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).