Ehrlichiosis and related infections.

Zoonosis Update T he term ehrlichiosis has been broadly applied to a variety of diseases of humans and animals caused by pathogens formerly classified in the genus Ehrlichia. However, a recently proposed taxonomic reorganization 1 has recommended the reclassifications of some former Ehrlichia species under the genera Anaplasma and Neorickettsia and Cowdria ruminantium under the genus Ehrlichia. 1 Therefore, the term ehrlichiosis is no longer accurately descriptive of the nature of infections caused by these diverse pathogens, but usage of the term persists, and it will be used here for simplicity. The purpose of this review is to summarize pertinent points regarding a few of these pathogens that may infect both humans and animals and are of zoonotic importance in the United States, specifically Anaplasma phagocytophilum (formerly Ehrlichia phagocytophila), E chaffeensis, E ewingii, and E canis (Table 1). Several related pathogens will not be discussed in this review; although E sennetsu causes infections in humans, this pathogen is not found in the United States, and additional pathogens of veterinary interest in the United States such as Neorickettsia risticii (formerly E risticii, the agent of Potomac horse fever or equine monocytic ehrlichiosis), E platys (the agent of canine cyclic throm-bocytopenia), and E bovis are not known to cause infections in humans. Pathogen Characteristics Ehrlichia and Anaplasma species belong to the family Anaplasmataceae. 1 These organisms are small (0.5-to 1.5-µm), gram-negative, pleomorphic, obligate intracel-lular bacteria that reside and replicate in membrane-bound vacuoles of eukaryotic cells. 2 Anaplasma phagocy-tophilum, E chaffeensis, E ewingii, and E canis primarily invade mammalian leukocytes, in which they multiply and form membrane-bound, intracytoplasmic colonies called morulae. 2 The diseases caused by these pathogens have traditionally been categorized by the type of blood cell most commonly infected. For example, E chaffeensis and E canis reside primarily in monocytes, and the disease caused by these agents is frequently called mono-cytic (or monocytotropic) ehrlichiosis. Anaplasma phago-cytophilum and E ewingii reside primarily in granulo-cytes, and the disease caused by these agents is often referred to as granulocytic (or granulocytotropic) ehrli-chiosis. However, some Ehrlichia and Anaplasma species have been found in cells other than their chief target cell type. In addition, more than 1 genus or species may be responsible for monocytic or granulocytic ehrlichiosis. Thus, the traditional cell-based classification scheme is not adequately descriptive of these diseases. The Ehrlichia and Anaplasma species discussed in this review are zoonotic tick-borne pathogens. These agents …