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?
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.
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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.
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.