Ehrlichia in humans refers to a group of related rickettsial bacterial diseases transmitted through a tick bite. The two main types of human ehrlichiosis are human monocytic ehrlichiosis (HME) and human granulocytic anaplasmosis (formerly known as human granulocytic ehrlichiosis or HGE). Ehrlichia chaffeensis causes monocytic ehrlichiosis. Anaplasma phagocytophilum (formerly Ehrlichia phagocytophilum) is responsible for anaplasmosis. A third emerging species, Ehrlichia ewingii, is also known to cause ehrlichiosis.
While each microorganism has its own way of affecting the body’s cells and functions, the ehrlichioses cause similar symptoms and are treated with the same antibiotics.
Humans acquire Ehrlichia chaffeensis through the bite of a tick, most often the Lone Star tick. Lone Star ticks are common in Texas and the Southeastern and mid-Atlantic United States. The name actually comes from the white spot on the tick’s back. Several other ticks transmit Ehrlichia chaffeensis including the dog tick, the deer tick, and possibly the Gulf Coast tick. A large number of reservoir animals carry Ehrlichia chaffeensis, namely white-tailed deer, goats, domestic dogs, red foxes, and some birds.
Various types of Ixodes ticks transmit anaplasma. In the United States, the deer tick, western black-legged tick or bear tick, and Ixodes spinipalpis most often pass on the bacterium. The reservoir animals for these bacteria are mainly deer, rabbits, lizards, and mice. Although the life cycle of anaplasma is poorly understood, it seems to mirror Borrelia burgdorferi and Babesia microti in many respects.
Both monocytic ehrlichia and anaplasma must find their way into white blood cells in order to survive and reproduce. Six different white blood cells protect the body against infectious diseases and foreign materials. Monocytic ehrlichia has a particular affinity for lymphocytes, monocytes, and macrophages. Anaplasma favors predominantly neutrophils, basophils, and eosinophils. Once the Ehrlichia bacteria are inside their respective white blood cells, they multiply by splitting to make two microorganisms. With enough splitting, the white blood cells become overwhelmed, dysfunctional, and die. Disabled and dying white blood cells weaken the body’s defenses, making it open to other pathogens.
Monocytic ehrlichiosis and granulocytic anaplasmosis are similar clinically. Ehrlichia symptoms first occur five days to three weeks after the offending tick bite. Symptoms of ehrlichiosis resemble those of influenza, i.e., fever, chills, cough, low energy, headache, and muscle pains. Nausea, abdominal pain, and lack of appetite may occur as well, although these symptoms are more common with the monocytic type. Unlike other tick-borne illnesses, skin rash is not typical in ehrlichiosis. Rash is rare in anaplasmosis and occurs about 30% of the time in monocytic ehrlichiosis. If a rash does occur, it can look like any rash, including flat/raised red marks or small pinpoint lesions spread over the body.
Doctors often find it hard to tell the difference between ehrlichioses and Rocky Mountain spotted fever, another rickettsial disease. Making the correct diagnosis is even more difficult if the person has a rash. Ehrlichiosis causes a low white blood cell count, low platelet count, and elevated liver enzymes, all of which can be seen on routine blood tests. Knowing this helps doctors look further into the cause of the disease and perhaps order more specific tests.
One such test is the immunofluorescent antibody or IFA test. If a patient’s blood contains antibodies against Ehrlichia chaffeensis or Anaplasma phagocytophilum, the IFA test will be positive. Sometimes diagnosis requires staining a sample of blood to view under a microscope for visible bacteria inside white blood cells. The most sensitive test is polymerase chain reaction or PCR. However, PCR is only effective for detecting Ehrlichia early in the infection, is not available in all clinics or hospitals, and can be expensive.
Ehrlichioses are treated with antibiotics, specifically doxycycline or tetracycline. Doxycycline is usually the drug of choice since it’s more effective, better tolerated, and safer for use in children than tetracycline. Since ehrlichia is an acute disease, doctors recommend starting antibiotic therapy as soon as possible to avoid complications, especially in patients with weak immune systems. Since doxycycline is also used to treat Rocky Mountain spotted fever, antibiotic therapy can be started even if the diagnosis is still uncertain or the results of advanced blood tests aren’t available yet.
Fortunately, serious complications in either monocytic ehrlichia or anaplasmosis are unlikely, but they do occur. The most common complication is an opportunistic infection. This means that a patient infected with ehrlichia acquires another infection at the same time. In extreme cases, ehrlichia can be fatal. While less than 1% of patients with anaplasmosis die from the disease, that percentage rises to 3% in patients with monocytic ehrlichiosis. Complications and fatalities are most common in people with depressed immune systems. In those patients, monocytic ehrlichiosis may cause brain inflammation, kidney failure, heart inflammation, adult respiratory distress syndrome (a serious lung illness), gastrointestinal bleeding, and abnormal blood clotting.
With proper treatment, Ehrlichia chaffeensis and Anaplasma phagocytophilum can be completely eliminated from the body. In fact, even without antibiotic treatment, the disease typically clears within one month in patients with healthy immune systems (Note: this is unlikely if the person also has Lyme Disease and other co-infections). The body does retain antibodies against Ehrlichia after the infection is gone. While the antibodies aren’t very helpful at preventing a second infection, they may influence subsequent tests. It’s therefore very important to tell your doctor if you’ve ever had either monocytic ehrlichiosis or anaplasmosis.
Ganguly S, Mukhopadhayay SK. Tick-Borne Ehrlichiosis Infection in Human Beings. (2008) Journal of Vector Borne Diseases, Vol. 45, pgs. 273-280. (PDF download)
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