What Is Human Granulocytic Anaplasmosis?

Medically Reviewed by Mahammad Juber, MD on October 14, 2022
Written by Sierra Hernandez
4 min read

Human granulocytic anaplasmosis (HGA) is a tick-borne infection that is caused by a bacterium called Anaplasma phagocytophilum. This type of bacteria attacks the white blood cells and is primarily transmitted by Ixodes ticks. Although animals are more commonly infected, this illness does affect a small number of humans, and it is spreading to more parts of the world. 

Human granulocytic anaplasmosis was first reported in 1990 when a patient in Wisconsin became ill following a tick bite and died two weeks later. Blood smears showed bacteria that were similar to another type, E. chaffeensis, but further testing proved them to be distinct species. 

Once more cases began to arise, it was concluded that a new species of Ehrlichia was emerging, which was first identified as human granulocytic ehrlichiosis (HGE). In 1994, DNA sequencing studies revealed that HGE was actually identical to another species known as Anaplasma phagocytophilum. Now, the condition this bacteria causes is known as human granulocytic anaplasmosis.

Human granulocytic anaplasmosis affects people in North America, eastern Asia, and most of Europe. 

Depending on where you live, different ticks may infect you with HGA. For example, Ixodes scapularis is found in the eastern U.S., while Ixodes pacificus is found on the Pacific coast. 

Those at the greatest risk of infection in these and other affected areas include:

  • People living in rural areas
  • Farm workers
  • Forest workers
  • Hunters
  • Dog owners

Once a patient presents symptoms of HGA, a standard blood test or complete chemistry panel can be used to confirm the infection. Test results are often similar to those associated with human monocytic ehrlichiosis (HME), including leukopenia, thrombocytopenia, and elevated liver enzymes. 

These abnormalities will typically clear up by the second week of symptoms, though, and may not be present if a patient seeks diagnosis after the disease has already progressed. Some alternative testing options include:

Indirect immunofluorescence (IFA) of IgM and IgG anti-A. phagocytophilum antibodies

Indirect immunofluorescence (IFA) is the most common form of diagnostic confirmation used by physicians and labs because it is the most sensitive. IgG tests are the more sensitive form, whereas IgM tests are only reactive during the first 40 days following infection, which can negatively affect a physician's ability to provide treatment options.

A Wright- or Giemsa-stained peripheral blood smear

This blood smear can be helpful to physicians when patients display morulae in the cytoplasm of peripheral blood neutrophils in the first week of illness. However, this method is not often preferred by clinics, as it has a wide variation of sensitivity (25% to 75%) when it comes to the presentation of morulae in the neutrophils. 

Additionally, prior antibiotic treatment reduces the sensitivity of this test.

Polymerase chain reaction (PCR) tests

Coming with a smaller yield and more helpful results, the polymerase chain reaction test (PCR) comes with a sensitivity of 67% to 90%. The downside to this test is its lack of accessibility in public health and commercial reference laboratories, so it may not be used in many cases.

Note that prior antibiotic treatment reduces the sensitivity of this test.

The incubation period for HGA is 5 to 14 days, so you can expect symptoms of HGA to manifest after that period. Some may include:

  • Fever
  • Chills
  • Severe headache
  • Muscle and joint pain
  • Anemia
  • Vomiting
  • Diarrhea
  • Hepatitis
  • Confusion
  • Skin rashes
  • Coughing

It is important to note that these symptoms vary in severity and combination. Additionally, the severity of symptoms may increase due to older age, low immunity, or the presence of other health complications.

Physicians will commonly recommend the use of doxycycline to treat HGA, and they will often move quickly, as delaying treatment increases the chances of severe illness and even death in a patient. 

The prevention of HGA disease is primarily dependent on the avoidance of the ticks that carry this infection. Here are some specific precautions you can take:

  • Wear proper clothing when outdoors in areas where ticks may be present, such as long-sleeve shirts, long pants, and close-toed shoes.
  • When outdoors, be sure to avoid dense grassy or woody areas.
  • Tuck in clothing to restrict access to the body.
  • Wear light-colored clothing so that it is easier to spot dark-colored ticks.
  • Use insect repellent that contains DEET or permethrin.
  • Tie up long hair so that attached ticks are more likely to be visible.
  • Consistently inspect pets that spend time outdoors.

Human granulocytic anaplasmosis is a tick-borne disease that occurs globally but is more prominent in North America, Europe, and eastern Asia. It can affect anyone who lives in, works at, or visits endemic tick regions. Symptoms can range from mild to severe, and expedient diagnosis and treatment are crucial to ensuring that a patient does not experience severe illness or death. Since there is no current vaccine to protect against HGA, the best way to prevent it is by preventing tick bites.