Ehrlichia species are bacteria of the family Anaplasmataceae. They are small, gram-negative, pleomorphic organisms that primarily infect circulating leukocytes. The 3 main ehrlichial species that currently cause infection in humans in the United States are Ehrlichia chaffeensis, Anaplasma phagocytophilum (formerly Ehrlichia phagocytophilum), and Ehrlichia ewingii.
E chaffeensis, the causative agent of human monocytotrophic ehrlichiosis (HME), is endemic to the southeastern and south central United States. The pathogen infects mononuclear phagocytes in blood and tissue.1 Strains of E chaffeensis, such as the Arkansas strain and strain 91HE17, differ pathogenically.2
Since it was first recognized in 1986, HME has become an important public health issue in the southeastern and south central United States. The average reported annual incidence is approximately 0.7 cases per million population.3 In 2007, 743 cases were reported.4
HME may cause morbidity and can result in severe illness if left untreated or if treatment is delayed. Death also is possible; the case fatality rate is approximately 2.7%.5-10 The history of a patient with HME typically includes a tick bite 2 to 3 weeks before presentation. An associated rash is uncommon, occurring in approximately a third of patients, whereas a rash develops in approximately 80% of patients with Rocky Mountain spotted fever.11 On average, patients are men (75%) who reside in rural areas and are about 44 years of age.10
Aphagocytophilum infects granulocytes and causes human granulocytic anaplasmosis (HGA). It is endemic to New England and the north central and Pacific states. The average reported annual incidence of HGAis 1.6 cases per million population. 3 A total of 672 cases were reported in 2007.4
Symptoms of HGA include headache, myalgia, rigors, and malaise. GI, respiratory, and CNS abnormalities occur in a minority of patients.12 Rash is rare. Opportunistic infections caused by Ehrlichia-associated immunosuppression may develop in the setting of HGA infection. These infections include disseminated candidiasis, herpetic esophagitis, cryptococcal pneumonitis, and invasive pulmonary aspergillosis.12,13
E ewingii is primarily a dog and deer pathogen, although it occasionally causes human illness in immunocompromised patients.14 E ewingii infection causes disease similar to HME but is milder and is associated with fewer complications; it has not been associated with deaths.12 The organism is endemic to the south central and south atlantic states.