General Characteristics and Taxonomy
Members of the family Anaplasmataceae are classic causative agents of zoonoses. Taxonomically, they belong to the class Alpha-proteobacteria and the order Rickettsiales.
The life cycle of these microorganisms is inextricably linked to host cells because they are obligate intracellular parasites. In mammals (humans, horses, cattle, sheep, deer), the bacteria target circulating blood cells—predominantly various leukocyte pools—as well as hematopoietic progenitor cells. In addition to mammals, hosts can include fish and insects.
Modern Classification of Pathogens
For many years, the classification of these microorganisms was revised. In 2001, based on accumulated clinical, epidemiological, and molecular-genetic data, the family Anaplasmataceae was definitively established as a separate taxon.
Pathogens affecting humans belong to three genera: Ehrlichia, Anaplasma, and Neorickettsia. They cause distinct clinical entities:
- Causative agent of human granulocytic anaplasmosis: Anaplasma phagocytophilum. Parasitizes neutrophils.
- Causative agents of human monocytic ehrlichiosis: Ehrlichia chaffeensis and Ehrlichia muris. Predominantly infect monocytes.
- Causative agent of Sennetsu fever: Neorickettsia sennetsu. Endemic to Japan.
History of the Discovery of Ehrlichioses
The genus name Ehrlichia was proposed in 1945. The bacteria were named in honor of the prominent immunologist Paul Ehrlich, highlighting their specific tropism for hematopoietic cells.
For a long time, these microorganisms were considered exclusively a veterinary problem, but the situation changed in the late 20th century:
- Human Monocytic Ehrlichiosis (HME). The first clinical case was described in 1986 in the USA by researcher K. Maeda. Subsequently, the disease was confirmed in European countries and parts of Africa.
- Human Granulocytic Anaplasmosis (HGA). Discovered in 1994 by a group of scientists led by D. Bakken. The researchers found the pathogen directly inside neutrophils. The clinical presentation of the first described case featured severe fever closely resembling monocytic ehrlichiosis. In Europe, the first case of anaplasmosis was recorded shortly after—in 1997 in Slovenia.