By Jessica Vogelsang, DVM
After an unusually wet winter, this spring promises many things: amazing wildflowers, tall grasses and plenty of ticks. Ticks are more than just gross: they transmit a variety of diseases that can infect both humans and pets. Rocky Mountain Spotted Fever is one particularly nasty tick-borne disease that affects both dogs and humans.
Rocky Mountain Spotted Fever (RMSF) is caused the bacterium Rickettsia rickettsii. RMSF is transmitted through the bite of an infected tick. In North America, the types of tick usually associated with the disease are Dermacentor and Rhipicephalus species; common names include Rocky Mountain wood tick, American dog tick and the brown dog tick. These ticks are widely distributed east of the Rocky Mountains and are also seen on the Pacific Coast.
Both dogs and humans can suffer from Rocky Mountain Spotted Fever. Although both species can develop the disease, a bite by an infected tick is required for transmission of disease, so dogs cannot infect humans directly and vice versa. While cats may test positive for exposure to Rickettsia, they are not thought to develop the clinical signs of RMSF.
In dogs, there are no telltale signs that specifically point to RMSF as a cause of illness. Because the organism damages the walls of small blood vessels, clinical signs develop throughout the body. The clinical signs can be vague and mimic other disease processes, which may delay diagnosis and the start of treatment. Common symptoms include:
As the Rickettsia organisms progress through the body, leaky blood vessels and decreased clotting can lead to a variety of bleeding abnormalities. Owners may notice hematuria, which is blood in the urine. Dogs may also have nosebleeds, a condition known as epistaxis. The third most common finding associated with bleeding is melena, or blood in the stool. Depending on the location of the GI bleed, a pet’s stool may show evidence of blood or may have a dark, tarry appearance.
Dogs who are normally voracious eaters may start to eat more slowly than usual or leave food behind in the bowl. In severe cases, the pet may stop eating entirely. Although less common, dogs with RMSF may also vomit after eating. Although this is a very general symptom associated with many other medical conditions, when combined with other symptoms, it may be an important clue if deciding whether to test for RMSF.
One of the most telltale signs of RMSF is bruising secondary to the leaky blood vessels and impaired blood clotting. Petechiae are small bruises, less than an eighth of an inch. Rather than being solitary spots, they usually occur over large patches of skin. Larger bruises, called ecchymoses, may also occur if the bleeds are bigger.
These types of lesions are most easily noted in the mucous membranes. If a dog’s lip is raised, petechiae are often found on the gums. Owners may also notice bruising on the genitalia, and around the eyes.
Dogs suffering from RMSF often exhibit an overall malaise. Reluctance to exercise and go on walks, lack of interest in interacting with the family, and increased time sleeping are often reported.
Elevated body temperature is one of the first symptoms to appear in dogs infected with RMSF. Affected dogs develop a fever about four days after the tick bite. A normal dog’s body temperature ranges from 100.5 to 102.5 degrees Fahrenheit. Temperatures between 102.5 and 104.9 degrees Fahrenheit are common with RMSF.
Joint pain is a well-documented symptom of tick-borne disease. Owners may incorrectly assume their pet is not in pain because he or she isn’t vocalizing, but most dogs show pain through body language, not yelping. Many dogs simply seem to move “slower” than usual, or are sensitive to touch around affected joints. Reluctance to use stairs, difficulty getting up and down off of furniture and taking a long time to stand up from a prone position are all associated with joint pain.
Various parts of the body, causing swelling we refer to as edema. Edema usually occurs in the extremities, lips, ears or scrotum. Affected areas may appear “puffy” or “squishy” due to the extra fluid accumulation in the tissue. The overlying skin is usually normal in appearance, unless the pet is also showing bruising. Pets are often sensitive to touch in those swollen regions. Pets may also develop swollen lymph nodes.
Your veterinarian may suspect RMSF based on the pet’s clinical signs and history of tick exposure and may order a complete blood cell count and chemistry panel in addition to a specific test to check for antibodies to the Rickettsia organism.
RMSF is treated with the antibiotic doxycycline, and supportive care to manage symptoms.
Unfortunately, no vaccine exists to prevent Rocky Mountain Spotted Fever. The best prevention for RMSF and all tick-borne diseases is through tick prevention. Because the clinical signs are so generic, it’s important to report tick bites to your veterinarian if your pet becomes suddenly ill. Early treatment is the key to a good outcome.
Learn more about one dog's diagnosis and treatment of Rocky Mountain Spotted Fever.