Rocky Mountain Spotted Fever

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Basics

Rocky Mountain spotted fever (RMSF) is one of the spotted fever rickettsioses (SFR) and is the most common of the rickettsial infections in North America. It is associated with the highest rates of severe and fatal outcomes of all reportable rickettsial diseases in the United States (1)[A],(2)[C].

Description

  • RMSF is a potentially fatal tick-borne systemic small and medium vessel vasculitis caused by the bacterium Rickettsia rickettsii (2)[C].
  • Symptoms include fever, headache, and myalgia followed by a macular rash; begins at wrists and ankles, spreading toward palms, soles, and the trunk
  • System(s) affected: cardiovascular, musculoskeletal, skin, central nervous system (CNS), renal, hepatic, and pulmonary
  • SFR are difficult to diagnose because early signs and symptoms are nonspecific and acute-phase diagnostic tests are not widely available; however, early intervention and treatment are key (3)[C].

Epidemiology

In the United States, ticks are both vectors and main reservoirs. They become infected either transovarially or by feeding on an infected animal (deer, dog, livestock). Important species in the U.S. include the American dog tick, Dermacentor variabilis in the eastern two-thirds of the U.S.; the Rocky Mountain wood tick, Dermacentor andersoni in the western U.S.; and the Brown dog tick, Rhipicephalus sanguineus is distributed throughout all states (1)[A],(3)[C].

Incidence
  • In the United States, the annual incidence of SFR increased from 1.7 cases per million persons in 2000 to 13.2 in 2016. Cases have been reported in all states except Hawaii and Alaska. RMSF also seen in Canada, Mexico, and throughout Central and South America (4)[B].
  • North Carolina, Arkansas, Missouri, Oklahoma, and Tennessee account for over 60% of cases (1)[A]
  • Cases occur year round. Most are reported during April to September during the peak of outdoor activity (1)[A].
  • All ages are susceptible; highest incidence occurs in age 60 to 69 years. Highest case-fatality rate is in children <10 years (1)[A].

Prevalence
In the United States, 4,470 cases were reported in 2012. <0.1% of ticks carry virulent rickettsial species.

Etiology and Pathophysiology

  • An adult tick releases R. rickettsii, an obligate intracellular gram-negative coccobacilli, from its salivary glands after 6 to 10 hours of feeding.
  • Pathogens infect vascular endothelial cells, causing small and medium vessel injury throughout the body leading to disseminated inflammation. This leads to microhemorrhages and increased vascular permeability causing pulmonary and cerebral edema. Local consumption of platelets results in the characteristic petechial rash.
  • Subsequent end-organ injury may also result in meningoencephalitis, ARF, acute respiratory distress syndrome, shock, arrhythmia, and seizure (1).
  • Symptoms appear 3 to 12 days after bite or between 4 to 8 days after discovery of an attached tick. Incubation period is typically 5 days or less.
  • It is unknown whether R. rickettsii crosses the placenta and causes in utero infection.
  • RMSF can rarely be caused by direct inoculation of tick blood into open wounds or conjunctivae.

Genetics
R. rickettsii has one of the smallest bacterial genomes (1.1 to 1.6 Mb), making the organism highly adapted to the intracellular environment.

Risk Factors

  • Known tick bite, engorged tick, or presence of tick for >20 hours. The likelihood of infection increases with the duration of tick attachment.
  • Tick crushed during removal
  • Accumulated outdoor exposure or residence in wooded areas
  • Contact with outdoor pets or wild animals

General Prevention

  • Limit tick exposure when possible in endemic areas, highest tick exposure with time spent in tall grasses, open areas of low bushy vegetation, or wooded areas.
  • Wear light-colored clothing, long sleeves, pants, socks, and closed-toed shoes.
  • Use DEET-containing insect repellents.
  • Permethrin spray on clothing
  • Regular tick checks
  • Prompt and proper tick removal
  • Wash hands and site of bite with soap and water after tick removal to avoid potential mucosal inoculation.
  • Nail polish, petrolatum jelly, and heat do not aid in tick removal. Do not use bare hands to remove ticks.
  • Protect pets through ectoparasite control (1)[A].
  • Prophylactic antibiotic treatment is not recommended.

-- To view the remaining sections of this topic, please or --

Basics

Rocky Mountain spotted fever (RMSF) is one of the spotted fever rickettsioses (SFR) and is the most common of the rickettsial infections in North America. It is associated with the highest rates of severe and fatal outcomes of all reportable rickettsial diseases in the United States (1)[A],(2)[C].

Description

  • RMSF is a potentially fatal tick-borne systemic small and medium vessel vasculitis caused by the bacterium Rickettsia rickettsii (2)[C].
  • Symptoms include fever, headache, and myalgia followed by a macular rash; begins at wrists and ankles, spreading toward palms, soles, and the trunk
  • System(s) affected: cardiovascular, musculoskeletal, skin, central nervous system (CNS), renal, hepatic, and pulmonary
  • SFR are difficult to diagnose because early signs and symptoms are nonspecific and acute-phase diagnostic tests are not widely available; however, early intervention and treatment are key (3)[C].

Epidemiology

In the United States, ticks are both vectors and main reservoirs. They become infected either transovarially or by feeding on an infected animal (deer, dog, livestock). Important species in the U.S. include the American dog tick, Dermacentor variabilis in the eastern two-thirds of the U.S.; the Rocky Mountain wood tick, Dermacentor andersoni in the western U.S.; and the Brown dog tick, Rhipicephalus sanguineus is distributed throughout all states (1)[A],(3)[C].

Incidence
  • In the United States, the annual incidence of SFR increased from 1.7 cases per million persons in 2000 to 13.2 in 2016. Cases have been reported in all states except Hawaii and Alaska. RMSF also seen in Canada, Mexico, and throughout Central and South America (4)[B].
  • North Carolina, Arkansas, Missouri, Oklahoma, and Tennessee account for over 60% of cases (1)[A]
  • Cases occur year round. Most are reported during April to September during the peak of outdoor activity (1)[A].
  • All ages are susceptible; highest incidence occurs in age 60 to 69 years. Highest case-fatality rate is in children <10 years (1)[A].

Prevalence
In the United States, 4,470 cases were reported in 2012. <0.1% of ticks carry virulent rickettsial species.

Etiology and Pathophysiology

  • An adult tick releases R. rickettsii, an obligate intracellular gram-negative coccobacilli, from its salivary glands after 6 to 10 hours of feeding.
  • Pathogens infect vascular endothelial cells, causing small and medium vessel injury throughout the body leading to disseminated inflammation. This leads to microhemorrhages and increased vascular permeability causing pulmonary and cerebral edema. Local consumption of platelets results in the characteristic petechial rash.
  • Subsequent end-organ injury may also result in meningoencephalitis, ARF, acute respiratory distress syndrome, shock, arrhythmia, and seizure (1).
  • Symptoms appear 3 to 12 days after bite or between 4 to 8 days after discovery of an attached tick. Incubation period is typically 5 days or less.
  • It is unknown whether R. rickettsii crosses the placenta and causes in utero infection.
  • RMSF can rarely be caused by direct inoculation of tick blood into open wounds or conjunctivae.

Genetics
R. rickettsii has one of the smallest bacterial genomes (1.1 to 1.6 Mb), making the organism highly adapted to the intracellular environment.

Risk Factors

  • Known tick bite, engorged tick, or presence of tick for >20 hours. The likelihood of infection increases with the duration of tick attachment.
  • Tick crushed during removal
  • Accumulated outdoor exposure or residence in wooded areas
  • Contact with outdoor pets or wild animals

General Prevention

  • Limit tick exposure when possible in endemic areas, highest tick exposure with time spent in tall grasses, open areas of low bushy vegetation, or wooded areas.
  • Wear light-colored clothing, long sleeves, pants, socks, and closed-toed shoes.
  • Use DEET-containing insect repellents.
  • Permethrin spray on clothing
  • Regular tick checks
  • Prompt and proper tick removal
  • Wash hands and site of bite with soap and water after tick removal to avoid potential mucosal inoculation.
  • Nail polish, petrolatum jelly, and heat do not aid in tick removal. Do not use bare hands to remove ticks.
  • Protect pets through ectoparasite control (1)[A].
  • Prophylactic antibiotic treatment is not recommended.

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