Rocky Mountain spotted fever (RMSF) is the most common rickettsial disease in the United States. Its causative organism, Rickettsia rickettsii, was identified in the early 1900s, although the disease was first described in the northwestern US in the late 19th century. (Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 5th ed. Philadelphia: Churchill Livingstone, 2000:2035-42).
Rickettsia are a group of primitive bacteria that are obligate intracellular parasites: in order to replicate, they must enter the cytoplasm of other living cells. R. rickettsii invades the endothelial and smooth muscle cells that line human blood vessels.
Rickettsia cause a variety of animal and plant diseases; they are transmitted to their hosts by ticks, mites, lice, or fleas. RMSF is transmitted by ticks: The wood tick (Dermacentor andersoni) is the principal vector in the western United States, whereas the dog tick (Dermacentor variabilis) is the usual vector in the eastern and southern United States.
Most tick bites, and therefore most cases of tick-borne diseases, occur during the spring and summer months; isolated infections occur later in the year in more southern locations . The majority of cases of RMSF occur in the southeast Atlantic coast states and in the Midwest. However, as of 2005, the disease had been diagnosed in every state except Maine, Hawaii, and Alaska. (Bratton R, Corey G. Tick-borne disease. Am Fam Phys 2005;71(12):2323-30)
Signs and Symptoms of Rocky Mountain Spotted Fever
Symptoms of RMSF begin five to seven days after the tick bite. Common initial symptoms include:
- Generalized malaise
- Muscle aches (especially the back and leg muscles)
- Fever
- Frontal headache
- Nausea
- Vomiting
- Less common symptoms include a dry cough, sore throat, pleurisy, and abdominal pain.
- A distinctive rash usually begins within the first week of symptoms (10% of patients do not develop a rash). It first appears on the wrists and ankles, and then spreads to the palms, soles, face and trunk.
- The rash is initially pink and flat and blanches with pressure. As it progresses, the rash may become raised and then petechial (resembling small blood blisters). Eventually, large areas of bruising and ulceration may develop.
- Laboratory abnormalities include low serum sodium, depressed white blood cell count, low platelet count, and elevated liver enzymes.
Complications of Rocky Mountain Spotted Fever
If left untreated, RMSF can be fatal in up to 25% of cases (mortality approaches 5% even in treated cases). Death may occur within eight to 15 days, and is usually due to respiratory and circulatory failure.
Other complications include:
- Encephalitis and neurologic compromise
- Pulmonary edema
- Acute respiratory distress syndrome
- Bleeding and clotting abnormalities
- Cardiac arrhythmias
- Gastrointestinal bleeding
- Skin ulceration and necrosis
Individuals with glucose-6-phosphate dehydrogenase deficiency are particularly prone to complications.
Diagnosis of Rocky Mountain Spotted Fever
The diagnosis of RMSF is based on clinical signs and symptoms, usually in conjunction with the history of a tick bite. Although highly specific, skin biopsy is only 60% sensitive, and serologic markers may only be elevated in the convalescent period. PCR may be useful, but immediate treatment is more important than waiting for laboratory confirmation of a diagnosis.
Treatment of Rocky Mountain Spotted Fever
In addition to supportive care, antibiotic therapy with tetracycline, doxycycline, or chloramphenicol for a minimum of seven days is essential. Other antibiotics may be useful as well. Optimal benefit is achieved if antibiotic treatment is started early.
Human forays into the outdoors will continue to be a source for tick-borne illnesses. Proper clothing, effective repellants, and frequent inspection for the presence of ticks will help to prevent these diseases.
For More Information about Rocky Mountain Spotted Fever
Stephen Allen Christensen - Dr. Steve Christensen's writing has appeared in magazines, professional journals, poetry anthologies, and children's books since 1976.
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