Rocky Mountain Spotted Fever (Canine)

Rocky Mountain Spotted Fever (RMSF) is a parasitic disease that is carried by ticks and can cause systemic illness in both dogs and humans. Although dogs cannot transmit the disease directly to humans, they can carry ticks that may transmit the disease to humans.

Common name: Rocky Mountain Spotted Fever, RMSF
Scientific name: Rocky Mountain Spotted Fever

Diagnosis

Signalment
Purebred dogs are more susceptible, especially German Shepherd Dogs. English Springer Spaniels with an enzyme (phosphofructokinase) deficiency are more likely to develop more severe clinical signs.

Incidence/prevalence
RMSF is more commonly seen between March and October.

Geographic distribution
RMSF is most often found in the Southeastern United States, however, cases have been reported in most states, western Canada, Mexico, and Central and South America.

Clinical signs (primary most to least frequent, scientific term, synonyms)
Presence of a tick or history of recent tick exposure, fever, inflammation of blood vessels (vasculitis).

Clinical signs (secondarymost to least frequent, scientific term, synonyms)
Depression, lack of appetite (anorexia), inflammation of the eye (conjuncitivitis, ocular discharge), swelling (edema) of the face, skin, scrotum, limbs, muscle and joint pain (myalgia, polyarthritis), neurological signs (head tilt, circling, incoordination, hypersensitivity to sound, seizures), coughing and difficulty breathing (dyspnea), rash, hemorrhage under the skin (petechia) and sloughing of skin (dermal necrosis), vomiting and diarrhea, organ failure, especially the liver or kidneys, irregular heart beats (cardiac arrhythmias), low platelet count (thrombocytopenia), low or high white blood cell count (leukopenia or leukocytosis), low red blood cell count (anemia), low blood protein (hypoproteinemia), low blood sodium (hyponatremia), protein in the urine (proteinuria), blood in the urine (hematuria).

Causes (scientific, common term)
Rickettsia rickettsii.

Organ systems affected (most to least affected)
Blood, skin, eyes, digestive tract, muscles, joints, respiratory system, nervous system, lymph nodes, heart, kidney, liver, reproductive system.

Diagnostic tests
Physical exam, complete blood count, blood chemistry analysis, radiographs (x-rays), urinalysis, serology comparing antibody titers (levels) in an ill dog to antibody levels from the same dog 3 weeks later.

Differential Diagnosis
Other tick borne disease, immune mediated disease.

Overview

Rocky Mountain Spotted Fever (RMSF) is a parasitic disease carried by ticks that can cause illness in both dogs and humans. Although dogs cannot transmit the disease directly to humans, they can carry ticks that may transmit the disease to humans. RMSF is most commonly seen in the Southeastern United States, but can occur in any state, Canada, Mexico, and Central and South America. It is usually diagnosed between March and October. Young dogs (less than 3 years of age), purebred dogs (especially German Shepherd Dogs), and dogs housed outdoors are more likely to become infected.

A few varieties of ticks can become infected with the parasite that causes RMSF, Rickettsia rickettsii, by acquiring the parasite from parents by passage through eggs or by feeding on infected small mammals (such as squirrels). An infected tick must remain attached to the dog for 5 to 20 hours to transmit R. rickettsii. Once R. rickettsii enters the dog's bloodstream, it begins to reproduce, causing inflammation of the blood vessels (vasculitis). Clinical signs can occur within 2-14 days of the tick bite.

Signs of RMSF in dogs can be vague. Initial fever may be followed by signs such as facial, scrotal, and limb swelling (edema), skin sloughing, anorexia, neurological signs (depression, head tilt, circling, incoordination, seizures), muscle or joint pain, coughing, difficulty breathing, hemorrhage under the skin (petechia), skin rash, irregular heart beats (cardiac arrhythmias) or organ failure. Blood work can show lowering of platelets, red blood cells, blood protein and blood sodium levels. White blood cells may be elevated or lowered. Radiographs (x-rays) of the lungs may show evidence of fluid in the lungs (pulmonary edema).

Treatment

Home Care
Tick control using an appropriate insecticide recommended by a veterinarian is the primary means of preventing exposure. Dogs should be checked for ticks promptly after being outside in areas known to harbor ticks, especially long-haired dog. Ticks should be carefully removed while wearing gloves and washing hands thoroughly as humans can become infected with exposure to contaminated tick fluids.

Professional Care
Veterinary treatment should involve diagnostic tests depending on the signs seen. To confirm a diagnosis of RMSF, a blood test should be performed before treatment and three weeks later to compare titers (antibody levels) for R. rickettsii. Treatment will involve starting the dog on a specific antibiotic (tetracycline, chloramphenicol, doxycycline, or enrofloxacin) for R. rickettsii, and supportive treatment as needed based on the dog's clinical signs.

Action
Ill dogs should be seen by a veterinarian. Veterinarians should be informed if the dogs have been in areas where tick exposure is possible or if ticks have recently been found on the dogs.

Outcome
When the appropriate treatment for RMSF is started early, the prognosis is generally good and signs rapidly improve within 24-48 hours. Some dogs may be incorrectly diagnosed due to the vague signs of illness and difficulty diagnosing RMSF. RMSF can be fatal without proper treatment.

References/Additional Readings
Warner, R.D. and Marsh, W.W. Rocky Mountain Spotted Fever. Journal of the American Veterinary Medical Association 2002; 221: 1413-7

Greene, C.E. and Breitschwerdt, E.B. Rocky Mountain Spotted Fever. In: Greene, C.E., ed. Infectious Diseases of the Dog and Cat, 2nd edition. Philadelphia: W.B. Saunders, 1990: 155-162.

Breitschwerdt, E.B. The Rickettsioses. In: Ettinger, S.J. and Feldman, E.C., ed. Textbook of Veterinary Internal Medicine, 5th edition. Philadelphia: W.B. Saunders, 2000: 400-402.

Author
Cristine Hayes, DVM

Editor
Sharon Gwaltney-Brant DVM, PhD
Diplomate, American College of Veterinary Toxicology
Diplomate, American College of Toxicology