Author: Marie Hartley, Staff Writer, 2009. Updated by Vanessa Ngan, Staff Writer, February 2014.
Ehrlichiosis (also called human monocytic ehrlichiosis) is an infection of white blood cells caused by bacteria called Ehrlichia chaffeensis and Ehrlichia ewingii.
Ehrlichiosis occurs in parts of the United States, Europe and Africa. Around 600 cases of ehrlichiosis are reported in the United States annually. However, it is likely that the actual incidence is much higher as many cases are unreported, and many more may be symptom-free and therefore undiagnosed.
Ehrlichiosis is transmitted via the bite of an infected tick; Amblyomma americanum (Lone Star tick) is the principle tick vector. Ehrlichiosis is most commonly diagnosed from spring through autumn.
Historically, the term ehrlichiosis also encompassed a very similar tick-borne disease caused by bacteria called Anaplasma phagocytophilum. This disease was previously known as human granulocytic ehrlichiosis (HGE) and later as human granulocytic anaplasmosis (HGA). Both of these names refer to the same disease, now known as anaplasmosis.
Anaplasmosis occurs in parts of the United States and Europe. About 600–800 cases of anaplasmosis are reported in the United States each year, but this is also likely to be an underestimate as some people do not become ill or experience only very mild symptoms and do not seek medical treatment. Ixodes ticks are the principle tick vectors of anaplasmosis.
Many people with ehrlichiosis and anaplasmosis may be symptom-free or have only very mild symptoms.
Symptomatic patients may experience fever, fatigue, chills, severe headaches, muscle aches, nausea, vomiting, and loss of appetite. These symptoms generally begin after a 5–21 day incubation period. Blood count abnormalities such as low white cell count and low platelet count may occur.
In patients with impaired immunity, ehrlichiosis and anaplasmosis may be more severe.
The proportion of patients with ehrlichiosis who develop a skin rash varies between 20 to 88% of cases. When present, the rash takes various forms. It has been described as red, petechial (small red or purple spots due to bleeding into the skin), macular (flat discolourations), and papular (small lumps). Less commonly, lesions are described as blistering, nodular (larger solid lumps), vasculitic, purpuric, mottled, blotchy, crusted, or ulcerated. A single patient may display multiple types of lesions.
In severe cases, a widespread rash and desquamation (shedding of the skin in scales) can fit criteria for toxic shock syndrome.
The rash appears from day 0 to day 13 of the illness. The rash has a variable distribution over the body, but the palms and soles are rarely involved.
Skin manifestations of anaplasmosis are rare, occurring in only 1 to 16% of cases. The lesions have been described as red, flat or raised, pustular (pus-filled blister), or papular. Compared to ehrlichiosis, the lesions associated with anaplasmosis are more often individual, localised, and may represent tick-bite lesions.
Ehrlichiosis and anaplasmosis are both diagnosed using serological testing and polymerase chain reaction (PCR) testing on blood samples:
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