Rocky Mountain Spotted Fever (RMSF)

RMSF is limited to the Western Hemisphere. Initially recognized in the Rocky Mountain states, it occurs in practically all of the United States and throughout Central and South America. RMSF cases outside the Americas are not well documented.

In humans, infection occurs mainly from March to September, when adult ticks are active and people are most likely to be in tick-infested areas.

Amblyomma cajennense (Cayenne Tick)

Image

This image shows a female cayenne tick, Amblyomma cajennense, which transmits Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever (RMSF).

CDC/James Gathany

In the southern United States, especially Arkansas, Missouri, North Carolina, Oklahoma, and Tennessee, sporadic cases occur throughout the year. In 2023, 1205 spotted fever rickettsioses, including RMSF, were reported to the United States Centers for Disease Control and Prevention (CDC); however, the exact proportion of cases caused by R. rickettsii is unclear (1). The reported incidence was highest in adults > 40 years old and in people who frequented tick-infested areas for work or recreation. Since 2020, the number of reported cases of all spotted fever rickettsioses has decreased. This decrease is primarily driven by changes in epidemiologic categorization (ie, higher serologic cutoffs of indirect immunofluorescence IgG from ≥ 1:64 to ≥ 1:128, elimination of latex agglutination and enzyme-linked immunosorbent assay, and removal of IgM). The case fatality rate is estimated to be < 0.5% of cases based on surveillance data. Mortality is highest in young children.

Dermacentor andersoni (Wood Tick)

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This image shows a male Rocky Mountain wood tick, Dermacentor andersoni, which transmits Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever.

CDC/James Gathany

Amblyomma aureolatum (Yellow Dog Tick)

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This image shows a female yellow dog tick, Amblyomma aureolatum, which transmits Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever.

CDC/James Gathany

The pathophysiology of RMSF is centered on systemic vasculitis caused by R. rickettsii. Small blood vessels are the sites of the characteristic pathologic lesions of Rocky Mountain spotted fever. Rickettsiae propagate within damaged endothelial cells, and vessels may become blocked by thrombi, producing vasculitis in the skin, subcutaneous tissues, central nervous system, lungs, heart, kidneys, liver, and spleen. Pathogen-mediated injury to vascular endothelium also results in increased capillary permeability, microhemorrhages, and platelet consumption. Disseminated intravascular coagulation can occur in severe cases, but it is rare (1).

In cases of severe infection, microvascular occlusion can lead to ischemia, which in turn can lead to necrosis and gangrene.

The incubation period for Rocky Mountain spotted fever varies from 3 to 12 days (1); the shorter the incubation period, the more severe the infection.

Dermacentor andersoni Bite

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This image shows a dorsal view of an engorged female tick, Dermacentor andersoni, in the process of obtaining a blood meal from a human.

Note the intense local reaction immediately surrounding the bite site.

CDC/National Institute of Allergy and Infectious Diseases (NIAID)

Onset is abrupt, with severe headache, chills, prostration, and myalgias. Fever reaches 39.5 to 40° C within several days and remains high (for 15 to 20 days in severe cases), although morning remissions may occur.

By the sixth day of fever, 90% of patients with RMSF develop a rash (1). Rash typically occurs on the wrists, ankles, palms, soles, and forearms and rapidly extends to the neck, face, axillae, buttocks, and trunk. Initially macular and pink, it becomes maculopapular and darker. The rash is typically nonpruritic. In approximately 5 or 6 days, the lesions become petechial and may coalesce to form large hemorrhagic areas that later ulcerate (1). Eschar is usually absent.

Examples of Rocky Mountain Spotted Fever (RMSF) Rash

Rocky Mountain Spotted Fever Rash (1)

This photo shows the maculopapular and petechial rash that is typical of Rocky Mountain spotted fever.

This photo shows the maculopapular and petechial rash that is typical of Rocky Mountain spotted fever.

CDC

Rocky Mountain Spotted Fever Rash (2)

This image shows the characteristic rash of Rocky Mountain spotted fever (RMSF).

This image shows the characteristic rash of Rocky Mountain spotted fever (RMSF).

CDC

Rocky Mountain Spotted Fever Rash (3)

This image shows the characteristic rash of Rocky Mountain spotted fever (RMSF).

This image shows the characteristic rash of Rocky Mountain spotted fever (RMSF).

CDC

Rocky Mountain Spotted Fever Rash (1)

This photo shows the maculopapular and petechial rash that is typical of Rocky Mountain spotted fever.

This photo shows the maculopapular and petechial rash that is typical of Rocky Mountain spotted fever.

CDC

Rocky Mountain Spotted Fever Rash (2)

This image shows the characteristic rash of Rocky Mountain spotted fever (RMSF).

This image shows the characteristic rash of Rocky Mountain spotted fever (RMSF).

CDC

Rocky Mountain Spotted Fever Rash (3)

This image shows the characteristic rash of Rocky Mountain spotted fever (RMSF).

This image shows the characteristic rash of Rocky Mountain spotted fever (RMSF).

CDC

Neurologic symptoms include headache, restlessness, insomnia, delirium, and coma, all indicative of encephalitis.

Hypotension develops in severe cases. Hepatomegaly may be present, but jaundice is infrequent. Nausea and vomiting are common. Localized pneumonitis may occur. Untreated patients may develop pneumonia, tissue necrosis, and circulatory failure, sometimes with brain and heart damage. Cardiac arrest with sudden death occasionally occurs in fulminant cases.

• History and physical examination

• Molecular diagnostic testing (polymerase chain reaction [PCR]) of infected tissue or blood specimens

• Biopsy of rash with immunohistochemistry to detect organisms

• Acute and convalescent serologic testing (serologic testing not useful acutely)

Clinicians should suspect Rocky Mountain spotted fever in any seriously ill patient who has travelled to or lives or works in or near a wooded area anywhere in the Western Hemisphere and has unexplained fever, headache, and prostration, with or without a history of tick contact (1). A history of tick bite within 14 days of illness onset occurs only in 55 to 60% of patients (2).

Clinicians must typically make treatment decisions before receiving the results of confirmatory testing because delaying treatment for RMSF may increase the risk of severe morbidity or mortality (3).

Rickettsia rickettsii (Smear)

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This photomicrograph of a Gimenez-stained yolk sac smear reveals the presence of Rickettsia rickettsii bacteria, visible as pink, rod-shaped organisms.

CDC

PCR testing on whole blood, rash biopsies, or tissue is a preferred method for diagnosing Rocky Mountain spotted fever; however, if testing results are negative, treatment must not be withheld (1). If patients have a rash, a skin biopsy should be taken from the rash site (1). PCR testing or immunohistochemical (IHC) staining, which can provide fairly rapid results, is used. However, a negative test result does not justify withholding treatment when clinical manifestations suggest RMSF.

The standard serologic diagnostic test for RMSF is an indirect immunofluorescence assay (IFA) for IgG using paired acute and convalescent serum samples collected 2 to 10 weeks apart to demonstrate seroconversion (usually ≥ 4-fold) (1). IgM testing is also available at select reference laboratories but is limited by lower specificity. Antibody titers are frequently negative in the first week of illness, limiting the usefulness of serology in the acute setting.

Culture of R. rickettsii is available only at specialized laboratories. For details, see Diagnosis of Rickettsial and Related Infections.

• DoxycyclineDoxycycline

• Rarely chloramphenicolRarely chloramphenicol

Doxycycline is the recommended first-line treatment for RMSF in all patients, including children and pregnant patients (1). If clinicians have a high index of clinical suspicion of RMSF, starting antibiotics early significantly reduces mortality and prevents most complications of Rocky Mountain spotted fever. I However, prophylactic antibiotics are not indicated for clinically asymptomatic patients who have a tick bite and have travelled to or live or work in an endemic area.

If fever, headache, and malaise occur with or without a rash, antibiotics should be started promptly.

Doxycycline is administered for at least 3 days after the fever has subsided, and there is evidence of clinical improvement (Doxycycline is administered for at least 3 days after the fever has subsided, and there is evidence of clinical improvement (1). The minimum course of treatment is 5 to 7 days. The United States Centers for Disease Control and Prevention (CDC) has found that short courses of doxycycline (5 to 10 days) can be used in children without causing tooth staining or weakening of tooth enamel (2).

Use of antibiotics other than doxycycline increases the risk of severe illness and death (3). For patients who do not tolerate doxycycline, desensitization is recommended.

The CDC recommends the use of doxycycline for suspected RMSF in pregnant patients because of the high case fatality rate and lack of effective alternative (The CDC recommends the use of doxycycline for suspected RMSF in pregnant patients because of the high case fatality rate and lack of effective alternative (1). The limited available data regarding doxycycline use in pregnant patients suggest that although the potential for risk exists, this medication is unlikely to have substantial teratogenic effects given the dosages used to target rickettsial disease (3). Thus, pregnant patients should be counseled on the potential risks versus benefits when making a decision about antibiotic treatment of RMSF.

Chloramphenicol is second-line treatment and given for 7 days. Chloramphenicol is second-line treatment and given for 7 days.Chloramphenicol can cause adverse hematologic effects, which require monitoring of blood indices, and, in neonates, gray baby syndrome.

Severely ill patients with RMSF may have a marked increase in capillary permeability in later stages; thus, IV fluids should be given cautiously to maintain blood pressure while avoiding worsening pulmonary and cerebral edema.

No effective vaccine is available to prevent Rocky Mountain spotted fever.

Measures can be taken to prevent tick bites (1).

Preventing Tick Bites

Preventing tick access to skin includes: Staying on paths and trails Tucking trousers into boots or socks Wearing long-sleeved shirts Applying repellents including diethyltoluamide (DEET), picaridin, IR3535, oil of lemon eucalyptus (OLE), para-menthane-diol (PMD), or 2-undecanone to skin surfacesApplying repellents including diethyltoluamide (DEET), picaridin, IR3535, oil of lemon eucalyptus (OLE), para-menthane-diol (PMD), or 2-undecanone to skin surfaces Treating clothing, shoes/boots, and gear with permethrinTreating clothing, shoes/boots, and gear with permethrin To avoid potential toxicity, DEET should not be used in infants under 2 months of age, and DEET concentrations ≤ 10% should be used in children under 12 years of age (2). OLE and PMD should not be used in children under 3 years of age (3). Permethrin on clothing effectively kills ticks. Frequent searches for ticks, particularly in hairy areas and on children, are essential in endemic areas.). Permethrin on clothing effectively kills ticks. Frequent searches for ticks, particularly in hairy areas and on children, are essential in endemic areas. Engorged ticks should be removed with care and not crushed between the fingers because crushing the tick may result in disease transmission. The tick’s body should not be grasped or squeezed. Gradual traction on the head with a small forceps dislodges the tick. The point of attachment and the person's hands should be disinfected with soap and water, rubbing alcohol, or hand sanitizer. Petroleum jelly, lit matches, and other irritants are not effective ways to remove ticks and should not be used. Complete elimination of tick populations is often not practically possible over large areas, but they may be reduced in certain zones (eg, residential) within endemic areas. To create a tick-safe residential environment, it is necessary to eliminate tick habitats and restrict host entry by clearing brush, tall grasses, and accumulated debris from around homes and lawn edges (1). Regular lawn maintenance, including frequent mowing and the installation of a 3-foot wide barrier of wood chips or gravel between lawns and wooded areas, is recommended to restrict tick migration into recreational zones. To reduce the presence of disease-carrying rodents (eg, rats, mice) and wildlife (eg, deer, raccoons), firewood may be stacked in dry, elevated areas. Physical fencing can also be installed. Recreational equipment and outdoor seating should be removed from shaded yard edges as should accumulated yard clutter to eliminate the potential for tick harborage.