Approach Considerations
Laboratory findings may be nonspecific in Rocky Mountain spotted fever (RMSF). On the complete blood cell count, the total leukocyte count may be normal, elevated, or decreased but usually shows a left shift.
Mild anemia and thrombocytopenia of less than 150 × 109/L (< 150 × 103/µL) occur in approximately one third of patients. Severe thrombocytopenia of less than 20 × 109/L (< 20 × 103/µL) occurs in approximately 10% of patients.
On a comprehensive metabolic panel, the following may be noted:
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Hyponatremia (serum sodium < 130 mEq/L) in 20% of patients
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The serum alanine aminotransferase level is usually increased
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Serum albumin values may be low
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The blood urea nitrogen (BUN) level is increased
Results of cerebrospinal fluid (CSF) analysis are generally normal. However, mild pleocytosis may be present, and approximately 50% of patients have a predominance of polymorphonuclear cells. An elevated CSF protein level may also be observed.
Serologic assays to detect anti–R rickettsii immunoglobulin G (IgG) antibodies are usually performed for definitive diagnosis. [10] Testing of acute-phase and convalescent-phase sera is recommended to demonstrate a 4-fold or higher increase in the titer.
Enzyme immunoassays (EIAs) and immunoglobulin M (IgM) antibody-capture immunoassays are new serologic tests that potentially allow for early diagnosis.
In research laboratories, isolation of rickettsiae from tissues or direct detection of rickettsiae in tissues by means of direct immunofluorescence is used to confirm the diagnosis. Polymerase chain reaction tests have been developed but are not widely available.
Imaging Studies
CT imaging is typically normal, whereas MRI seems more sensitive at revealing abnormalities. Published findings include diffuse edema, effacement of the sulci, arterial infarctions, prominent perivascular spaces, and enhancement of the meninges (ie typical findings of meningoencephalitis). In a case series of 3 pediatric cases of Rocky Mountain spotted fever (RMSF) with encephalitis, scattered nonenhancing punctate lesions were described throughout the cerebral white matter, visible on T2- and diffusion-weighted MRI. [11, 12, 13]
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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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