Ehrlichiosis, is an acute disease of humans and animals caused by bacteria called Ehrlichia that attack white blood cells. The bacteria are transmitted to humans by the bite of an infected tick. Ehrlichiosis is found in almost any area of the United States and in many foreign countries as well. The first human case in the U.S. was reported in 1986. Since 1986, there have been as many as 13 cases reported in one year in Virginia.
Ehrlichiae are small, gram-negative bacteria that primarily invade leukocytes (white blood cells), the same cells which fight disease by destroying microorganisms that enter the body. One type of ehrlichiosis, caused by the bacterium Ehrlichia chaffeensis, infects white blood cells called monocytes. This species of Ehrlichia has been known to exist and cause human illness for several years. A newly recognized species of Ehrlichia (not yet named) infects another type of white blood cell known as a granulocyte.
Ehrlichia chaffeensis primarily infects mononuclear
(predominantly monocytes and macrophages), but may also be seen occasionally in the granulocytes of some patients with severe disease.
Photo courtesy of CDC.
The ticks that spread ehrlichiosis are the deer tick, the dog tick and the Lone Star tick.
Adult female lone star tick (Amblyomma americanum). Photo courtesy of CDC.
Patients with ehrlichiosis generally visit a physician in their first week of illness, following an incubation period of about 5-10 days after the tick bite. Initial symptoms generally include the sudden onset of moderate to high fever, headache, malaise, and muscle aches. Other signs and symptoms may include nausea, vomiting, diarrhea, cough, joint pains, confusion, and occasionally rash. Unlike Lyme disease or Rocky Mountain spotted fever, a rash is not common. However, rash has been described in approximately 60% of pediatric patients infected with E. chaffeensis.
Blood tests may show a low white blood cell count or a low platelet count. Because diagnostic tests are not readily available, the diagnosis of ehrlichiosis should be made based on clinical symptoms and a history of exposure to ticks.
Appropriate antibiotic treatment should be initiated immediately when there is a strong suspicion of ehrlichiosis on the basis of clinical and epidemiologic findings. Treatment should not be delayed until laboratory confirmation is obtained.
The disease responds well to treatment with doxycycline or other
tetracyclines. Because these antibiotics can cause dental staining in
children, physicians should consult with an infectious disease expert
when treating children.