Description

• Rocky Mountain spotted fever (RMSF) is a potentially fatal tick-borne systemic small vessel vasculitis caused by the bacterium Rickettsia rickettsii.
• RMSF is typically characterized by fever, headache, and myalgias followed by a centripetal rash.

• System(s) affected: Cardiovascular, Musculoskeletal, Skin, CNS, Renal, Hepatic, Pulmonary

Epidemiology

• In the US, the incidence of RMSF has increased from <2 cases per million persons in 2000 to >8 per million in 2008.
• Although RMSF has been reported in almost all states, most cases occur in the mid-Atlantic and southern states. North Carolina, Oklahoma, Arkansas, Tennessee, and Missouri account for ∼60% of cases. RMSF has also been found in areas of Canada and in Central and South America.
• Predominant age: All ages are susceptible, but highest prevalence is in those age 50–64 years. Children age 5–9 years and elderly >70 years are at higher risk of of significant morbidity and/or mortality.
• Predominant sex: Male > Female, likely a product of outdoor exposure.
• Peak incidence occurs with tick exposure, typically in late spring and summer.

• <0.1% of ticks carry virulent Rickettsial species.

Risk Factors

• Known tick bite, engorged tick or presence of tick for >20 hours
• Crushed tick during its removal
• Accumulated outdoor exposure or residence in a wooded area
• Contact with outdoor pets or wild animals

General Prevention

People who will be exposed to the outdoor environment, especially in known tick-prone areas, can take measures to prevent infection (1,2,3)[A]:

• Limit time spent in tall grasses, open areas of low bushy vegetation, and wooded areas.
• Cover exposed skin; wear a hat, long sleeves and pants, closed-toed shoes.
• Use DEET-containing insect repellents on exposed skin. Permethrin can be used in conjunction on clothing.
• Carefully inspect the entire body after possible exposure, especially the scalp, neck, and axillae. Ticks are commonly hidden by hair. Closely inspect legs, groin, external genitalia, and waistlines.
• Likelihood of infection increases with the duration of tick attachment
• Remove attached ticks in their entirety with tweezers immediately. Wear gloves if possible.
• Wash hands and site of bite thoroughly after tick removal to avoid potential mucosal inoculation.
• Prophylactic antibiotic treatment is not recommended.

Pathophysiology

• An adult tick releases R. rickettsii from its salivary glands after 6–10 hours of feeding. Bacteria spread hematogenously.
• Rickettsiae proliferate inside endothelial cells by binary fission and invade contiguous vascular endothelial cells, causing a small-vessel vasculitis and the characteristic rash.
• Subsequent increased vascular permeability can lead to edema, hypovolemia, and hypoalbuminemia, with subsequent end-organ injury.
• Platelets are consumed locally due to vascular injury, but coagulopathy or disseminated intravascular coagulation (DIC) is rare.
• Incubation time: 2–14 days, with a median of 4–7 days
• Transplacental transmission of infection has not been demonstrated.

Etiology

• RMSF is caused by R. rickettsii; it is transmitted by attachment and prolonged feeding of an infected tick, primarily the American dog tick, Dermacentor variabilis, in the eastern US, the Rocky Mountain wood tick, Dermacentor andersoni, in the western US, and the Brown dog tick, Rhipicephalus sanuineus, in the southwest.
• R. rickettsii is an aerobic obligate intracellular gram-negative coccobacilli.
• RMSF can rarely be caused by direct inoculation of tick blood into open wounds or conjunctivae.