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.
Canine Ehrlichiosis and Anaplasmosis in dogs:
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 sub- clinical 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 (see Ticks, page 123). 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.
Dogs living in areas where the disease is endemic can be protected by giving a low dose of oral tetracycline (1.3 mg per pound or .45 kg of body weight) or doxycycline (0.45 to 0.90 mg per pound or .45 kg of body weight) every 24 hours. However, this is rarely necessary.
Using Frontline or Advantix to control fleas also kill ticks for up to 30 days following a single application. Collars containing amitraz are also effective in controlling ticks.