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Rocky Mountain Spotted Fever

Rocky Mountain spotted fever (RMSF), also known by various names like blue disease, Brazilian spotted fever, and tick-borne typhus fever, is a bacterial infection caused by Rickettsia rickettsii. This disease is primarily transmitted to humans through tick bites, notably from the American dog tick, Rocky Mountain wood tick, and brown dog tick. RMSF is characterised by early symptoms of fever and headache, followed by a distinctive rash.

Petechial rash on the arm caused by Rocky Mountain spotted fever
Petechial rash on the arm caused by Rocky Mountain spotted fever

Signs and Symptoms

RMSF can be challenging to diagnose in its early stages due to nonspecific symptoms that resemble other diseases. Symptoms typically appear following an incubation period of one to two weeks after a tick bite.

Early Symptoms:

  • Fever
  • Nausea
  • Vomiting
  • Severe headache
  • Muscle pain
  • Malaise

Later Symptoms:

  • Maculopapular rash
  • Petechial rash
  • Abdominal pain
  • Joint pain
  • Conjunctivitis
  • Forgetfulness

The rash generally starts on the wrists and ankles and spreads inwards towards the trunk. It appears two to five days after fever onset and is most common in younger patients. The rash involves the palms or soles in about 80% of cases, but some patients may never develop a rash.


Long-term complications can be severe, including cognitive deficits, ataxia, hemiparesis, blindness, deafness, or gangrene leading to limb amputation.


RMSF is caused by Rickettsia rickettsii, transmitted primarily through tick bites. Ticks serve as both reservoirs and vectors for the bacteria. Transmission can also occur through contact with tick tissues, fluids, or faeces. Female ticks can transmit the bacteria to their eggs, maintaining the pathogen across generations.

The lifecycle of Dermacentor variabilis and Dermacentor andersoni ticks (family Ixodidae)
The lifecycle of Dermacentor variabilis and Dermacentor andersoni ticks (family Ixodidae)
American dog tick (Dermacentor variabilis) range
American dog tick (Dermacentor variabilis) range
Rocky Mountain wood tick (Dermacentor andersoni) range
Rocky Mountain wood tick (Dermacentor andersoni) range


Rickettsia rickettsii initially infects blood vessel endothelial cells and spreads to essential organs like the brain, skin, and heart. The bacteria enter host cells via receptor-mediated invasion, use the host's nutrients for growth, and escape from the phagosome using phospholipase D and hemolysin C. The bacteria use actin-based motility to move through the cytosol and spread from cell to cell.


Diagnosis is based on clinical symptoms, as laboratory tests may not be reliable in the early stages. Abnormal laboratory findings may include low platelet count, low blood sodium concentration, or elevated liver enzyme levels. Serology testing and skin biopsy are considered the best diagnostic methods, although most antibodies are undetectable during the first seven days of infection.


Immediate antibiotic treatment is very important and should not be delayed for laboratory confirmation. Doxycycline is the drug of choice for all age groups, including children and pregnant women. Treatment typically consists of 100 milligrams every 12 hours for adults or 4 mg/kg per day in two divided doses for children. The course should continue for at least three days after the fever subsides and until clinical improvement is evident. Chloramphenicol is an alternative for pregnant women but requires careful monitoring due to potential side effects.


RMSF can be severe and often requires hospitalisation. Complications include partial paralysis, gangrene, hearing loss, loss of bowel or bladder control, and movement or language disorders. These are more common in patients recovering from severe, life-threatening disease.


RMSF is reported in fewer than 5,000 cases per year in the United States, most often in June and July. The disease has been diagnosed throughout the contiguous United States, Western Canada, and parts of Central and South America.

US distribution of spotted fever rickettsiosis, of which RMSF is a type, in 2014
US distribution of spotted fever rickettsiosis, of which RMSF is a type, in 2014

Self-assessment MCQs (single best answer)

What is the causative agent of Rocky Mountain Spotted Fever (RMSF)?

Which of the following ticks is NOT a known vector for RMSF?

What is a distinctive characteristic of the rash associated with RMSF?

Which of the following is NOT a common early symptom of RMSF?

What is the drug of choice for treating RMSF?

Which laboratory finding is often associated with RMSF?

How is Rickettsia rickettsii primarily transmitted to humans?

Which age group is most likely to develop a rash when infected with RMSF?

What is a potential long-term complication of RMSF?

In which months are RMSF cases most frequently reported in the United States?


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