Ehrlichiosis causes

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Ehrlichiae
Scientific classification
Domain: Bacteria Kingdom: Prokaryote Phylum: Proteobacteria Class: Alphaproteobacteria Order: Rickettsiales Family: Anaplasmataceae Genus: Ehrlichia

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Ilan Dock, B.S.

Overview

Ehrlichiosis is a tickborne bacterial infection, caused by bacteria of the family Anaplasmataceae, genera Ehrlichia and Anaplasma. These obligate intracellular bacteria infect and kill the white blood cells. Ehrlichiae are transmitted to humans by the bite of an infected tick. Usually, symptoms occur within 1-2 weeks following a tick bite.

Taxonomy

Classifications
Domain	Bacteria
Kingdom	Prokaryote
Phylum	Proteobacteria
Class	Alpha Protobacteria
Order	Rickettsiales
Family	Anaplasmataceae
Genus	Ehrlichia
Species	E. canis E. chaffeensis E. ewingii

Phylogeny

• Small intracellular bacteria
• Infect vacuoles of host eukaryotic cells
• Gram-negative cell wall
• Grow within a host cells endosomes in morulae clusters
• Manipulate the endosomes mechanical function in order to resist lysosomal fusion
• Ehrlichiae exits the cell as morulae attaches to cell wall
• Target cells include neutrophils, platelets, erythrocytes, and endothelium

General Tick Life Cycle

This image displays an example of the tick lifecycle, based on stages and the months that they are most likely to occur during.

• A tick's life cycle is composed of four stages: hatching (egg), nymph (six legged), nymph (eight legged), and an adult.
• Ticks require blood meal to survive through their life cycle.
• Hosts for tick blood meals include mammals, birds, reptiles, and amphibians. Ticks will most likely transfer between different hosts during the different stages of their life cycle.
• Humans are most often targeted during the nymph and adult stages of the life cycle.
• Life cycle is also dependent on seasonal variation.
• Ticks will go from eggs to larva during the summer months, infecting bird or rodent host during the larval stage.
• Larva will infect the host from the summer until the following spring, at which point they will progress into the nymph stage.
• During the nymph stage, a tick will most likely seek a mammal host (including humans).
• A nymph will remain with the selected host until the following fall at which point it will progress into an adult.
• As an adult, a tick will feed on a mammalian host. However unlike previous stages, ticks will prefer larger mammals over rodents.
• The average tick life cycle requires three years for completion.
• Different species will undergo certain variations within their individual life cycles.

Transmission

• Ehrlichia are transported between cells through the host cell filopodia during initial stages of infection, whereas, in the final stages of infection the pathogen ruptures the host cell membrane.
• Most of the symptoms of ehrlichiosis can likely be ascribed to the immune dysfunction that it causes.
• Early in infection, production of TNF-alpha, a cellular product that promotes inflammation and immune response, is suppressed.
• Experiments in mouse models further supports this hypothesis, as mice lacking TNF-alpha I/II receptors are resistant to liver injury caused by ehrlichia infection.
• Late in infection, however, production of this substance can be upregulated by 30 fold, which is likely responsible for the "toxic shock-like" syndrome seen in some severe cases of ehrlichiosis.