Five species of ehrlichia have been identified as the etiologic agents in humans. Most cases of HME have occurred in southeastern and south central United States. HGE has been linked to the bites of Ixodes scapularis , but clear-cut evidence is lacking. Patients with ehrlichiosis are typically older than those with other tick-borne diseases and have peak occurrence of illness from May through July. These morulae rupture and release elementary bodies into the circulatory system, where they infect other leukocytes.
Intracellular infection is seen in liver, spleen, lymph nodes, bone marrow, lungs, and kidneys.
Vasculitis is not observed. The clinical features of HME and HGE are similar, and from a case management point of view there is no need to differentiate between the two. The incubation period for human ehrlichiosis is from 1 to 3 weeks. The disease presents with fever, headache, anorexia, and myalgia. Vomiting, diarrhea, abdominal pain, and cough are less often seen.