Ehrlichiosis is caused by rickettsial-like bacteria of the genus Ehrlichia transmitted to humans by ticks. Symptoms resemble those of Rocky Mountain spotted fever except that a rash is much less common. Onset of illness, with fever, chills, headache, and malaise, is abrupt.
Most cases have been identified in the southeastern and south central US. Three species of Ehrlichia are human pathogens in the US: E. chaffeensis causes human monocytic ehrlichiosis; Anaplasma phagocytophila (formerly E. phagocytophila) and E. ewingii cause human granulocytic ehrlichiosis. The difference in the primary target cell results in only minor differences in clinical manifestations.
These obligate, intracellular bacteria appear as small cytoplasmic inclusions in lymphocytes and neutrophils. Infections are transmitted to humans via tick bites, sometimes via contact with animals that carry the brown dog tick or deer tick.
Symptoms and Signs
Although some infections are asymptomatic, most cause an abrupt onset of illness with fever, chills, headache, and malaise, usually beginning about 12 days after the tick bite. Some patients develop a maculopapular or petechial rash involving the trunk and extremities, although rash is rare with E. ewingii. Abdominal pain, vomiting and diarrhea, disseminated intravascular coagulation, seizures, and coma may occur.
Diagnostic serologic tests are available, but PCR of blood is more sensitive and specific and can result in an early diagnosis. Cytoplasmic ehrlichial inclusions in monocytes or neutrophils may be detected. Blood and liver functions tests may detect hematologic and hepatic abnormalities, such as leukopenia, thrombocytopenia, and elevated aminotransferase levels.
Treatment is best started before laboratory results return. When treatment is started early, patients generally respond rapidly and well. A delay in treatment may lead to serious complications, including viral and fungal superinfections and death in 2 to 5%. Primary treatment is doxycycline 200 mg po once followed by 100 mg bid until the patient improves and has been afebrile for 24 to 48 h but is continued for at least 7 days. Chloramphenicol 500 mg po or IV qid for 7 days is 2nd-line treatment. Measures can be taken to prevent tick bites (see Sidebar 1: Rickettsiae and Related Organisms: Tick Bite Prevention).
Primary treatment is doxycycline 200 mg po once followed by 100 mg bid until the patient improves, has been afebrile for 24 to 48 h, and has received treatment for at least 7 days. Chloramphenicol 500 mg po or IV qid for 7 days is 2nd-line treatment.
Measures can be taken to prevent tick bites (see Sidebar 1: Rickettsiae and Related Organisms: Tick Bite Prevention).
Last full review/revision November 2007 by William A. Petri, Jr., MD, PhD