Rocky Mountain Spotted Fever in Dogs

By Malcolm Weir, DVM, MSc, MPH; Ryan Llera, BSc, DVM; Cheryl Yuill, DVM, MSc, CVH

Dog walking along log in the forestWhat is Rocky Mountain Spotted Fever?

Rocky Mountain Spotted Fever (RMSF) is a disease caused by an infectious bacterium called Rickettsia rickettsii, which can only survive within its host’s cells. RMSF occurs in North, South, and Central America and is widespread throughout the Midwest, the Pacific Northwest, and the eastern United States, as well as in parts of Canada.

How does a dog become infected with Rocky Mountain Spotted Fever?

This disease is transmitted through tick bites. The species of tick that is involved in its transmission varies with the geographical area. In the eastern US, the most common tick to transmit this disease is the American dog tick, and in the western US, the wood tick, with the exception of Arizona, where the brown dog tick transmits the disease. In Canada, RMSF is less common but can occur wherever ticks responsible for the transmission of the disease are found.

If your dog runs in wooded areas or if you live in an area with a high tick population, there is a higher probability that your dog will have contact with infected ticks. The number of cases identified increases between March and October each year.

"If your dog runs in wooded areas or if you live in an area with a high tick population, there is a higher probability that your dog will have contact with infected ticks."

What are the clinical signs of Rocky Mountain Spotted Fever?

In dogs, the signs of RMSF can be vague and non-specific. Typically, a dog that has become infected may have one or more of the following clinical signs: poor appetite, muscle or joint pain, fever, coughing, abdominal pain, vomiting, diarrhea, swelling of the face or legs, or depression.

Focal hemorrhages may occur in the eyes and gums, as well as nosebleeds in severe cases. Neurological signs such as wobbling when walking (ataxia) and painful hypersensitivity can also be seen. In severe cases, where there are a lot of parasites present in the body, extensive damage to blood vessels can cause necrosis (tissue death) of the extremities.

How is Rocky Mountain Spotted Fever diagnosed?

When your dog is examined, your veterinarian may find the clinical signs listed above. Since these signs are non-specific, a history of tick exposure or possible tick exposure will help in the diagnosis of this disease.

The first step that your veterinarian will take to determine what is causing your dog’s illness will be to perform basic blood tests and possibly a urinalysis or X-rays. Abnormal findings on a complete blood count (CBC) usually include low numbers of platelets or red blood cells (anemia), and abnormal white blood cell counts. In the early stages, the white blood cell count will be low, but in the later stages of the disease, the white cell count may be increased. Tests to measure blood clotting ability may also be run. In addition, biochemical tests will often show low protein levels, abnormal calcium levels, electrolyte abnormalities, and abnormal liver or kidney values.

"...a history of tick exposure or possible tick exposure will help in the diagnosis of this disease."

Confirmatory testing for RMSF requires the submission of blood samples to an outside laboratory. The gold standard confirmatory test is called an Indirect Immunofluorescent Assay (IFA) test. This test requires the submission of two samples of blood; one obtained at the time of illness, and a second test obtained several weeks later. The diagnosis of RMSF is confirmed if the antibody titer increases four-fold between the first and second samples, although a high titer on the first sample can increase suspicion for RMSF.

Other tests such as a PCR or a spinal fluid tap can be done but are less sensitive to picking up a diagnosis.

How fast do symptoms of Rocky Mountain Spotted Fever develop?

An infected tick must feed on your dog in order to transmit the parasite. While it used to be thought that the tick needed to remain attached for several hours to transmit the infection, more recent research has shown that this can occur within minutes. Once the parasite enters the dog’s bloodstream, it reproduces in the cells of the blood vessels, causing inflammation and constriction of the affected blood vessels. Symptoms usually develop after an incubation period of two to fourteen days.

Could the clinical signs be caused by something else?

Yes, similar clinical signs are caused by other diseases such as ehrlichiosis, Lyme disease, leptospirosis, immune-mediated diseases, canine distemper, other bacterial infections, or organ failure.

What is the treatment for Rocky Mountain Spotted Fever?

The definitive treatment for RMSF is a course of antibiotics. Doxycycline (Vibramycin®, Oracea®, Monodox®, Periostat®, Doryx®, Acticlate®) is the preferred antibiotic for most cases and may be given from anywhere from 7-21 days depending on the dose. Tetracycline (Achromycin®, Medicycline®, Sumycin®, Tetracyn®) is also effective but requires more frequent administration and is given for 14-21 days. Neither of these drugs should be given to young animals or females that could become pregnant. Two other antibiotics that may be used are enrofloxacin (Baytril®) and chloramphenicol (Chloromycetin®, Viceton®) for similar durations. Each medication has its pros and cons, and your veterinarian will discuss these with you.

Hospitalization may be necessary if your dog is not eating, is showing evidence of organ failure, or if supportive treatment such as intravenous fluid therapy is needed. As soon as your dog’s condition becomes stable, he or she can be discharged and the antibiotic treatment can be continued at home.

"If RMSF is diagnosed in its early stages and treatment is started immediately, the prognosis for successful treatment is excellent."

If RMSF is diagnosed in its early stages and treatment is started immediately, the prognosis for successful treatment is excellent. If the diagnosis is delayed, or if the dog is infected with a severe number of parasites, the prognosis becomes less favorable.

Is there anything I can do to prevent my dog from getting Rocky Mountain Spotted Fever?

In areas where RMSF occurs, you should use an effective tick control product to prevent ticks from attaching to your dog. There are both topical (Advantix®, Frontline Plus®, Bravecto®) or oral (Nexgard®, Simparica®, Bravecto®) options available. Your veterinarian can advise you on the best tick control products to minimize the risk of exposure.

Removing tick from dog's skin with a Tick Twister while wearing glovesIn addition, after your dog has been running in long grass or wooded areas, check to see if any ticks have become attached. Since you could become infected from the saliva of an infected tick, be sure to wear disposable gloves before attempting to remove any tick that is attached to your pet. If you find a tick attached to your pet, grasp the tick with fine tweezers near the dog's skin and firmly pull it straight out. There are also useful tools available called Tick Twisters or Tick Keys. However, take care to use them cautiously as twisting or jerking the tick may cause the mouthparts to break off and remain in the skin. If you are unable to remove the tick or are unsure of how to do so, contact your veterinarian.

Can people get Rocky Mountain Spotted Fever?

Yes, both people and dogs can become infected with RMSF if they are bitten by an infected tick. People cannot get this infection directly from dogs. However, if a dog has become infected, any people or other animals that share the dog’s environment could become infected by being bitten by an infected tick.

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