Rocky Mountain Spotted Fever in Dogs

What is Rocky Mountain Spotted Fever?

Rocky Mountain Spotted Fever is a disease caused by an infectious organism with the scientific name of Rickettsia rickettsii (often shortened to R. rickettsii). R. rickettsii is an “obligate intracellular parasite,” meaning that it can only survive when it is within its host’s cells. Rocky Mountain Spotted Fever occurs in North, South, and Central America.

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

Picture of a German Shepherd Dog outdoorsThis disease is transmitted through tick bites. The species of tick that is involved in its transmission varies with the geographical area. In the eastern states, the most common tick to transmit this disease is the American dog tick (Dermacentor variabilis). In most parts of the western states, the wood tick (Dermacentor andersoni) causes most infections, with the exception of Arizona, where the brown dog tick (Rhipicephalus sanguineus) transmits the disease. In Canada, Rocky Mountain Spotted Fever is less common, but can occur wherever the ticks responsible for transmission of the disease are found.

How do ticks get infected with Rickettsia rickettsii?

Infected female ticks can pass the rickettsial organisms to their eggs, and male ticks can transmit the organism to females during mating. Ticks can also become infected by feeding on an infected animal. 

Can people get this disease?

Yes, both people and dogs can become infected with Rocky Mountain Spotted Fever 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. Clusters of cases in both dogs and people are often reported in a particular area.

What risks are there for my dog to become infected with Rocky Mountain Spotted Fever?

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

What are the symptoms?

In dogs, the symptoms of Rocky Mountain Spotted Fever can be vague and non-specific. Typically, a dog that has become infected with Rocky Mountain Spotted Fever may have one or more of the following symptoms: poor appetite, non-specific muscle or joint pain, fever, coughing, abdominal pain, vomiting, diarrhea, swelling of the face or legs, or depression. Since these symptoms are non-specific, a history of tick exposure, or possible tick exposure, will help in the diagnosis of this disease.

In severe cases where there are a lot parasites present in the body, extensive damage to blood vessels can cause necrosis (tissue death) of the extremities due to gangrene.

How is Rocky Mountain Spotted Fever diagnosed?

When your dog is examined, your veterinarian may find a fever (up to 105°F or 40.5°C), enlarged lymph nodes, swollen joints, swelling of the lower legs, abnormal nervous system findings such as a head tilt or circling, hemorrhages on the gums or conjunctiva of the eyes, and retinal hemorrhages.

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

Confirmatory testing for Rocky Mountain Spotted Fever requires submission of blood samples to an outside laboratory.  The “gold standard” confirmatory test is called an Indirect Immunoflourescent Assay (IFA) test. This test requires submission of 2 samples of blood, one obtained at the time of illness and a second test obtained several weeks later. The diagnosis of Rocky Mountain Spotted Fever is confirmed if the antibody titer increases by four-fold between the first and second samples.

How fast do symptoms develop?

An infected tick must feed on your dog for at least 5-20 hours in order to transmit the parasite. 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 4-5 days, but can take as long as 2 weeks to develop in milder cases.

Could the symptoms be caused by something else?

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

What is the treatment?

The definitive treatment for Rocky Mountain Spotted Fever is a course of antibiotics, lasting 2-3 weeks. Doxycycline is the preferred antibiotic for the majority of cases. Tetracycline is also effective but requires more frequent administration and should not be given to young animals or females that may become pregnant.

What is the success rate for treating?

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

How long will my dog need treatment?

Antibiotic treatment is required for a minimum of 2-3 weeks. 

Will my dog need to be hospitalized for treatment?

Hospitalization may be necessary if your dog is anorectic (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.

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

In areas where Rocky Mountain Spotted Fever occurs, you should use an effective tick control product to prevent ticks from attaching to your dog. Your veterinarian can advise you on the best tick control products to minimize the risk of exposure.

"Wear disposable gloves before attempting
to remove any tick that is attached to your pet."

In 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.

This client information sheet is based on material written by: Cheryl Yuill, DVM, MSc, CVH

© Copyright 2015 LifeLearn Inc. Used and/or modified with permission under license.

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