These disorders are caused by rickettsial-like bacteria. Ehrlichiosis is caused mainly by Ehrlichia chaffeensis; anaplasmosis is caused by Anaplasma phagocytophilum. Both are 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.
E. chaffeensis causes human monocytic ehrlichiosis. Most cases of ehrlichiosis have been identified in the southeastern and south central US, where its arthropod vector (the lone star tick) is endemic. Anaplasma phagocytophilum (formerly E. phagocytophila) causes human granulocytic anaplasmosis, which occurs in the Northeast, mid-Atlantic, upper Midwest and West Coast of the US, where its arthropod vector (ixodid ticks) are endemic. Several cases of anaplasmosis have been reported after blood transfusions from asymptomatic or acutely infected donors.
The difference in the primary target cell (monocytes for ehrlichiosis and granulocytes for anaplasmosis) results in only minor differences in clinical manifestations.
Symptoms and Signs
Clinical features of ehrlichiosis and anaplasmosis are similar. Although some infections are asymptomatic, most cause abrupt onset of an influenza-like illness with nonspecific symptoms such as fever, chills, myalgias, weakness, nausea, vomiting, cough. headache, and malaise, usually beginning about 12 days after the tick bite. Rash is uncommon in anaplasmosis. Some patients infected with E. chaffeensis develop a maculopapular or petechial rash on the trunk and extremities.
Ehrlichiosis may result in disseminated intravascular coagulation, multiorgan failure, seizures, and coma.
Disease appears to be more severe and have a higher mortality rate in patients with compromised immunity caused by immunosuppressants (eg, corticosteroids, cancer chemotherapy, long-term treatment with immunosuppressants after organ transplantation), HIV infection, or splenectomy.
Diagnostic serologic tests are available, but PCR of blood is more sensitive and specific and can result in an early diagnosis because serologic tests require comparison of serial titers. 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 is no longer effective. Some patients continue to experience headache, weakness, and malaise for weeks after adequate treatment.
Measures can be taken to prevent tick bites (see Sidebar 1: Rickettsiae and Related Organisms: Tick Bite Prevention).
Last full review/revision November 2012 by William A. Petri, Jr., MD, PhD