Diagnostic Considerations
Ehrlichiosis should be diagnosed based on clinical and epidemiological findings. It can later be confirmed using specialized confirmatory laboratory tests. Treatment should be initiated immediately pending the receipt of laboratory test results.
Ehrlichiosis is a difficult infectious disease to diagnose because it manifests as an acute, undifferentiated, febrile, RMSF-like illness with few or no physical findings. Most patients who are diagnosed with RMSF without rash probably have ehrlichiosis. Co-infections of various tick-borne pathogens transmitted by the same vector are rare, but they do occur.
Ehrlichiosis has the same distribution as RMSF and is transmitted by the same tick species (eg, Amblyomma, Dermacentor). However, RMSF causes physical findings that ehrlichiosis does not, including bilateral periorbital edema, edema of the dorsum of the hands and feet, and conjunctival suffusion. The maculopapular/petechial rash of RMSF on the wrists/ankles is usually absent in ehrlichiosis.
Laboratory findings associated with RMSF and ehrlichiosis are similar (eg, thrombocytopenia, relative lymphopenia, increased levels of serum transaminases, atypical lymphocytes). However, neutropenia is more common in ehrlichiosis than in RMSF.
Most patients with ehrlichiosis present with fever and a severe headache. Nuchal rigidity may occur but it is not common. The cerebrospinal fluid (CSF) profile in patients with ehrlichiosis is mostly within normal limits but lymphocytic pleocytosis as well as elevated protein has occured in a few patients. [14]
Other differential diagnostic possibilities include typhoid fever, malaria, and babesiosis. All of these infectious diseases manifest as acute, undifferentiated, febrile illnesses with a paucity of physical signs. The diagnosis of typhoid fever and malaria are suggested by an appropriate epidemiologic profile and/or travel history. Exposure to large Dermacentor ticks would suggest RMSF, whereas exposure to small Ixodes ticks would suggest the possibility of babesiosis.
Other diseases that share clinical and laboratory findings of ehrlichial disease, particularly in patients with rash, include meningococcemia, toxic shock syndrome, murine typhus, Q fever, typhoid fever, leptospirosis, hepatitis, enteroviral infection, influenza, bacterial sepsis, endocarditis, and Kawasaki disease.
If a history of tick bite and outdoor activities exist with these symptoms, the physician should consider other tickborne febrile illnesses such as Rocky Mountain spotted fever, relapsing fever, tularemia, Lyme borreliosis, Colorado tick fever, and babesiosis.
Differential Diagnoses
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Female Lone Star tick, Amblyomma americanum, found in the southeastern and Midatlantic United States. It is a vector of several zoonotic diseases, including human monocytic ehrlichiosis and Rocky Mountain spotted fever. Courtesy of the CDC/Michael L. Levin, PhD.
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Map of the United States showing the distribution of the Lone Star Tick, which is the principle vector for ehrlichiosis.
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Established and reported distribution of anaplasmosis vectors Ixodes scapularis and Ixodes pacificus, by county, in the United States from 1907-1996. Courtesy of the Division of Vector-Borne Infectious Diseases at the Centers for Disease Control and Prevention.
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Anaplasmosis incidence by age. Courtesy of the Centers for Disease Control and Prevention.
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Ehrlichiosis incidence by age. Courtesy of the Centers for Disease Control and Prevention.
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This graph displays the number of human cases of ehrlichiosis caused by Ehrlichia chaffeensis reported to the Centers for Disease Control and Prevention (CDC) annually from 2000 through 2016. *From 2000 to 2008, ehrlichiosis was included in the reporting category “human monocytic ehrlichiosis” in reports to the National Notifiable Diseases Surveillance System (NNDSS). **Since 2008, ehrlichiosis has been reported to the NNDSS under the categories “Ehrlichia chaffeensis infections,” “Ehrlichia ewingii infections,” and “Undetermined ehrlichiosis/anaplasmosis infections”, which include infections caused by Ehrlichia muris eauclairensis. Only E chaffeensis infections are shown above. Courtesy of the CDC (https://www.cdc.gov/ehrlichiosis/stats/index.html).
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This graph shows the number of ehrlichiosis cases caused by Ehrlichia chaffeensis reported from 2000 through 2016 by month of onset to illustrate the seasonal trends. Cases are reported in each month of the year, although most are reported in June and July. Courtesy of the CDC (https://www.cdc.gov/ehrlichiosis/stats/index.html).
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This graph shows the number of US ehrlichiosis cases caused by Ehrlichia chaffeensis and reported to the CDC from 2000 to 2018. Courtesy of the CDC (https://www.cdc.gov/ehrlichiosis/stats/).
Tables
|
Human monocytic ehrlichiosis (HME) |
Human granulocytic anaplasmosis (HGA) |
Cell type Affected |
Monocytes |
Granulocytes |
Organism |
E chaffeensis |
A phagocytophilum |
Vector |
Amblyomma americanum (Lone Star tick) |
Ixodes scapularis (black-legged tick), Ixodes pacificus (Western black-legged tick) in California, Ixodes ricinus in Europe, and probably Ixodes persulcatus in parts of Asia |
Location |
Southeastern and south-central United States |
Wisconsin and Minnesota, less active in New York and Connecticut, also California |
Rash |
30% of adults, 60% of children |
Rare |
Prognosis |
~3% mortality |
< 1% mortality |
