Pet Health Library – Marin Pet Hospital, San Rafael, CA

Rocky Mountain Spotted Fever

Kathi Smith, RVT

Definition: RMSF is a tick-transmitted rickettsial disease of dogs, and people. Untreated, this disease can be fatal. It is found on the eastcoast, Midwest and plains states. With the mortality rate in humans at approximately 5% each year, dogs may serve as a “sentinel species” for human. Understanding the epidemology, transmission, clinical disease, and treatment of canine RMSF may help with the human awareness of the disease in prevalent areas.

Etiologic Agent involved with RMSF is Rickettsia rickettsii and is the most pathogenic of the rickettsial species identified with the spotted fever group (SFG). The vectors include the American dog tick (Dermacentor variabilis) found in the eastern United States, and the wood tick (Dermacentor andersoni) found in areas from the Rocky Mountains to the Cascades. These are 3 host ticks; the permanent hosts are humans, dogs, and cats. The ticks can also serve as reservoirs and hosts although rodents are the primary reservoir. Ticks usually acquire the organism through feeding on infected animals, although vertical transmission can occur. Attachment to the host for a minimum of 5-20 hours is necessary for the tick to transmit infection. However, Iatrogenic transmission can also occur through contaminated blood transfusions.

Clinical Signs can be observed in natural and experimentally induced infections of RMSF and are classified by the subclinical and acute stages. In the subclinical stage, dogs are usually asymptomatic with possible mild laboratory abnormalities.

The acute stage can have presentation of clinical signs as early as 2-3 days after tick attachment and the incubation period can run from 2-14 days.