MICROBIOLOGY

• Cause of tick-borne infection in humans; obligate intracellular pathogens that infect human macrophages and monocytes.
• Species described as causing human infection:
  
  • Human monocytic ehrlichiosis (HME): due to Ehrlichia chaffeensis, transmitted by Amblyomma americanum (lone star tick, Fig 1) and possibly other tick vectors such as Dermacentor variabilis (American dog tick).
    • Lone star is the most common cause of tick bite in southern U.S. Ticks generally in woodland habitats with white-tail deer (thought to be the main reservoir).
  • E. ewingii (Ee): a canine pathogen that rarely infects humans, infection now termed "human ewingii ehrlichiosis [HEE]."
    • Human cases increasingly described over a wider range (10 states), though most to date in Missouri.
  • E. muris: human infection noted in Europe, Russia, Japan/Asia, and described in the Western U.S.
    • This occurs via tick vector Ixodes persulcatus complex.
  • Ehrlichia species, E. muris eauclairensis
    • Previously termed E. muris-like (EML) agent.
    • Recently identified[13] in Wisconsin, this organism is a close relative of E. muris; likely vector is the Ixodes scapularis tick (same as HGA and B. burgdorferi).
    • Sero-crossreactivity is seen with antibodies to E. chaffeensis which may confuse an accurate diagnosis.
      • For reporting purposes, remains categorized in CDC reporting as "Undetermined ehrlichiosis/anaplasmosis."
    • Existing PCR primers for E. chaffeensis and E. ewingii may fail to detect.
    • Cases to date described in Minnesota and Wisconsin.
• See separate module for human granulocytic anaplasmosis [HGA] (formerly called human granulocytic ehrlichiosis) caused by the distantly related organism, Anaplasma phagocytophilum, transmitted by Ixodes scapularis (deer tick) and on the West Coast, the western black-legged tick (Ixodes pacificus) in the same distribution as Lyme disease (Borrelia burgdorferi).