Ehrlichiosis overview
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# 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 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. 

# Historical Perspective
Ehrlichiosis was first observed in the 19th century by the German microbiologist, Paul Ehrlich. Originally it was classified under the genus Rickettsia, however further observation led to a reclassification of the bacteria as a separate genus, Ehrlichia. The first Ehrlichia based infection was reported in South Africa in 1900. In the 1980's, Ehrlichiosis was recognized as a disease present in the United States. By 1999, Ehrlichia became a disease reportable to the Centers for Disease Control and Prevention.

# Classification
Three strains of Ehrlichia (E. ewingii, E. chaffeensis, and E. Muris) are responsible for human infection. These resulting infections present themselves with very similar, if not the same, clinical manifestations. Endemic regions are different among the different diseases which may be used to diagnose the organism of infection.

# Pathophysiology
The pathophysiological process of ehrlichiosis begins with the inoculation of the disease from a tick vector. Following inoculation ehrlichiae and anaplasmosis enter the circulatory system in an attempt to infect a target cell. The infectious agents will then enter the cell via a receptor-mediated endocytosis. This particular endocytosis process is facilitated by a glycophoshoinositol anchored receptor. Both the Ehrlichiae and Anaplasma complete their reproduction process within the host cell's endosome.Infectious agents of both disease are then able to reprogram a host cell's defense mechanisms in order to silently proliferate.

# Differentiating Ehrlichiosis from other Diseases
Ehrlichiosis must be differentiated from other diseases that cause fever, chills, headaches, body ache, and rash. When trying to differentiate Ehrlichiosis from other infections, it is important to recognize that the clinical manifestations of Ehrlichiosis greatly resemble those of other tick-borne illnesses, especially those caused by the Rickettsiae family. Examples of misdiagnoses, with the umbrella of tick-borne diseases include typhus-spotted fevers and Colorado tick fever.

# Epidemiology and Demographics
Ehrlichiosis was deemed a reportable disease by the United States Center for Disease Control and Prevention in 1999. Since the year 2000 the number of reported cases has increased from 200 (in 2000) to 961 in 2008. However it should be noted that amount of reported cases was lessened in 2010. The majority of cases are currently reported in Oklahoma, Missouri, and Arkansas. Other endemic areas include the South central and Southeastern United States. Incidents are highest among males and adults between the ages of 60 to 64 years of age. Cases have also been reported at higher rates among individuals with compromised immune systems.

# Risk Factors
The primary risk factors associated with Ehrlichiosis are exposure to endemic environment and the time of that exposure. Lone Star ticks have been identified as the primary vector of E. ewingii and E. chaffeensis infections, thus being bitten in an endemic area may result in the contraction of the disease. (The primary vector of E. muris has not yet been verified.)

# Natural History, Complications and Prognosis
The prognosis is usually good for human granulocytotropic anaplasmosis (HGA). Individuals suffering from HGA should fully recover without treatment in 2 months. However, proper treatment of HGA will expedite the recovery process. Symptoms that are commonly associated with HGA include sudden onset of fever and intense pains. Progression of the disease may lead to serious complications including neurological disorders and ARDS. Co-infection remains the greatest threat associated with death as a result of HGA. Thus individuals with compromised immune systems or the elderly should be closely monitored in order to reduce the likelihood of an opportunistic infection. Human monocytotropic ehrlichiosis (HME) presents a very different scenario than that of HGA. HME is far more dangerous as well as deadly. An incubation period of 7-10 days will often follow an infected tick bite. Initial symptoms include fever, headache, and malaise. Further complications will follow the onset of infection, these complications may prove to be extremely dangerous and should be closely monitored for the patients safety.

# History and Symptoms
Clinical manifestations will present themselves differently depending on the infectious agent. HME will display far more severe symptoms than HGE, and thus requires immediate and more closely monitored medical attention. However both have overlapping symptoms including fever, headache, and nausea.

# Physical Examination
Patients with Ehrlichiosis will often display a fever and myalgia, along with heightened blood pressure and an increase heart rate. Some populations of patients have displayed a mauculopapular rash which is often indicative of an HME infection. It is important for the physician to differentiate between different Ehrlichiosis infection agents since certain strains of Ehrlichiae result in a higher degree of infection severity.

# Laboratory Findings
There are three primary laboratory methods to diagnose an ehrlichiosis infection, polymerase chain reaction (PCR), peripheral blood smear, and an immunofluorescence assay (IFA). Polymerase chain reaction and peripheral blood smear exams are most effective when conducted early on in the diagnoses. The gold standard serologic test for ehrlichiosis is the immunofluorescence assay. The test is most effective when conducted once early on in the infection and a second time, later in the infection. Infection rates will show in increase within an IFA as the illness progresses. 

# Chest X-ray
A chest x-ray may be helpful in the diagnosis of ehrlichiosis. Findings on a chest x-ray indicating acute respiratory distress syndrome may be suggestive of an ehrlichiosis infection.

# Other Diagnostic Studies
There are no further diagnostic studies associated with Rocky Mountain spotted fever.

# Medical Therapy
The mainstay of therapy in ehrlichiosis is antimicrobial therapy.  Doxycycline is the drug of choice to treat ehrlichiosis.

# Prevention
Ehrlichiosis prevention strategies are based on avoiding potential, infected, tick bites. Avoiding tick bites may be accomplished through limited exposure to endemic areas. However if it is impossible or impractical to avoid these areas, several preventative strategies may be implemented. These strategies are indicated within the Prevention microchapter. Other prevention strategies include a proper removal of the tick. This process is also outlined under the title, the best way to remove a tick, within the ehrlichiosis, prevention, microchapter.