Microbe of the Week: Anaplasma phagocytophilum

Arrows indicate Anaplasma phagocytophilum residing inside host cells. Photo Credit: CDC Public Health Image Library

Microbe of the Week is written by undergraduate researcher, Maria Bergquist.

Microbe of the Week: Anaplasma phagocytophilum

 

 

Welcome to microbe of the week, where we break down the different disease-causing microbes that lurk inside our tick vectors! This week’s microbe is Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis (HGA) or more simply referred to as Anaplasmosis.

What is Anaplasma phagocytophilum?

Anaplasma phagocytophilum (referred to as Anaplasma here on out) is a tiny, round bacterial species that forms grape-like clusters. It lives and reproduces inside host immune cells—you know, the cells that are supposed to protect against foreign invaders. This behavior makes Anaplasma just as tricky as Borrelia burgdorferi (the bacterial species we covered last week). Both Anaplasma and Borrelia are transmitted by the black-legged tick (aka deer tick) in the United States, and other Ixodid ticks around the world. Further, the reservoir for both bacterial species is the white-footed mouse (Peromyscus leucopus).

More details on Anaplasma epidemiology and statistics available on the CDC website at: https://www.cdc.gov/anaplasmosis/stats/index.html

Variants of the Species

Anaplasma phagocytophilum was originally three separate species under a different genus (Ehrlichia phagocytophila, Ehrlichia equi, and the agent of human granulocytic ehrlichiosis (HGE)) until reclassification in 2001. Despite this reclassification, infectivity of Anaplasma depends on the variant of the species. The variant that infects humans (causing HGA) generally is not the same as the variant that infects animals (a disease known as Tick-borne Fever or TBF in cattle, sheep, and other ruminants).

Symptoms

Symptoms of Anaplasmosis occur about 1 to 5 days after tick bite exposure and may be confused with other common illnesses. These symptoms include fever, chills, severe headache, nausea, vomiting, diarrhea, and loss of appetite. Serious symptoms are rare but may present if treatment is delayed or if there are other health issues. Severe late-stage symptoms include respiratory failure, bleeding problems, organ failure, and sometimes even death.

See the CDC’s website for more information on symptoms of Anaplasmosis: https://www.cdc.gov/anaplasmosis/symptoms/index.html

Treatment

The CDC recommends the antibiotic Doxycyline for treating Anaplasmosis.

Condensed history

• 1932 — Tick-borne fever (TBF) recognized as a distinct tick-borne illness in Scotland. 
• 1940 — Anaplasma phagocytophilum animal variant (not yet named such) recognized as causative agent of TBF. 
• 1949 — Animal variant of Anaplasma initially coined Rickettsia phagocytophila. 
• 1950 to 1989 — TBF recognized in sheep and cattle throughout Europe. 
• 1962 — Animal variant of Anaplasma renamed Cytoecetes phagocytophila. 
• 1969 — Equine granulocytic ehrlichiosis (now equine granulocytic anaplasmosis (EGA)) found in horses in California. 
• 1984 — Animal variant of Anaplasma renamed (again) to Ehrlichia phagocytophila. 
• 1994 — First human case of Anaplasmosis recorded, known then as human granulocytic ehrlichiosis. 
• 2001 — Increased research reclassifies granulocytic ehrlichiosis diseases to Anaplasma phagocytophilum. 
• 2008 — Reported cases of Anaplasmosis experience a steady rise in the US according to the CDC. 
• 2017 — 5,762 reported cases to the CDC, a sharp peak from previous years. 
• 2018 — CDC reports a reduced case load compared to previous years (4,008). This is the most recent data for cases in the US.

An in-depth breakdown of the history of Anaplasma was written by Zerai Woldehiwet in 2010 and is available at: https://www-sciencedirect-com.ezp1.lib.umn.edu/science/article/pii/S0304401709005469