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).
Treatment: Tetracycline and its derivative, doxycycline, are the antibiotics of choice. Enrofloxacin is also effective. Antibiotics should be started as soon as Rocky Mountain spotted fever is suspected, even if the diagnosis is not confirmed. Mortality is high if treatment is delayed. Furthermore, dogs with Rocky Mountain spotted fever respond dramatically in one to two days, which confirms the presumptive diagnosis. Antibiotics are continued for two to three weeks. Supportive treatment is the same as that described for Canine Ehrlichiosis.
Canine Ehrlichiosis and Anaplasmosis
This is a relatively common rickettsial disease caused by the organisms E. canis and E. ewingii, although several other rickettsia are capable of causing ehrlichiosis. The disease is transmitted by the bite of the brown dog tick and, occasionally, other tick species. Ehrlichiosis occurs mainly in the Gulf Coast area, the eastern seaboard, the Midwest, and California. Outside the United States it is distributed worldwide. Some of the Ehrlichia species have been renamed and are now listed in scientific literature as Anaplasma platys.
Ticks acquire the rickettsia by feeding on an infected host. A variety of wild and domestic animals serve as reservoirs. Because of its chronic nature, cases of ehrlichiosis are seen year-round, not just during the tick season.
The disease occurs in three phases. During the acute phase, the dog develops fever, depression, loss of appetite, shortness of breath, enlarged lymph nodes, and, occasionally, signs of encephalitis. These symptoms may suggest Rocky Mountain spotted fever, Lyme disease, or canine distemper.
Two to four weeks after the onset of the acute phase, the dog enters a subclinical phase that lasts weeks to months. Some dogs eliminate the infection during the subclinical phase; others progress to the chronic phase. There appears to be a breed disposition for developing chronic ehrlichiosis; German Shepherd Dogs and Doberman Pinschers, for example, are at increased risk.
During the chronic phase, which appears one to four months after the tick bite, the disease attacks the dog's bone marrow and immune system, producing weight loss, fever, anemia, a hemorrhagic syndrome with spontaneous bleeding and nose bleeds, swelling of the limbs, and various neurological signs. These signs may suggest leukemia. Infections of E. ewingii usually show arthritis as well.
A serologic blood test (IFA) is sensitive for E. canis. However, the test may not be positive until two to three weeks after the tick bite. A new ELISA test has been developed that checks for Lyme disease, ehrlichiosis, anaplasmosis, and heartworm.
Treatment: Tetracycline and doxycycline are highly effective against rickettsiae, and should be given for at least one month. Improvement in the acute phase begins within one to two days. Supportive treatment involves intravenous fluids and blood transfusions. The outlook for recovery is excellent if treatment is started before the dog develops bone marrow suppression.
Prevention: Tick control is the mainstay of prevention. Ticks do not infect dogs until they have been attached for 5 to 20 hours. Therefore, examining pets who have been roaming in tick-infested areas, and promptly removing ticks, can prevent many infections.
This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.