Ehrlichia chaffeensis

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Scientific classification
Alpha Proteobacteria

Ehrlichia chaffeensis is an obligate intracellular [1] gram-negative species of rickettsiales bacteria. [2] It is a zoonotic pathogen transmitted to humans by the lone star tick (Amblyomma americanum). [3] It is the causative agent of human monocytic ehrlichiosis. [4]

Rickettsiales Order of bacteria

The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria that are endosymbionts of eukaryotic cells. Some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans, and Ehrlichia, which causes diseases in livestock. Another genus of well-known Rickettsiales are the Wolbachia, which infect approximately two-thirds of all arthropods and nearly all filarial nematodes. Genetic studies support the endosymbiotic theory according to which mitochondria and related organelles developed from members of this group.

Bacteria A domain of prokaryotes – single celled organisms without a nucleus

Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about half of the bacterial phyla have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

<i>Amblyomma americanum</i> species of arachnid

Amblyomma americanum, also known as the lone star tick, the northeastern water tick, or the turkey tick, is a type of tick indigenous to much of the eastern United States and Mexico, that bites painlessly and commonly goes unnoticed, remaining attached to its host for as long as seven days until it is fully engorged with blood. It is a member of the phylum Arthropoda, class Arachnida. The adult lone star tick is sexually dimorphic, named for a silvery-white, star-shaped spot or "lone star" present near the center of the posterior portion of the adult female shield (scutum); adult males conversely have varied white streaks or spots around the margins of their shields.


Genetic studies support the endosymbiotic theory that a subset of these organisms evolved to live inside mammalian cells as mitochondria to provide cellular energy to the cells in return for protection and sustenance. ATP production in the rickettsia is biochemically identical to that in mammalian mitochondria.

Human monocytic ehrlichiosis caused by E. chaffeensis is known to spread through tick infection primarily in the southern, southcentral and mid-Atlantic regions of the United States. [5] In recent years the lone star tick has expanded its range up the East Coast to New England, putting more humans at risk for tick-borne infections. [6]

It is named for Fort Chaffee where the bacteria was first discovered in blood samples of infected patients. [2]

Transmission cycle

E. chaffeensis is maintained in nature through a complex zoonotic relationship. The white-tailed deer (Odocoileus virginianus) is known to be the main competent reservoir for E. chaffeensis [1] and the lone star tick (Amblyomma americanum) is the principal vector for human transmission. [3] There is some evidence that other organisms may serve as reservoirs for the bacteria such as domestic goats, domestic dogs, raccoons, [1] and coyotes. [5]

White-tailed deer species of mammal

The white-tailed deer, also known as the whitetail or Virginia deer, is a medium-sized deer native to the United States, Canada, Mexico, Central America, and South America as far south as Peru and Bolivia. It has also been introduced to New Zealand, Cuba, Jamaica, Hispaniola, Puerto Rico, the Bahamas, the Lesser Antilles, and some countries in Europe, such as Finland, the Czech Republic, Romania and Serbia. In the Americas, it is the most widely distributed wild ungulate.

E. chaffeensis can be transmitted to non-infected tick larvae when feeding on the blood from an infected host. [7] The infection is then maintained and can be transmitted to a reservoir organism or humans at the nymphal stage. Adult ticks can maintain the infection or be infected from feeding on the blood of an infected reservoir organism and may also pass E. chaffeensis to humans or other non-infected reservoir organisms. [1] Transovarial transmission is not known to occur so eggs and unfed larvae are not believed to be infected. [7]


Ehrlichia chaffeensis Echaff.jpg
Ehrlichia chaffeensis

E. chaffeensis causes human monocytic ehrlichiosis and is known to infect monocytes. [1] It has also been known to infect other cell types such as lymphocytes, atypical lymphocytes, myelocytes, and neutrophils, but monocytes appear to best harbor the infection. [1]

E. chaffeensis has also been shown to infect canines both naturally [6] and artificially. [8] Symptoms in canine infections are hard to differentiate between E. chaffeensis infection and Ehrlichia canis, which is the species of Ehrlichia that most commonly affects canines. [8]

Signs and symptoms

Patients display early symptoms within 1 to 2 weeks after tick infection. Early symptoms include fever, headache, [9] malaise, low-back pain, or gastrointestinal symptoms. [3] Some patients may also have myalgias, arthralgias, and an estimated 10–40% of patients may develop coughing, pharyngitis, diarrhea, vomiting, abdominal pain, and changes in mental status. [1]

Diagnosis or detection

A variety of procedures have been used to detect E. chaffeensis in humans and reservoir organisms. Most commonly serologic testing and PCR amplification are used. [1] [3]


E. chaffeensis is susceptible to tetracyclines. [1] Doxycycline treatment is suggested for any patients presenting symptoms of an Ehrlichia infection during the appropriate season and potential tick exposure. [9]

See also

Related Research Articles

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<i>Bartonella</i> genus of bacteria

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Human monocytotropic ehrlichiosis human disease

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  2. 1 2 Ehrlichia+chaffeensis at the US National Library of Medicine Medical Subject Headings (MeSH)
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  9. 1 2 Baddour, L. M. (2011). Newly discovered ehrlichia species implicated in human infection. Journal Watch Infectious Diseases