''Ehrlichia chaffeensis'' is an obligate intracellular, Gram-negative species of

Rickettsiales The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are obligate intracellular parasites, and some are notable pathogens, including ''Rickettsia'', which causes a variety of diseases in humans, and ' ...

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

. It is a zoonotic pathogen transmitted to humans by the lone star tick (''Amblyomma americanum''). It is the causative agent of human monocytic ehrlichiosis. 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 rickettsiae is biochemically identical to that in mammalian mitochondria; all multi-cellular eukaryotes have mitochondria in their cells, including birds, fish, reptiles, invertebrates, plants, and fungi in addition to mammals. Human monocytic ehrlichiosis caused by ''E. chaffeensis'' is known to spread through tick infection primarily in the Southern, South-central and Mid-Atlantic regions of the United States. In recent years, the lone star tick has expanded its range along the East Coast to New England, putting more humans at risk for tick-borne infections.Little, S. E. (2007, January). ''New developments in managing vector-borne diseases''. Retrieved from http://www.iknowledgenow.com/tocnavc2007smallanimal.cfm It is named for

Fort Chaffee Fort Chaffee Joint Maneuver Training Center is an Army National Guard installation in western Arkansas, adjacent to the city of Fort Smith. Established as Camp Chaffee in 1941, renamed to Fort Chaffee in 1956, Fort Chaffee has served as a United ...

, where the bacterium was first discovered in blood samples of infected patients.

Transmission cycle

''E. chaffeensis'' is maintained in nature through a complex zoonotic relationship. The

white-tailed deer The white-tailed deer (''Odocoileus virginianus''), also known as the whitetail or Virginia deer, is a medium-sized deer native to North America, Central America, and South America as far south as Peru and Bolivia. It has also been introduced t ...

(''Odocoileus virginianus'') is known to be the main competent reservoir for ''E. chaffeensis'' and the lone star tick (''Amblyomma americanum'') is the principal vector for human transmission. Some evidence shows that other organisms may serve as reservoirs for the bacteria such as domestic goats, domestic dogs, raccoons, and coyotes. ''E. chaffeensis'' can be transmitted to uninfected tick larvae when feeding on the blood from an infected host. 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 uninfected reservoir organisms. Transovarial transmission is not known to occur, so eggs and unfed larvae are not believed to be infected.

Pathogenesis

''E. chaffeensis'' causes human monocytic ehrlichiosis and is known to infect monocytes. 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. ''E. chaffeensis'' has also been shown to infect canines both naturally and artificially.Baneth, G. (2010). ''Ehrlichia and anaplasma infections''. Paper presented at World small animal veterinary congress. Retrieved from http://www.ivis.org/proceedings/wsava/2010/d12.pdf Symptoms in canine infections are hard to differentiate between ''E. chaffeensis'' infection and ''E. canis'', which is the species of ''Ehrlichia'' that most commonly affects canines.

Signs and symptoms

Patients display early symptoms within 1 to 2 weeks after tick infection. Early symptoms include fever, headache,Baddour, L. M. (2011). Newly discovered ehrlichia species implicated in human infection. ''Journal Watch Infectious Diseases'' malaise, low-back pain, or gastrointestinal symptoms. Some patients may also have myalgias or arthralgias, and an estimated 10–40% of patients may develop coughing, pharyngitis, diarrhea, vomiting, abdominal pain, and changes in mental status.

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.

Treatment

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

References