There are two kinds of ehrlichiosis, both of which are caused by tick-borne rickettsial parasites called Ehrlichia that infect different kinds of white blood cells. In HME (human monocytic ehrlichiosis), they infect monocytes. In HGE (human granulocytic ehrlichiosis), they infect granulocytes. HGE was renamed anaplasmosis in 2003. Ticks carry many Ehrlichia-like parasites that have not been identified yet. It is likely that the lone star tick transmits HME and that the deer tick transmits HGE.
Ehrlichiosis (HME) was originally thought to be only an animal disease. It was described in humans in 1987 and is now found in 30 states, predominately in the southeast, south-central, and mid-Atlantic states, Europe and Africa. Anaplasmosis (HGE)in humans was first identified in 1990 in a Wisconsin man. Before that it was known to infect horses, sheep, cattle, dogs and cats. It occurs in the upper midwest, northeast, the mid-Atlantic states, northern California, and many parts of Europe. Studies suggest that in endemic areas as much as 15% to 36% of the population has been infected, though often it is not recognized.
The clinical manifestations of ehrlichiosis and anaplasmosis are the same. Each is often characterized by sudden high fever, fatigue, muscle aches, headache. The disease can be mild or life-threatening. Severely ill patients can have low white blood cell count, low platelet count, anemia, elevated liver enzymes, kidney failure and respiratory insufficiency. Older people or people with immune suppression are more likely to require hospitalization. Deaths have occurred.
Diagnosis is limited by our current ability to test for only two species. Ehrlichia parasites multiply inside host cells, forming large mulberry-shaped clusters called morulae which doctors can sometimes see on blood smears. The infection still can easily be missed. The doctor may suspect ehrlichiosis/anaplasmosis in a patient who does not respond well to treatment for Lyme disease.
The treatment of choice for ehrlichiosis/anaplasmosis is doxycycline, with rifampin recommended in case of treatment failure.
OTHER TICK-BORNE DISEASES
Besides the diseases described above, ticks in different geographic areas may be infected with one or more of the following: Colorado tick fever virus; Mycoplasmas; Powassan encephalitis virus; Q Fever; Rocky Mountain spotted fever (Rickettsia); tickborne relapsing fever Borrelia; Tularemia (bacteria). The Tick Chart tells where these diseases are found.
It is certain that we have not yet identified all the diseases that ticks carry and transmit. Coinfections complicate diagnosis and treatment and make recovery even more difficult. Doctors may suspect coinfections in patients who do not respond satisfactorily to antibiotics prescribed for Lyme disease.
There are other possible explanations for treatment failures. People with chronic tickborne infections often have a weakened immune response. This allows other opportunistic infections to flourish, such as HHV-6, CMV, and EBV. These diseases are not necessarily carried by ticks but are widespread in the environment. PCR rather than antibody tests should be used to diagnose these infections. Some people may also have exposure to toxic metals. Specialists should evaluate these cases.
Colorado Tick Fever
Colorado tick fever is caused by a virus carried by Rocky Mountain wood ticks. Symptoms are acute high fever, severe headache, chills, fatigue, and muscle pain.
Mycoplasma species have been identified in ticks. Smaller than bacteria, they invade human cells and disrupt the immune system, causing fatigue, musculoskeletal symptoms, and cognitive problems. Mycoplasmas can be treated with antibiotics.
Powassan virus causes tick-borne encephalitis (TBE). Symptoms may include fever, convulsions, headache, disorientation, lethargy, partial coma and paralysis. Ten percent of patients die and survivors may have permanent damage.
Q fever is caused by Coxiella burnetii, a kind of bacteria carried by cattle, sheep, and goats. Symptoms are similar to those of Lyme disease. Q fever is likely to start with a high fever. Pneumonia and abnormal liver function also suggest Q fever. Doxycycline is the treatment of choice.
Rocky Mountain Spotted Fever
Rocky Mountain spotted fever is caused by bacteria called Rickettsia rickettsii that are transmitted by the bite of a tick. Patients develop high fever, rash, headache and bleeding problems. Thirty percent of untreated patients die. It is treatable with antibiotics, often doxycycline.
Certain ticks secrete a toxin that causes a progressive paralysis, which is reversed when the tick is removed.
Tickborne Relapsing Fever
The agent of tickborne relapsing fever, Borrelia hermsi, is carried by soft ticks of the western United States. It is characterized by cycles of high fever and is treated with antibiotics.
Tularemia, or rabbit fever, occurs throughout the United States. It is caused by the bacterium Francisella tularensis. Symptoms may include skin ulcers, swollen and painful lymph glands, inflamed eyes, sore throat, mouth sores, pneumonia, diarrhea and vomiting. The most effective treatment is with fluorinated quinolones