MICROBIOLOGY

• The cause of tick-borne infection in humans is 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 bites in the southern U.S. Ticks are generally found in woodland habitats with white-tail deer (considered the main reservoir).
  • E. ewingii (Ee): a canine pathogen that rarely infects humans, an infection now termed "human ewingii ehrlichiosis [HEE]."
    • Human cases are increasingly described from a broader range (10 states range similar to E. chaffeensis), though most to date have been in Missouri.
    • ~20-30 cases are reported in the U.S. annually.
  • 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[15] in Wisconsin, this organism is a close relative of E. muris; the likely vector is the Ixodes scapularis tick (the same as HGA and B. burgdorferi).
    • Sero-crossreactivity is seen with antibodies to E. chaffeensis, which may confuse an accurate diagnosis.
      • For reporting purposes, it remains categorized in CDC reporting as "Undetermined ehrlichiosis/anaplasmosis."
    • Existing PCR primers for E. chaffeensis and E. ewingii may fail to detect EML.
    • Cases to date have been described in Minnesota, Wisconsin, and Michigan.
• 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).