Morphological and Staining Properties
The pathogens are characterized by small dimensions and marked pleomorphism—microscopic preparations reveal both coccoid and ovoid forms. Electron microscopy confirms that the ultrastructure of these microorganisms shares significant similarities with rickettsiae.
However, unlike typical rickettsiae, Ehrlichia and Anaplasma exhibit a distinct cellular tropism. When blood smears are stained with Romanowsky stain, these intracellular microorganisms acquire an intense dark blue or purple hue. Bacterial cell replication occurs via classical binary fission.
Intracellular Parasitism and Morula Formation
The lifecycle of these pathogens occurs strictly within cytoplasmic vacuoles (phagosomes or endosomes) of infected cells. Monocytes, macrophages, and circulating blood cells are primarily affected.
A hallmark of Anaplasma and Ehrlichia biology is the formation of dense, compact bacterial clusters inside phagosomes that visually resemble a mulberry fruit. These specific structures are termed morulae.
Quantitative characteristics of the infectious process vary:
- A single vacuole may contain from 3 to 50 microbial cells.
- A single infected host cell may occasionally harbor dozens to hundreds of infected vacuoles.
This feature is critical for laboratory diagnosis. In blood smears, the proportion of infected leukocytes is typically very low, ranging from 0.3% to 6%, requiring meticulous examination during screening.
Cultivation and Antigenic Structure
Isolation of Anaplasma and Ehrlichia from patients is challenging due to their fastidious culture requirements. These bacteria grow exclusively in cell cultures, such as macrophage-like lines (e.g., canine macrophages) and standard continuous cell lines like Vero and HeLa. Biomass accumulation is extremely slow and prolonged, with cultivation taking anywhere from 20 to 40 days.
The antigenic structure of members of the family Anaplasmataceae features important characteristics. They share no specific cross-reacting antigens with the typhus group or spotted fever group rickettsiae. However, significant antigenic cross-reactivity is observed within the family among related species pathogenic to animals.
Laboratory animal models exist for studying certain species. For instance, Neorickettsia sennetsu can be studied using white mice, in which the pathogen causes a severe generalized infection characterized by massive bacterial accumulation in the spleen and peritoneal macrophages.
Classification of Major Species
Modern classification is based on the type of host cells infected and the specific species of pathogen.
1. Monocytic Ehrlichiosis
- Pathogens: Ehrlichia chaffeensis (USA, Eurasia), Ehrlichia muris (Europe, Russia).
- Reservoir: Ixodid ticks (I. ricinus, I. persulcatus, A. americanum), dogs, rodents, and wild animals (deer, roe deer, coyotes).
- Vector: Various species of ixodid ticks.
2. Granulocytic Anaplasmosis
- Nomenclature: Until 1994, the disease was termed human granulocytic ehrlichiosis; it was reclassified as human and animal granulocytic anaplasmosis in 2001.
- Pathogen: Anaplasma phagocytophilum (widespread globally).
- Reservoir: Ticks, wild ungulates, small rodents, and sheep.
- Vector: Ixodid ticks.
3. Sennetsu Neorickettsiosis (Infectious Mononucleosis)
- Pathogen: Neorickettsia sennetsu (endemic to Japan and Malaysia).
- Transmission Route: The definitive vector has not been firmly established. Infection occurs via the alimentary route—ingestion of raw fish contaminated with marine mollusks, which serve as the natural reservoir.