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Human Monocytic Ehrlichiosis

Human monocytic ehrlichiosis (HME) is a relatively newly documented disease (1987) caused by a bacterium that develops within white blood cells of mammals. Over 750 human cases have been reported in the U.S. HME is similar to two diseases (HGE and HGA) discussed below in being caused by closely related bacteria that develop within white blood cells, in being tick-borne, in being the cause of an initial flu-like illness, and in being especially serious in people with weakened immune systems. The three diseases differ with regard to the tick vectors and the mammalian reservoir hosts.

Causative agent

  • A bacterium with the scientific name Ehrlichia chaffeensis.
  • It is an intracellular parasite that undergoes development within white blood cells, especially monocytes, but also in granulocytes and macrophages.

Geographical distribution of cases

  • Most cases have occurred in the southeastern and south central U.S.

Symptoms of infection

  • Fever, headache, joint pain, and muscle ache.
  • 20-30% of patients develop a body rash, but it typically does not the involve the hands and feet.

Reservoir hosts of Ehrlichia chaffeensis

  • White-tailed deer and possibly other wild animals.

Vector of Ehrlichia chaffeensis

  • Amblyomma americanum, lone star tick.

Modes of transmission

  • From infected lone star ticks (nymphs and adults) to humans via their bite.
  • From infected tick larvae to nymphs to adults via transstadial transmission.
  • NOTE: there is no human-to-human transmission.

Diagnosis of infection

  • Symptoms, together with onset in late spring into summer.

Treatment of infection

  • Antibiotic prescribed by a physician.

Prevention of infection

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