Diagnostic Considerations
Rocky Mountain spotted fever (RMSF) is a life-threatening disease. Therefore, it is important to consider the possibility of RMSF in any patient whose clinical presentation is consistent with the disease. Clinicians should have a low threshold for treatment if RMSF is clinically suspected.
Signs and symptoms can mimic those of other diseases. Therefore, a history of traveling to endemic areas, having tick bites, or having exposures to ticks is an important clue. Absence of a history of tick bites should not exclude the diagnosis if the index of clinical suspicion is high.
The differential diagnosis of RMSF includes other infections with fever and rash, especially those involving rash including palms and soles, such as the following:
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Enteroviral infection
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Secondary syphilis
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Meningococcemia
An enteroviral infection is typically seen in summer months, the child is relatively well-appearing, and the screening laboratory tests usually do not reveal significant derangements. In a sexually active individual, secondary syphilis can present with fevers and rash, including on the palms and soles. Invasive West Nile virus infection causes maculopapular rash with a petechial component. Sepsis, especially meningococcemia, should be considered in a toxic-appearing child who presents with fever and a purpuric rash. Toxin-mediated gram-negative sepsis should be considered in an immunocompromised host with fever and rash.
Other tick-borne infections, such as monocytic ehrlichiosis and granulocytic anaplasmosis, can present strikingly similarly to RMSF. However, ehrlichiosis usually is associated with leucopenia; thrombocytopenia and rash are often absent. It should be kept in mind that tick-borne co-infections are possible in the same patient bitten by the same tick.
Other infectious and inflammatory entities that may have clinical features that can be confused with those of RMSF include the following:
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Infectious mononucleosis
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Parvovirus infection
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Roseola
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Scarlet fever
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Leptospirosis
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Kawasaki disease
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Typhoid fever and dengue should be in the differential if the patient has a history of travel
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The new Phlebovirus (Heartland virus) seen mostly in Missouri, and a new rickettsial species ( Rickettsia amblyommii) seen in North Carolina, may present similarly to infection with R rickettsii
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Fever and rash can also be present with noninfectious conditions, such as drug reactions, and vasculitis
Differential Diagnoses
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Enterovirus
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Meningococcal Infections
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Parvovirus
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Rosela
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Geographic distribution of Rocky Mountain spotted fever incidence in 2010, cases per million: Courtesy of the US Centers for Disease Control and Prevention.
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Annual incidence (per million persons) for Spotted Fever Rickettsiosis (SFR) in the United States, 2017. Courtesy of the CDC.
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Under a magnification of 98X, this scanning electron microscopic (SEM) image depicts the dorsal view of the head region from an American dog tick, Dermacentor variabilis, magnified 98X. D. variabilis is a known carrier of Rocky Mountain Spotted Fever (RMSF) caused by the bacterium, Rickettsia rickettsii. Courtesy of the Public Health Image Library (PHIL), CDC, photo credit Janice Haney Carr.
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Rocky Mountain Spotted Fever (RMSF) rash in a child. It appears day 3-5 of illness, begins in ankles and wrists, and typically involves palms and soles. In early stages it is macular and later it is petechial. Courtesy of the Morbidity and Mortality Weekly Report (MMWR), CDC.
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Tick identification. (i) American dog tick (Dermacentor variabilis), ii) Rocky Mountain wood tick (Dermacentor andersoni), and, iii) brown dog tick (Rhipicephalus sanguineus) iv) Lone star tick (Amblyomma americanum). Courtesy of the CDC.
Tables
Organism |
Disease or Presentation |
Geographic Location |
Rickettsia rickettsii |
Rocky Mountain spotted fever |
North, Central and South America |
Rickettsia akari |
Rickettsialpox |
Worldwide |
Rickettsia conorii |
Mediterranean spotted fever, boutonneuse fever, Israeli spotted fever, Astrakhan fever, Indian tick typhus |
Europe, Asia, Africa, India, Israel, Sicily, Russia |
Rickettsia sibirica |
Siberian tick typhus, North Asian tick typhus |
Siberia, People's Republic of China, Mongolia, Europe |
Rickettsia australis |
Queensland tick typhus |
Australia |
Rickettsia honei |
Flinders Island spotted fever, Thai tick typhus |
Australia, South Eastern Asia |
Rickettsia africae |
African tick-bite fever |
Sub Saharan Africa, Caribbean |
Rickettsia japonica |
Japanese or Oriental spotted fever |
Japan |
Rickettsia felis |
Cat flea rickettsiosis, flea borne typhus |
Worldwide |
Rickettsia slovaca |
Necrosis, erythema, lymphadenopathy |
Europe |
Rickettsia heilongjaiangensis |
Mild spotted fever |
China, Asian region of Russia |
Rickettsia parkeri |
Mild spotted fever |
US |
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