Microbiology & infectious diseases

Gram negative bacteria


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PubMed Search: Anaplasma

Dragos C. Luca, M.D.
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Cite this page: Luca DC. Anaplasma. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/microbiologyanaplasma.html. Accessed April 14th, 2024.
Definition / general
  • Anaplasma phagocytophilia: human granulocytotropic anaplasmosis (HGA, formally termed human granulocytic ehrlichiosis)
  • Vector borne disease transmitted through bite of Ixodes ticks
  • Bacteria is obligate intracellular pathogen that binds to P selectin glycoprotein ligand 1 (PSGL1 / CD162)
  • Susceptibility also associated with expression of CD15s (J Clin Invest 1999;103:407)
  • First described in USA in 1994
  • Geographic distribution of A. phagocytophilia (HGA) reflects regions of US where their hard tick vectors reside: northeastern states, northwest Wisconsin, eastern Minnesota and Pacific northwest states<
Clinical features
  • Presents with fever, leukopenia, thrombocytopenia (70 - 90%) and elevated liver enzymes
  • Mortality rate is 0.5 - 1% for HGA
  • Particularly severe infections occur in elderly / immunocompromised
  • Characteristic intracytoplasmic morulae (morula is Latin for mulberry): cytoplasmic membrane bound vacuoles with irregular edges containing hundreds to thousands of clustered gram negative bacteria
  • Infected cells typically contain only 1 or 2 morulae although as many as 15 may be seen in immunosuppressed individuals
  • Greatly variable percentage of peripheral blood films with detectable morulae in the literature (3 - 80%) with a higher number seen with HGA infection (50 - 80%) and in immunosuppressed individuals
Case reports
  • Most patients are seronegative during first few weeks of acute infection (60 - 97%), so therapeutic decisions must be based on clinical suspicion, peripheral blood findings and PCR (sensitivity is 60 - 85%, high degree of false positive results)
  • Became a nationally reportable disease to US Centers for Disease Control in 1999
  • Organisms are susceptible to tetracyclines and their derivatives, particularly doxycycline
Microscopic (histologic) description
  • Peripheral blood: buffy coat examination may reveal intracytoplasmic inclusions (morulae - spherical structures with irregular edges) within neutrophils or monocytes
  • Bone marrow: epithelioid granulomas; usually normo or hypercellular with intact trilineage maturation; rare hypoplasia; possible increased megakaryocytes
  • Histopathologic bone marrow findings: inconsistent and likely to change during the course of the disease
  • HGA organisms preferentially infect more mature rather than immature granulocytic cells in bone marrow
Peripheral smear images

Contributed by Patricia Tsang, M.D.

Case of the Month #486

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HGA: inclusions in granulocytes

Left: HGA (HGE)

Board review style question #1
What of the following is true about anaplasmosis?

A. Anaplasmosis is typically transmitted by exposure to respiratory droplets.
B. Immunocompromised patients traveling to endemic regions should be vaccinated against Anaplasma.
C. A common presentation for patients with anaplasmosis includes relapsing fevers, neutrophilia and reactive thrombocytosis.
D. A diagnostic feature of anaplasmosis is the presence of neutrophilic morulae.
E. Anaplasma and malaria are common coinfections.
Board review style answer #1
D. Neutrophilic morulae are a characteristic feature of anaplasmosis on peripheral blood smear examination.

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Reference: Anaplasmosis
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