Rickettsiae are an unusual type of bacteria that cause several diseases, including Rocky Mountain spotted fever. Rickettsiae differ from most other bacteria in that they can only live and multiply inside the cells of another organism (host) and cannot survive on their own in the environment.
Each of the rickettsiae has its own hosts and vectors (organisms that carry parasites to other organisms). Although some rickettsiae are found primarily in people, the usual host for most species is an animal. The population of animals that can serve as hosts is called the reservoir of infection. Animals in the reservoir may or may not be ill from the infection. People usually become infected through the bites of ticks, mites, fleas, and lice (vectors) that previously fed on an infected animal (see Introduction to Diseases Spread between Animals and People (Zoonoses)).
The signs of a rickettsial infection vary among host species. In dogs, signs of infection may include fever, lack of appetite, depression, loss of stamina, lameness, and coughing. Such infections are usually seen during the warmer months and are not generally fatal. If the infection becomes chronic (longterm), severe problems can develop in the kidneys, lungs, brain, spleen, and bone marrow. Depression and weight loss are common, and death can result.
A history of exposure to ticks and fleas raises suspicion for a rickettsial infection, but such contact may not always be apparent. Confirming the diagnosis is difficult, because rickettsiae cannot be identified using commonly available laboratory tests. Special cultures and blood tests for rickettsiae are not routinely available and take so long to process that treatment is usually needed before the test results are available. Serology tests and newer, polymerase chain reaction (PCR) assays are useful for diagnosis. Suspicion of rickettsial infection remains the most important factor in seeking a diagnosis.
Rickettsial infections respond promptly to early treatment with antibiotics known to be effective against them. Improvement is usually noticeable in 1 to 2 days, and fever usually disappears in 2 to 3 days. Antibiotic treatment is needed for at least 1 week to clear the infection, or longer if the fever persists. Supportive care may be needed. When treatment begins late, improvement is slower and the fever lasts longer. Death can occur if the animal is not treated, or if treatment is begun too late.