Rocky Mountain spotted fever

Author: Marie Hartley, Staff writer, 2009.


Rocky Mountain spotted fever (RMSF) is a tick-borne infection caused by micro-organisms (small bacteria) called Rickettsia rickettsii. Various species of Dermacentor ticks are the typical vectors. Rickettsiae are introduced into humans after an infected tick is attached to the skin for at least 24 hours. RMSF occurs throughout the United States, Canada, Mexico, and South America. Most infections occur in the spring and summer.

Around 350-1500 cases of RMSF are reported in the United States per year with the highest incidence in children aged 5 to 9 years old. However seropositivity studies (blood tests) in patients without a previous history of the disease indicate some RMSF infections go unnoticed.

Rickettsiae cause disease by damaging blood vessels in various tissues and organs. Other species of rickettsiae are responsible for a range of spotted fever diseases in different geographic locations.

Clinical features of Rocky Mountain spotted fever

Symptoms generally appear within 14 days of a tick bite. However the tick bite is painless and frequently goes unnoticed. The classic symptoms are fever, severe headache, and a rash. Fever and headache generally precede the rash by 2 to 5 days. Myalgias (muscle aches) are also common. Other symptoms that may be present include:

More severe illness is experienced when treatment is delayed. The case-fatality rate of RMSF is 1-4%. Patients aged younger than 5 years or older than 70 years are at highest risk of death.

Skin manifestations

Although the majority of patients with RMSF have a rash, in 4-26%, the rash is absent.



Tetracyclines are the preferred treatment for RMSF. Doxycycline should be used for children of any age, including those less than 9 years old (the risk of stained teeth is outweighed by the improved efficacy of doxycycline in treating this potentially life-threatening disease). Chloramphenicol is an alternative drug and can be used to treat pregnant women.

Treatment should be continued until there has been no fever present for at least 2 or 3 days.


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