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Rocky Mountain Spotted Fever (RMSF) in Dogs

Reviewed by Dr. Robert M. DuFort, DVM, DACVIM on Tuesday, September 22, 2015
Posted February 14, 2014 in Dog Diseases & Conditions A-Z

Rocky Mountain spotted fever (RMSF) is a zoonosis which means it can be spread between animals and humans. Many such zoonotic diseases are transmitted through insects like mosquitoes and ticks. RMSF is caused by a very small type of bacteria called Rickettsia rickettsii which is injected into people and dogs by feeding ticks. However, direct transmission of organisms from dogs to people is not known to occur. 

Once established inside the animal, the rickettsial organisms cause damage and inflammation in tiny blood vessels resulting in the ‘spots’ referred to in the common name for the disease. These little areas of hemorrhage are often visible on the skin but they also occur in other internal organs like the heart, brain and kidney and can result in serious, potentially life-threatening/fatal illness.

The incubation period from infection to onset of clinical signs can vary from just a few days to two weeks; often times the connection may not be made between the presence of a tick and the later development of illness.

Symptoms of Rocky Mountain spotted fever
Symptoms can be very non-specific in dogs and in people:

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  • Fever
  • Nausea
  • Vomiting
  • Lack of appetite
  • Muscle pain

Diagnosing Rocky Mountain spotted fever
True diagnostic testing is based on demonstrating RMSF DNA in the patient’s blood by polymerase chain reaction (PCR) testing. Serology, which tests for a rising immune response over time, can also be performed. That means the results are not available until well into the illness. Unfortunately, Not every patient develops a rash, and when they do it is often a few days after they become clinically ill.

Treatment of Rocky Mountain spotted fever
Since treatment with the antibiotic doxycycline is most effective when started before the fifth day of symptoms, any delay in diagnosis and treatment can be profoundly significant in terms of outcome and prognosis. As a result, treatment is typically instituted based on clinical symptoms, location, and risk of exposure alone (with or without known tick bites).

What regions are at risk for Rocky Mountain spotted fever?
First and foremost, do not let the name of the disease fool you into a false sense of security based on where you live. Even though the disease was first recognized in the Rocky Mountains, only a small percentage of all cases are now found there. The CDC says, “RMSF cases have been reported throughout most of the contiguous United States, with

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Mike has more than 35 years of experience in companion animal veterinary practice and is a valued member of IDEXX’s Pet Health Network team since 2013.