Morphological and tintorial properties
The pathogens are small in size and exhibit pronounced pleomorphism—microscopic preparations may reveal both coccoid and ovoid forms. Electron microscopy confirms that the ultrastructure of these microorganisms closely resembles rickettsiae.
However, unlike typical rickettsiae, ehrlichiae and anaplasmas have a different cellular tropism. When blood smears are stained using the Romanowsky method, these intracellular microorganisms acquire an intense dark blue or purple hue. Bacterial cells multiply through the classic pathway of simple binary fission.
Intracellular parasitism and morula formation
The life cycle of the pathogens proceeds strictly inside cytoplasmic vacuoles (phagosomes or endosomes) of infected cells. Monocytes, macrophages, and cells directly in the bloodstream are affected primarily.
A characteristic feature of the biology of Anaplasma and Ehrlichia is the formation of dense, compact groups of bacteria within phagosomes that visually resemble a mulberry. These specific structures are called morulae.
Quantitative characteristics of the infectious process are variable:
- A single vacuole may contain from 3 to 50 microbial cells.
- A single infected host cell may sometimes harbor anywhere from several dozen to a hundred infected vacuoles.
This feature is of critical importance for diagnosis. During microscopy of blood smears, the proportion of infected leukocytes is usually extremely low, accounting for only 0.3–6%, which requires special attention during laboratory detection of the pathogen.
Cultivation and antigenic structure
Isolation of anaplasmas and ehrlichiae from sick patients is a challenging task due to their specific cultural properties. Bacteria can grow exclusively in cell cultures, such as macrophage-like lines (e.g., canine macrophages), as well as in standard continuous Vero and HeLa cell lines. Biomass accumulation is extremely slow and time-consuming—cultivation can take from 20 to 40 days.
The antigenic structure of members of the family Anaplasmataceae has important features. They share no common specific antigens with the rickettsiae of the epidemic typhus and spotted fever groups. At the same time, pronounced antigenic cross-reactivity is observed within the family itself among related species pathogenic to animals.
Laboratory models exist for studying certain species. For example, Neorickettsia sennetsu can be studied using white mice. In these animals, the pathogen causes a severe generalized infection with massive accumulation of bacteria in the spleen and peritoneal fluid macrophages.
Classification of main representatives
Modern classification is based on the type of infected cells and the specific species of the pathogen.
1. Monocytic ehrlichiosis
- Pathogens: Ehrlichia chaffeensis (USA, Eurasia), Ehrlichia muris (Europe, Russia).
- Reservoir: Ixodid ticks (I. ricinus, I. persulcatus, A. americanum), dogs, murid rodents, as well as wild animals (deer, roe deer, coyotes).
- Vector: Various species of ixodid ticks.
2. Granulocytic anaplasmosis
- Nomenclature: Until 1994, the disease was called granulocytic ehrlichiosis; in 2001, it was renamed human and animal anaplasmosis.
- Pathogen: Anaplasma phagocytophilum (worldwide distribution).
- 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 vector has not been definitively established. Infection occurs via the alimentary route—through the consumption of raw fish contaminated with marine mollusks, which serve as the natural reservoir of infection.
Briefly: answers to topical questions
What laboratory diagnostic methods are used to confirm ehrlichiosis and anaplasmosis?
Serological, molecular genetic, and microscopic methods are used to confirm ehrlichiosis and anaplasmosis.
- Serological methods — examination of paired sera using IFA, indirect IFA (IFA/RIF), or ELISA with a specific antigen.
- Molecular genetic methods — detection of pathogen DNA (Anaplasma phagocytophilum, Ehrlichia muris, Ehrlichia chaffeensis) in blood via PCR.
- Microscopy — Romanowsky-Giemsa staining of smears to search for pathogen clusters (morulae) inside the cytoplasm of monocytes or neutrophils.
Which antibacterial drugs are the drugs of choice for treating ehrlichiosis and anaplasmosis?
The drugs of choice for etiotropic therapy of ehrlichiosis and anaplasmosis are tetracycline antibiotics, primarily doxycycline. Chloramphenicol is a backup drug, used less frequently.
What virulence and pathogenicity factors are characteristic of anaplasmas and ehrlichiae?
Pathogenicity factors of anaplasmas and ehrlichiae include immunopathological mechanisms, life cycle features, and specific molecular factors.
- Cytokine storm/aggression — infected endothelial cells produce excess cytokines (TNF-$\alpha$, IFN-$\gamma$, IL-10), leading to target cell damage.
- Life cycle features — formation of "spore-like" corpuscles and cell exit via "budding," allowing infection of neighboring cells without immediate host destruction.
- Molecular factors — outer membrane proteins (44 and 153 kDa) that act as adhesins and regulate host cell gene expression.
To avoid confusing the main diseases: Monocytic ehrlichiosis affects monocytes (M-E), and Granulocytic anaplasmosis affects granulocytes (G-A). Clusters of bacteria inside cells are called morulae because they resemble a mulberry (Latin *morus*).
Class and exam questions
Why are anaplasmas and ehrlichiae so difficult to isolate from a patient?
This is due to their strict intracellular parasitism: they grow only on cell cultures, and growth is very slow (20–40 days) accompanied by minimal accumulation of the pathogen.
What do the pathogens look like under light microscopy?
When stained with Romanowsky stain, they appear as coccoid or ovoid dark blue or purple structures. They are localized inside cytoplasmic vacuoles as characteristic clusters—morulae.
How does sennetsu neorickettsiosis differ from other ehrlichioses in terms of transmission?
Unlike monocytic ehrlichiosis and granulocytic anaplasmosis, which are transmitted through tick bites, the vector of Neorickettsia sennetsu has not been established. Infection occurs through the alimentary route by eating raw fish.
What else to study on this topic
- Pathogenesis of tick-borne infections and interaction with the immune system
- Laboratory diagnostic methods: PCR and ELISA in ehrlichiosis
- Antigenic cross-reactivity within the family Anaplasmataceae
- Epidemiological features of natural foci of ixodid ticks
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