Ehrlichia chaffeensis (Human Monocytotropic Ehrlichiosis), Anaplasma phagocytophilum (Human Granulocytotropic Anaplasmosis), and Other Anaplasmataceae

Ehrlichiosis and anaplasmosis are tick-borne zoonoses caused by obligately intracellular bacteria in the order Rickettsiales, family Anaplasmataceae. Nearly all of the species that cause human disease result in what are considered emerging infections; most develop within an endosome of cells derived from bone marrow, usually leukocytes such as monocytes or macrophages and granulocytes. Within leukocytes, the bacteria survive by subverting signaling pathways and cytoskeletal rearrangement mechanisms, and by transcriptional reprogramming of the host cell. Part of the disease pathogenesis relies on induction of an inflammatory and immunopathologic process. The numbers of infections recorded have markedly increased in recent years and rival many other domestic life-threatening infections in incidence and prevalence. Infections by these agents result in an acute, sudden-onset febrile illness that occurs during the months in which peak tick feeding occurs. Headache, myalgia, arthralgia, and nausea are common clinical findings, and frequent laboratory manifestations include thrombocytopenia, leukopenia, and elevations in serum hepatic transaminase activities. Life-threatening complications include renal failure, a sepsis- or disseminated intravascular coagulation-like syndrome, acute respiratory distress syndrome, and in some cases, central nervous system involvement with meningoencephalitis. Patients with preexisting illnesses or immune compromise are at higher risk for severe disease or death. Diagnosis is best made by rapid nucleic acid amplification tests or by paired serology, and treatment is almost always successful with the use of doxycycline; these rickettsial agents are resistant to many other broad-spectrum antibacterial pharmaceuticals. Prevention is best achieved by avoidance of ticks or by use of personal protection against tick bites, and vaccines are not currently available.