Rocky Mountain Spotted Fever in Dogs

By Ryan Llera, BSc, DVM; Cheryl Yuill, DVM, MSc, CVH

Infectious Diseases, Parasites, Pet Services

What is Rocky Mountain Spotted Fever?

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

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

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 states, the most common tick to transmit this disease is the American dog tick, the wood tick in the western states (Dermacentor andersoni), with the exception of Arizona, where the brown dog tick 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.

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 clinical signs of Rocky Mountain Spotted Fever?

In dogs, the signs 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 clinical signs: poor appetite, non-specific muscle or joint pain, fever, coughing, abdominal pain, vomiting, diarrhea, swelling of the face or legs, or depression. Since these signs are non-specific, a history of tick exposure, or possible tick exposure, will help in the diagnosis of this disease. 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 parasites present in the body, extensive damage to blood vessels can cause necrosis (tissue death) of the extremities due to gangrene.female_brown_dog_tick

How is Rocky Mountain Spotted Fever diagnosed?

When your dog is examined, your veterinarian may find the clinical signs listed above.

The first step that your veterinarian will take in order 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, 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 two 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 four-fold between the first and second samples. 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 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 two days, but can take as long as two weeks to develop in milder cases.

Could the clinical signs be caused by something else?

Yes, similar clinical signs 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. Doxycycline is the preferred antibiotic for the majority of cases and may be given from anywhere to 7-21 days depending on the dose. Tetracycline 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 may become pregnant.

Two other antibiotics that may be used are enrofloxacin and chloramphenicol for similar durations. Each medication has its pros and cons and your veterinarian will discuss these with you.

What is the success rate of treatment?

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.

Will my dog need to be hospitalized for treatment?

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.

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. 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.

In addition, after your dog has been running in long grass or wooded areas, check to see if any ticks have tick_removalbecome 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 or your finger nails near the dog's skin and firmly pull it straight out.

There are also tools available called Tick Twisters or Tick Keys which can be useful. However, take care to use them cautiously as twisting or jerking the tick may cause the mouth parts 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 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.

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