Clinical Pearls & Morning Reports
Published February 15, 2023
Ehrlichiosis and anaplasmosis are tickborne illnesses that are characterized by fever, leukopenia, thrombocytopenia, and elevated aminotransferase levels. Read the NEJM Case Records of the Massachusetts General Hospital here.
Q: How are anaplasmosis and ehrlichiosis similar and how do they differ?
A: Ehrlichiosis is associated with rash in 33% of affected patients and with encephalopathy in approximately 20% of patients; rash and encephalopathy are rare in patients with anaplasmosis. The vector for ehrlichia species is the lone star tick (Amblyomma americanum), and ehrlichiosis is most common in the southern and central United States. Anaplasma phagocytophilum, the bacterium that causes anaplasmosis, is transmitted by the deer tick (Ixodes scapularis) in the northeastern and upper midwestern United States. Clinically, ehrlichiosis and anaplasmosis are often indistinguishable, and they are treated the same way.
Q: What are some of the laboratory features of anaplasmosis?
A: Increased aminotransferase levels are observed in approximately 80% of affected patients. Thrombocytopenia and leukopenia occur in 75% and 55% of patients, respectively, and C-reactive protein elevations are seen in more than 90% of patients. Thrombocytosis and leukocytosis are highly unlikely to occur in patients with anaplasmosis, unless such abnormalities are caused by an alternative process. Anemia occurs in less than one third of patients, and therefore this finding can help to distinguish anaplasmosis from babesiosis, a parasitic infection transmitted by the same ixodes tick species in a similar geographic distribution.
A: The diagnosis of anaplasmosis can be achieved by means of direct and indirect methods. Direct diagnostic methods include nucleic acid amplification testing (NAAT) and microscopy. The sensitivity of NAAT ranges from 70 to 90% and is highest when the test is performed early in the disease course. The specificity approaches 100%. Given these testing characteristics, NAAT is considered to be the test of choice, particularly during the early stages of disease. Microscopic examination of a peripheral-blood or buffy-coat smear (stained with Wright-Giemsa stain) can detect intracytoplasmic inclusions (morulae) in granulocytes, which can support the diagnosis. However, microscopy is time-consuming and highly operator-dependent. The overall sensitivity of microscopic analysis ranges from 25 to 75%. Indirect diagnostic tests include serologic testing as well as evaluation of the peripheral-blood count and tests of liver function. Serologic testing requires paired serum samples that are obtained during the acute and convalescent phases of infection; test sensitivity is particularly limited in early infection.
A: Antimicrobial therapy is warranted in all patients with a confirmed diagnosis of anaplasmosis. Doxycycline is the preferred treatment for adults with anaplasmosis, and there is no documented resistance to this agent. This observation is based primarily on retrospective data, since data from randomized, controlled trials are lacking. Tetracyclines are bactericidal against anaplasma species and are associated with relatively few toxic effects. In addition, coinfection with Borrelia burgdorferi, the causative agent of Lyme disease, which is transmitted by the same ixodes tick species that transmits A. phagocytophilum, can occur in up to 12% of patients. The risk of coinfection offers additional rationale for the use of doxycycline, which is active against both bacterial species. Persistent infection after treatment is extremely uncommon. Rifamycins have shown in vitro activity against anaplasma species and can be considered if an alternative to doxycycline is needed.