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Rocky Mountain Spotted Fever in Dogs

Rickettsia are various disease-causing parasites (about the size of bacteria) that are carried by fleas, ticks, and lice. They live within cells. The majority are maintained in nature by a cycle that involves an insect vector, a permanent host, and an animal reservoir.

Rocky Mountain Spotted Fever

Rocky Mountain spotted fever is a rickettsial disease caused by Rickettsia ­rickettsii and transmitted by several species of ticks. It is the most significant rickettsial disease in humans. Most cases occur in the southeastern United States, Midwest, Plains States, and Southwest, but it has been seen outside those regions. The Rocky Mountain area, where the disease was first discovered at the turn of the 20th century, now accounts for only a small percentage of cases.

Unlike canine ehrlichiosis, Rocky Mountain spotted fever coincides with the tick season (April through September). The two main reservoirs for Rocky Mountain spotted fever are rodents and dogs. Adult ticks transmit the disease to dogs when they attach and feed.

Signs of acute infection appear during the tick season and include listlessness, depression, high fever, loss of appetite, cough, conjunctivitis, difficult breathing, swelling of the legs, and joint and muscle pains. Ocular signs, such as uveitis, may be present. Rarely, a rash will be noticed around the area of a tick bite. These symptoms may suggest canine ehrlichiosis, Lyme disease, or distemper. Central nervous system signs include unstable gait, altered mental state, and seizures. Inflammation of the heart muscle (myocarditis) can cause cardiac arrhythmias, resulting in sudden death.

One to two weeks after the onset of illness, some dogs develop a hemorrhagic syndrome similar to that seen with canine ehrlichiosis. Various bleeding problems, such as nosebleeds, subcutaneous hemorrhaging, and blood in the urine and stools, may develop. This can cause shock, multiple organ failure, and death.

Rocky Mountain spotted fever should be suspected in a sick dog with a history of tick infestation during April through September. Serologic diagnosis is best achieved by noting a rise in micro-IFA antibody titer in paired serum tests (done at the time of illness and two to three weeks later).

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