Rocky Mountain Spotted Fever

Rocky Mountain spotted fever is the most severe and most frequently reported rickettsial illness in the United States. The disease is caused by Rickettsia rickettsii, a species of bacteria that is spread to humans by ixodid (hard) ticks. Rocky Mountain spotted fever is characterized by a sudden onset of symptoms and can be fatal if not treated. Between 1974 and 1982, an annual average of 112 cases and 3 deaths were reported in Virginia. In recent years, only about 30 cases and less than 1 death per year have been reported. Nearly all cases occur in spring and summer months.

The Pathogen

Rocky Mountain spotted fever is caused by Rickettsia rickettsii, a very small bacterium that must live inside the cells of its hosts. In humans, rickettsiae live and multiply primarily within cells that line small- to medium-sized blood vessels. Once inside the host the rickettsiae multiply, resulting in damage and death of these cells. This causes blood to leak through tiny holes in vessel walls into adjacent tissues. This process causes the rash that is traditionally associated with Rocky Mountain spotted fever and causes damage to organs and tissues.

Gimenez stain of tick hemolymph cells infected with R. rickettsii. Photo courtesy of CD.i

The Tick

In Virginia, the dog tick (Dermacentor variabilis) commonly carries the organism that causes Rocky Mountain spotted fever. The tick is usually attached for 4 to 6 hours before it transmits the disease.

American dog tick (Dermacentor variabilis). Photo courtesy of CDC

Symptoms

Rocky Mountain spotted fever can be very difficult to diagnose in its early stages, even among experienced physicians who are familiar with the disease. Early symptoms of Rocky Mountain spotted fever, which start 2 to 14 days after the tick bite, include:

• fever
• nausea
• vomiting
• severe headache
• muscle pain
• lack of appetite

Later signs and symptoms include:

• rash
• abdominal pain
• joint pain
• diarrhea

Around the third day a red, spotted rash usually appears beginning on the wrists and ankles. It spreads quickly to the palms and soles and then to much of the rest of the body.

The classic triad of symptoms for this disease includes fever, rash, and history of tick bite. However, this combination is often not identified when the patient initially presents for care.

Blood tests can confirm the presence of the disease, but two tests spaced several weeks apart may be required.

Long-term health problems following acute Rocky Mountain spotted fever infection include partial paralysis of the lower extremities, gangrene requiring amputation of fingers, toes, or arms or legs, hearing loss, loss of bowel or bladder control, movement disorders, and language disorders. These complications are most frequent in persons recovering from severe, life-threatening disease, often following lengthy hospitalizations.

Treatment

Treatment should start as soon as possible based on symptoms and history of tick exposure rather than waiting for blood test confirmation. Antibiotics in the tetracycline family are usually administered to treat the disease in adults. Other antibiotics are prescribed for children.