Tick species that transmit anaplasmosis: Deer tick (Ixodes scapularis), western black-legged tick (Ixodes pacificus)

What is Anaplasmosis?

Human granulocytic anaplasmosis (HGA), previously known as human granulocytic ehrlichiosis (HGE), is a tick-associated disease caused by a species of bacteria called Anaplasma phagocytophilum. HGA is transmitted to humans by the bite of the deer tick and western black-legged tick. People exposed to the disease agent often have difficulty being diagnosed because of the non-specific nature of the symptoms. Most experience headaches, fever, chills, myalgia, and malaise that can be confused with other infectious and non-infectious diseases. Rashes are rarely reported by people exposed to HGA.

Where is Anaplasmosis Prevalent?

HGA is transmitted to humans by the deer tick and western black-legged tick. Deer ticks infected with A. phagocytophilum are found in the New England and North Central United States while black-legged ticks infected with A. phagocytophilum are found in northern California. States reporting the highest incidence of HGA in 2001-2002 were Rhode Island, Minnesota, Connecticut, New York, and Maryland. Most cases occur in the spring and summer months. During this time, nymphal deer ticks are most likely to come into contact with humans in New England and North Central United States. Adult deer ticks may transmit the infection in the fall. The main animal reservoir for A. phagocytophilum in Eastern regions is the white-footed mouse.


Clinical manifestations of HGA can range from mild to life-threatening depending on the patient’s age and general health. Onset of anaplasmosis generally begins within a week of a tick bite, and often includes fever, severe headaches, malaise, muscle pains, and chills. Other symptoms may include confusion, hemorrhages, and renal failure.


An initial diagnosis is based on the patient’s symptoms and laboratory tests. Routine laboratory results include a low white blood cell count, low platelet count, and elevated levels of specific liver enzymes. Blood smears may be used to look for characteristic morulae (microcolonies) of A. phagocytophilium in affected blood cells. However, a confirmed diagnosis can only be made using the PCR (Polymerase Chain Reaction) or by immunostaining methods.


According to the Center for Disease Control (CDC), people exposed to HGA should be treated with a tetracycline antibiotic (usually doxycycline) for 10 to 14 days. A longer treatment period may be needed to account for the possibility of a coinfection with Lyme disease.