Rocky Mountain Spotted Fever

Los Angeles County West Vector & Vector-Borne Disease Control District

Rocky Mountain Spotted Fever


How do people get Rocky Mountain spotted fever?

    The organism that causes Rocky Mountain spotted fever is transmitted by the bite of an infected tick. 

American dog tickRocky Mountain Wood Tick    The American dog tick (Dermacentor variabilis-photo left) and Rocky Mountain wood tick (Dermacentor andersoni-photo right) are the primary vectors of Rocky Mountain spotted fever bacteria in the United States.  Less commonly, infections may occur following exposure to crushed tick tissues, fluids, or tick feces.

What is Rocky Mountain spotted fever?

    Rocky Mountain spotted fever (RMSF) is the most severe tick-borne rickettsial illness in the United States. This disease is caused by infection with the bacterial organism Rickettsia rickettsiiIt was first recognized as a disease in the 1890's.  This disease is so named because it was discovered in the Rocky Mountains and infected persons had a rash with red-purple-black spots.

What are the symptoms of Rocky Mountain spotted fever?

    Patients infected with R. rickettsii usually visit a physician in their first week of illness, following an incubation period of about 5-10 days after a tick bite. The early clinical presentation of Rocky Mountain spotted fever is often nonspecific and may resemble many other infectious and non-infectious diseases. Initial symptoms may include fever, nausea, vomiting, muscle pain, lack of appetite and severe headache.  Later signs and symptoms include rash, abdominal pain, joint pain, and diarrhea. Three important components of the clinical presentation are fever, rash, and a previous tick bite, although one or more of these components may not be present when the patient is first seen for medical care. Rocky Mountain spotted fever can be a severe illness, and the majority of patients are hospitalized.

In the United States, where do most cases of Rocky Mountain spotted fever occur?

    Rocky Mountain spotted fever is a seasonal disease and occurs throughout the United States during the months of April through September. Over half of the cases occur in the south-Atlantic region of the United States (Delaware, Maryland, Washington D.C., Virginia, West Virginia, North Carolina, South Carolina, Georgia, and Florida). The highest incidence rates have been found in North Carolina and Oklahoma. Although this disease was reported most frequently in the Rocky Mountain area early after its discovery, relatively few cases are reported from that area today.

Number of reported cases of Rocky Mountain spotted fever by state and region, 1994-1998
(Click on Map)

Map- Number of reported cases of Rocky Mountain spotted fever by state and region, 1994-1998

How is Rocky Mountain spotted fever diagnosed?

    A diagnosis of Rocky Mountain spotted fever is based on a combination of clinical signs and symptoms and specialized confirmatory laboratory tests.  Other common laboratory findings suggestive of Rocky Mountain spotted fever include thrombocytopenia, hyponatremia, and elevated liver enzyme levels.

How is Rocky Mountain spotted fever treated?

    Rocky Mountain spotted fever is best treated by using a tetracycline antibiotic, usually doxycycline. This medication should be given in doses of 100 mg every 12 hours for adults or 4 mg/kg body weight  per day in two divided doses for children under 45 kg (100 lbs). Patients are treated for at least 3 days after the fever subsides and until there is unequivocal evidence of clinical improvement.  Standard duration of treatment is 5 to 10 days.

Can a person get Rocky Mountain spotted fever more than once?

    Infection with R. rickettsii is thought to provide long lasting immunity against re-infection.  However, prior illness with Rocky Mountain spotted fever should not deter persons from practicing good tick-preventive measures or visiting a physician if signs and symptoms consistent with Rocky Mountain spotted fever occur, especially following a tick bite.


How Can Tick Borne Illnesses be Prevented?



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