HGA is caused by the gram-negative intracellular bacterium Anaplasma phagocytophilum (previously known as Ehrlichia phagocytophilum), which is transmitted by the bite of a tick, Ixodes scapularis, commonly known as the black-legged tick. The incubation period is about five days - to 2 weeks. The bacteria invade granulocytes, blood progenitors, and endothelial cells, but they flourish in human neutrophils.
Usual early symptoms include fever, malaise, myalgia, and headache. If not treated, the illness can become chronic or severe and cause neuropathies, renal or respiratory failure, hemorrhage, DIC, etc.
Those at most risk for severe disease are older or immunocompromised patients or those who have had delayed treatment.
Incidence is most common in the Northeast and Upper Midwest United States. It is not uncommon for patients to also have gotten another tick-transmitted illness at the same time. According to the CDC, although previously quite rare, the incidence has increased rapidly in the last ten years. Men seem to get the disease more often than women.
