What is Rocky Mountain spotted fever?
Rocky Mountain spotted fever (RMSF) is a tick-borne infection caused by 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 5–9 years old. Seropositivity studies 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 2–5 days. Myalgias (muscle aches) are also common. Other symptoms that may be present include:
- Gastrointestinal involvement producing abdominal pain, nausea, and vomiting.
- Central nervous system involvement which may cause confusion, lethargy, seizures, blindness, deafness, or coma.
- Any organ may become involved including the lungs, heart, kidneys, and liver.
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 of Rocky Mountain spotted fever
Although the majority of patients with RMSF have a rash, in 4–26%, the rash is absent.
- The rash initially appears as red macules (flat spots). The macules are 1–5mm in size and may be itchy.
- Within days the lesions progress to become papules (small lumps), petechiae (small red or purple spots due to bleeding into the skin), and ecchymoses (bruises).
- The rash may become haemorrhagic (bloody) in around 50% of cases; or necrotic (blackened skin due to death of tissue) in 4%. These complications typically occur on the legs, scrotum, or vulva.
- The rash typically appears on days 3–5 of the illness, but this can be highly variable.
- The rash typically begins on the ankles and wrists, then spreads to the palms and soles (in around 50% of patients). The rash then spreads up the limbs, to the trunk. The face usually remains rash-free, but may become affected later in the course of the illness.
- As the patient recovers, the skin may be tender and may shed off in powdery scales. In severe cases, there may be sloughing of the skin, particularly the skin of the extremities and external genitals. This may resemble disseminated intravascular coagulation.
- Areas of petechiae may result in tiny scars. In rare cases, severe necrosis and gangrene may require amputation.
Diagnosis of Rocky Mountain spotted fever
- Early treatment reduces mortality, so the diagnosis of RMSF is often made based on clinical observations before the results of laboratory tests are available.
- Serology is the mainstay to confirm diagnosis of rickettsial diseases. These are blood tests that detect the presence of antibodies to rickettsial antigens. The indirect fluorescent antibody test is the most reliable, with antibodies typically appearing 10–14 days after infection.
- Organisms may be seen by direct immunofluorescence of skin biopsies, but false-negatives are common. So if clinical suspicion is high, treatment should commence even if the test is negative.
Treatment of Rocky Mountain spotted fever
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.
Prevention of Rocky Mountain spotted fever
- Avoid areas such as forests or fields where ticks are found.
- Use DEET insect repellents on the skin, and permethrin on the clothes.
- Wear long-sleeved clothing that fits tightly around the wrists, waist, and ankles.
- Check twice daily for attached ticks and remove immediately. While wearing protective gloves, gently grasp the tick with tweezers as close as possible to the skin and slowly, gently pull it away.