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Human Granulocytic Anaplasmosis

Human granulocytic anaplasmosis (HGA) was documented as a distinct disease in Minnesota and Wisconsin in 1994. The bacterium that causes HGA has a confusing history. Initially, it was known as Ehrlichia phagocytophila or E. equi, and at one time it was thought to be the cause of human granulocytic ehrlichiosis. However, HGA differs from the two types of ehrlichiosis in having the black-legged tick as the vector, and, accordingly, the same geographical distribution as Lyme disease.
Causative agent
  • A bacterium with the scientific name Anaplasma phagocytophilum.
  • It is an intracellular parasite that undergoes development within white blood cells, especially granulocytes.
Geographical distribution of cases
  • U.S. and Europe.
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.
  • Infections usually are mild, but severe cases can result in death.
Reservoir hosts of Anaplasma phagocytophilum
  • A wide range of mammalian hosts, including domestic pets, livestock, deer, elk, and rodents.
Vectors of Anaplasma phagocytophilum
  • Ixodes scapularis, black-legged tick, in the eastern U.S.
  • Ixodes pacificus, Western black-legged tick, in the Pacific Coast states.
Modes of transmission
  • From infected Ixodes ticks to humans via their bite.
  • From infected Ixodes 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
  • There is no vaccine.
  • Avoid tick habitat and contact with tick-infested vegetation ( see E-243-W "The Biology of Indiana Ticks").
  • Wear clothing consisting of a long sleeved shirt, long pants, and boots.
  • Use a repellent recommended by CDC and treat clothing with permethrin.
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