Etiology and Epidemiology

HME is caused by Ehrlichia chaffeensis and, in the United States, generally occurs in southeastern, south-central, and mid-Atlantic states from May to July. The incidence can be as high as 414 cases per 100,000 population.

• E. chaffeensis is transmitted by the Lone Star tick (Amblyomma americanum), and white-tailed deer are the major reservoir.
• The median age of pts is 52 years; 60% of pts are male.
• E. ewingii and E. muris eauclairensis cause an illness similar to, but less severe than, that due to E. chaffeensis.

Clinical Manifestations

Clinical findings are nonspecific and include fever (97%), headache (70%), myalgia (68%), and malaise (77%). Nausea, vomiting, diarrhea, cough, rash, and confusion may be noted.

• The median incubation period is 8 days.
• Disease can be severe: up to 77% of pts are hospitalized and 2% die. Complications include renal failure, a DIC-like syndrome, pneumonia, a septic shock-like syndrome, adult respiratory distress syndrome, cardiac failure, hepatitis, meningoencephalitis, and hemorrhage.
• Leukopenia (66%), thrombocytopenia (86%), and elevated serum aminotransferase levels (89%) are common.

Diagnosis

Because HME can be fatal, empirical antibiotic therapy based on clinical diagnosis is required. PCR testing before initiation of antibiotic therapy or retrospective serodiagnosis to detect increased antibody titers can be performed. Morulae are seen in <10% of peripheral-blood smears.

Etiology and Epidemiology

HGA is caused by Anaplasma phagocytophilum and, in the United States, occurs mainly in northeastern and upper midwestern states.

• The geographic distribution is similar to that of Lyme disease and Babesia microti infection, given the shared Ixodes scapularis tick vector.
• HGA incidence peaks in May through July, but the disease can occur year-round.
• The epidemiology of HGA is similar to that of HME, with males (59%) and older persons (median age, 51 years) more often affected.

Clinical Manifestations

Given high seroprevalence rates in endemic areas, it appears that most people develop subclinical infections.

• After an incubation period of 4-8 days, pts develop fever (75-100%), myalgia (75%), headache (83%), and malaise (97%).
• Severe complications-renal failure, respiratory distress, a toxic shock-like syndrome, pneumonia, and a DIC- or sepsis-like syndrome-occur most often in elderly pts.
• On laboratory examination, pts are found to have leukopenia (60%), thrombocytopenia (79%), and elevated serum aminotransferase levels (91%).

Diagnosis

HGA should be considered in pts with influenza-like illness during May through December and in pts with atypical severe presentations of Lyme disease.

• Peripheral-blood films may reveal morulae in neutrophils in 20-75% of infections.
• PCR testing before antibiotic therapy or retrospective serologic testing demonstrating a ≥4-fold rise in antibody titer can confirm the diagnosis.