe systemic decompensation and vasculitis such as sepsis and SIRS due to other causes may mimic RMSF. Leptospirosis and other causes of vasculitis and thrombocytopenia may also mimic RMSF. Other differential diagnoses for the neurologic signs seen in RMSF must also be considered. Appropriate antimicrobial treatment with doxycycline must begin before the diagnosis is confirmed by laboratory testing. Misdiagnosis and delayed or inappropriate antimicrobial drug therapy increase morbidity and mortality. • Human Health Significance: Owners and their physicians should be contacted whenever RMSF is diagnosed in any canine patient, because illness in dogs can coincide with or precede illness in humans. Dogs also are sentinels for MSF in people. Control of flea and tick vectors on dogs and cats is important to prevent rickettsioses in people.
