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Ehrlichiosis and anaplasmosis are a group of emerging zoonotic infections caused by Ehrlichia and Anaplasma species, respectively, which are obligate intracellular, gram-negative rickettsial organisms that infect human leukocytes.
Human granulocytic anaplasmosis (HA) is caused by the tick-borne rickettsia, Anaplasma phagocytophilum, which is transmitted by contact with Ixodes ticks. The white-footed mouse is the animal reservoir, and the epidemiology of this infection is very much like that of Lyme disease (caused by Borrelia burgdorferi) and babesiosis (caused primarily by Babesia microti), which all have the same tick vector. HA is most prevalent in the upper Midwest and in other areas of the United States that are endemic for Lyme disease.
Human monocytic ehrlichiosis (HE) is caused by Ehrlichia chaffeensis, which is transmitted by the Lone Star tick, Amblyomma americanum. The deer is believed to be the animal reservoir, and most cases of HE have been reported from the southeastern and south-central regions of the United States.
Ehrlichia ewingii, the known cause of canine granulocytic ehrlichiosis, can occasionally cause an HE-like illness in humans. Clinical features and laboratory abnormalities are similar to those of Ehrlichia chaffeensis infection, and antibodies to Ehrlichia ewingii cross-react with current serologic assays for detection of antibodies to Ehrlichia chaffeensis.
In 2009, Mayo Medical Laboratories detected a new species of Ehrlichia DNA in 4 patients (3 from Wisconsin, 1 from Minnesota) using PCR.(5) Sequenced portions of the groEL and rss (18S rRNA) genes show 98% homology to analogous regions of Ehrlichia muris, a species not previously identified in North America. The Ehrlichia muris-like (EML) organism has since been found in Ixodes scapularis ticks and white footed mice in Wisconsin and Minnesota, although it is not known whether these play a role in the life cycle of this organism.
Infective forms of Ehrlichia and Anaplasma are injected during tick bites and the organisms enter the vascular system where they infect leukocytes. They are sequestered in host-cell membrane-limited parasitophorous vacuoles known as morulae. Asexual reproduction occurs in leukocytes where daughter cells are formed and liberated upon rupture of the leukocytes.
Most cases of ehrlichiosis are probably subclinical or mild, but the infection can be severe and life-threatening in some individuals. Fever, fatigue, malaise, headache, and other "flu-like" symptoms, including myalgias, arthralgias, and nausea, occur most commonly. Central nervous system involvement can result in seizures and coma. The 4 patients infected with the EML organism presented with fever, headache, myalgia, and leukopenia.
Diagnosis of ehrlichiosis may be difficult since the patient's clinical course is often mild and nonspecific. This symptom complex is easily confused with other illnesses such as influenza, or other tick-borne zoonoses such as Lyme disease, babesiosis, and Rocky Mountain spotted fever. Clues to the diagnosis of ehrlichiosis in an acutely febrile patient after tick exposure include laboratory findings of leukopenia or thrombocytopenia and elevated serum aminotransferase levels. However, while these abnormal laboratory findings are frequently seen, they are not specific. Rarely, intragranulocytic or monocytic morulae may be observed on peripheral blood smear, but this is not a reliable means of diagnosing cases of human ehrlichiosis or anaplasmosis.
Definitive diagnosis is usually accomplished through PCR and serologic methods. PCR techniques allow direct detection of pathogen-specific DNA from patients' whole blood during the acute phase of disease. This is currently the test of choice for the newly described EML organism.
Serologic testing is usually done only for confirmatory purposes, by demonstrating a 4-fold rise or fall in specific antibody titers to Ehrlichia species or Anaplasma antigens. There is not currently a commercially available specific serologic test for the EML organism, and cross-reactivity with the other Ehrlichia species by serology may be unreliable.
It is important to note that concurrent infection with Anaplasma phagocytophilum, Borrelia burgdorferi, and Babesia microti is not uncommon as these organisms share the same Ixodes tick vector, and additional testing for these pathogens may be indicated.
Evaluating patients suspected of acute anaplasmosis or ehrlichiosis
Positive results indicate presence of specific DNA from Ehrlichia chaffeensis, Ehrlichia ewingii, Ehrlichia muris-like organism, or Anaplasma phagocytophilum and support the diagnosis of ehrlichiosis or anaplasmosis.
Negative results indicate absence of detectable DNA from any of these 4 pathogens in specimens, but it does not exclude the presence of the organism or active or recent disease.
Since DNA of Ehrlichia ewingii is indistinguishable from that of Ehrlichia canis by this rapid PCR assay, a positive result for Ehrlichia ewingii/canis indicates the presence of DNA from either of these 2 organisms.
This assay should not be used for screening asymptomatic individuals, and should only be used to test patients with signs and symptoms of ehrlichiosis or anaplasmosis.
A negative result does not indicate absence of disease.
Inadequate specimen draw or improper conditions for storage or transport may invalidate test results.
This test may detect DNA of Ehrlichia canis (reported to cause asymptomatic infection in Venezuela only).
This PCR test does not detect DNA of Rickettsia (formerly Ehrlichia) sennetsu, which has been reported to cause a rare mononucleosis-like illness in humans (in Japan and Malaysia).
1. Bakken JS, Dunler JS: Human granulocytic ehrlichiosis. Clin Infect Dis 2000 Aug;31(2):554-560
2. Dunler JS, Bakken JS: Human ehrlichioses: newly recognized infections transmitted by ticks. Ann Rev Med 1998;49:201-213
3. Krause PJ, McKay K, Thompson CA, et al: Disease-specific diagnosis of coinfecting tickborne zoonoses: babesiosis, human granulocytic ehrlichiosis, and Lyme disease. Clin Infect Dis 1999 May 1;34(9):1184-1191
4. McQuiston JH, Paddock CD, Holman RC, Childs JE: The human ehrlichioses in the United States. Emerging Infect Dis 1999 Sept-Oct;5(5):635-642
5. Pritt BS, Sloan LM, Johnson DK, et al: Emergence of a new pathogenic Ehrlichia species, Wisconsin and Minnesota, 2009. N Engl J Med 2011 Aug 4;365(5):422-429