Rocky Mountain spotted fever (RMSF) is a potentially fatal disease that affects both dogs and humans. It is caused by the parasite Rickettsia rickettsii, which is transmitted by ticks, particularly the American dog tick and the wood tick. If left untreated, RMSF can have severe consequences.
Pathophysiology
When a tick bites a dog, the parasite enters the bloodstream and spreads throughout the body. It damages blood vessel cells, causing vessel constriction and tissue inflammation. This triggers coagulation, disrupts circulation, and leads to fluid accumulation in vital organs such as the lungs, brain, and skin. The kidneys, heart, and brain are particularly vulnerable. In severe cases, systemic inflammation worsens organ damage.
Clinical Signs
Symptoms can be clinical or subclinical, meaning they may not always be visible. When symptoms do appear, they are often vague and make diagnosis difficult:
- Depression
- Anorexia
- Muscle and joint pain
- Vomiting
- Fever (typically 2–3 days post-exposure)
- Coughing and difficulty breathing
- Skin lesions: petechiae (small red dots on the skin or mucous membranes), bruising, edema, and vesicles
Differential Diagnosis
Several parasitic and non-parasitic diseases can resemble RMSF, including:
- Acute ehrlichiosis
- Babesiosis
- Lyme disease (borreliosis)
- Leptospirosis
- Immune-mediated diseases
- Distemper
- Bacterial discospondylitis
- Pneumonia
- Acute renal failure
- Pancreatitis
- Colitis
- Meningitis
Diagnostic Testing
General tests:
Blood tests may reveal anemia (mild to severe), low platelet count, and abnormal white blood cell levels. Additional findings may include decreased proteins, albumin, sodium, and calcium, along with elevated liver and kidney enzymes. In dogs with neurological signs, cerebrospinal fluid may show increased protein and white cells.
Specific tests:
- Antibody titers
- Immunofluorescence assay (IFA) to detect antigens in tissues (via biopsy or necropsy)
- DNA testing
- Organism culture (rarely performed)
Treatment
RMSF is primarily treated with antibiotics for 2–3 weeks. Corticosteroids may be used to reduce immune system complications. Supportive care such as intravenous fluids may be necessary to improve organ perfusion.
Prevention
Tick prevention is key. Once an animal recovers, it often develops lifelong immunity to the disease.
Prognosis
The prognosis is excellent with early diagnosis and effective treatment. However, late diagnosis, severe disease, or ineffective antibiotics increase the risk of mortality.
Public Health Implications
There is no direct transmission of RMSF from dogs to humans, but infected dogs can indicate a higher environmental risk of tick exposure for humans.
Endemic Regions
RMSF is mainly found in North America, particularly in the southeastern United States. However, its geographic range has expanded in recent decades. Cases are now reported in parts of South and Central America and in Canadian provinces like Ontario, Quebec, and British Columbia.
This spread is largely due to climate change, which creates warmer, more humid environments favorable to tick populations. American dog ticks (Dermacentor variabilis) and wood ticks (Ixodes scapularis) are now surviving in regions where they were previously absent, increasing the risk of RMSF.
While once confined to specific areas of the U.S., RMSF now poses a broader threat, emphasizing the importance of tick bite prevention and increased vigilance in newly affected areas.
Other Species
Cats can test positive for RMSF, but they typically do not show symptoms.
