29: Ehrlichioses

Classification: ICD-9 083.8; ICD-10 A77.4

Syndromes and synonyms: Human monocytic ehrlichioses (HME), human granulocytic anaplasmosis (HGA), Sennetsu fever.

Agent: Obligate intracellular, pleomorphic bacteria of the family Anaplasmataceae, which invade host leukocytes. Ehrlichia chaffeensis infects mononuclear phagocytes; E. ewingii and Anaplasma phagocytophilum infects neutrophils. E. canis may cause human disease in Venezuela. In 2009 an E. muris-like organism was discovered in USA. Neorickettsia sennetsu causes Sennetsu fever.

Reservoir: Deer and dogs (E. chaffeensis, E. ewingii). The white-tailed deer is a complete host for maintaining the transmission cycle of E. chaffeensis. Deer, ruminants, and field rodents (A. phagocytophilum); unknown for N. sennetsu; other members of the genus parasitize the trematode worms of aquatic vertebrates and invertebrates.

Vector: Ticks of the genera Amblyomma and Ixodes.

Transmission: By tick bites, except for N. sennetsu, which is probably transmitted through ingestion of uncooked, parasitized aquatic hosts. No person-to-person spread except by blood transfusion.

Cycle: Tick to mammalian host to tick for Ehrlichia and Anaplasma; trematode to aquatic host to trematode for Neorickettsia

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