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Potentially life-threatening emergency
Rocky Mountain spotted fever - Chem-Bio-Rad Suspicion
See also in: Overview
Other Resources UpToDate PubMed
Potentially life-threatening emergency

Rocky Mountain spotted fever - Chem-Bio-Rad Suspicion

See also in: Overview
Contributors: Michael Sean Antonis DO, Alex Garza MD, MPH, Susan Burgin MD
Other Resources UpToDate PubMed


*Highest fatality rate of tick-borne diseases*

Rocky Mountain spotted fever (RMSF) is caused by the gram-negative bacterium Rickettsia rickettsii. It is the most severe rickettsial illness of humans; without treatment, the case fatality rate is 20%-30%.

The disease is transmitted most commonly via the tick bite of the Dermacentor (American dog and Rocky Mountain wood ticks), Rhipicephalus (brown dog tick), or Amblyomma (cayenne tick). Mucosal transmission can occur when contaminated by a crushed tick or by tick fecal matter. RMSF occurs over a wide distribution throughout the contiguous United States, but cases are most commonly reported from Arkansas, Missouri, North Carolina, Oklahoma, and Tennessee. RMSF can also be seen in northern Mexico and Central and South America. Over 90% of cases occur during April through September. The disease is more frequent in males and children.

The incidence of RMSF has been steadily increasing to an estimated 2000 cases per year, but the case fatality rates (at least in the United States) have been decreasing (attributed to enhanced recognition and early treatment).

Early clinical manifestations of RMSF include high fever, severe headache, myalgias, nausea, and vomiting. Later manifestations include rash, photophobia, confusion, ataxia, seizures, cough, dyspnea, arrhythmias, jaundice, and severe abdominal pain. Rash occurs more frequently in children and young adults than in older individuals. It is also seen earlier in the course of the disease than in older individuals. Thrombocytopenia and hyponatremia and acute respiratory distress syndrome with organ failure may also be seen. A serious complication of RMSF is rhabdomyolysis. Long-term sequelae include central nervous system deficits and amputations.

RMSF is infectious as an aerosol, and if weaponized, that would be the most likely method of dispersal.

Classic triad of RMSF:
  1. Fever
  2. Rash
  3. History of tick exposure in last 12 days


A77.0 – Spotted fever due to Rickettsia rickettsii

186772009 – Rocky Mountain spotted fever

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Differential Diagnosis & Pitfalls

  • Meningococcemia typically occurs in the late winter to early spring with fever and rash appearing within 24 hours of infection. There is also marked lymphadenopathy.
  • Measles typically occurs in the winter to spring and has associated symptoms of cough, coryza, conjunctivitis, and Koplik spots.
  • Enteroviral infections typically occur in the summer to fall. The fever and rash often appear together. Sick contacts are common.
  • Dengue fever, also known as "breakbone fever," has severe arthralgias.
  • Vasculitis is marked by palpable purpura rather than petechiae.
  • Drug eruptions will have a history of exposure.
  • Secondary syphilis can also present with a palm and sole rash; occasionally, the rash imparts a rust-colored hue.
  • Gonococcemia may present with asymmetric monoarticular arthritis and a pustular or petechial rash.
  • Viral or bacterial enterocolitis presents with nausea and vomiting.
  • Acute surgical abdomen has severe abdominal pain.
  • Hepatitis presents with jaundice.
  • Meningitis has prominent neurological signs.
  • Idiopathic thrombocytopenic purpura (ITP) presents with a petechial rash.
  • Thrombotic thrombocytopenic purpura (TTP) is characterized by fever, anemia, thrombocytopenia, renal impairment, and neurological deficits.

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Last Reviewed:08/16/2018
Last Updated:08/16/2018
Copyright © 2023 VisualDx®. All rights reserved.
Potentially life-threatening emergency
Rocky Mountain spotted fever - Chem-Bio-Rad Suspicion
See also in: Overview
A medical illustration showing key findings of Rocky Mountain spotted fever : Abdominal pain, Headache, Nausea/vomiting, Tick bite, Arthralgia, Myalgia, Splenomegaly, High fever, PLT decreased
Clinical image of Rocky Mountain spotted fever - imageId=273650. Click to open in gallery.  caption: 'Myriad petechiae on the arm.'
Myriad petechiae on the arm.
Copyright © 2023 VisualDx®. All rights reserved.