Diseases > Ticks > Spotted Fever

Rocky Mountain Spotted Fever

Rocky Mountain Spotted Fever (RMSF) was first recognized as a human disease in Idaho in 1896. It is a very serious, potentially fatal disease, caused by a bacterium that is an intracellular parasite of cells lining blood vessels. Since its discovery, between 250-1,200 cases of RMSF have been diagnosed each year, with only 1-3% of cases in the Rocky Mountains. Nearly all cases over the past several decades have occurred east of the Mississippi River. RMSF derives its name from the characteristic rash that develops in from 35-60% of diagnosed patients. The rash (non-itching) consists of countless tiny red spots that initially appear on the hands and feet, but later become more widespread, including on the soles of the feet.

Causative agent

• A bacterium with the scientific name Rickettsia rickettsii.
• Rickettsia species multiply inside cells lining small and medium blood vessels, eventually destroying them.

Geographical distribution of cases

• South America, Central America, Mexico, southern Canada, and the continental United States.
• The vast majority of cases in the U.S. occur in the mid-Atlantic and southeastern states, especially in North Carolina and Oklahoma, but cases are documented throughout Indiana.

Symptoms of infection

• Fever, headache, joint pain, muscle ache, nausea, and possibly abdominal pain and diarrhea.
• 35-60% of patients develop a rash of non-itchy, tiny reddish spots that initially appear on hands and ankles, but eventually develop on the forearms and soles of the feet.
• Severe cases involve delirium and coma, with a fatality rate of 3-5% (nearly 30% in untreated patients).

Reservoir hosts of Rickettsia rickettsii

• Small rodents, including deer mice and meadow voles.

Vectors of Rickettsia rickettsii

• Dermacentor variabilis, American dog tick, in the eastern U.S.
• Dermacentor andersoni, Rocky Mountain wood tick, in the western U.S.
• NOTE: Rhipicephalus sanguineus, brown dog tick, may be a vector in Arizona (see E-243-W "The Biology and Medical Importance of Ticks in Indiana" (PDF 757KB)).

Modes of transmission

• From infected ticks, usually female adults, to humans via their bite.
• From infected tick larvae to nymphs to adults via transstadial transmission.
• From infected female ticks to their eggs via transovariole transmission.
• NOTE: there is no human-to-human transmission.

Diagnosis of infection

• Symptoms, especially the characteristic rash, together with onset in late spring-summer following tick bite.

Treatment of infection

• Antibiotic prescribed by a physician.

Prevention of infection

• There is no vaccine for the general public, but one is available for people in high-risk occupations such as veterinarians and wildlife biologists.
• Avoid tick habitat and contact with infested vegetation (see E-243-W "The Biology and Medical Importance of Ticks in Indiana" (PDF 757KB)).
• Wear clothing consisting of a long sleeved shirt, long pants, and boots.
• Use a repellent recommended by CDC and treat clothing with permethrin.