Signs and symptoms may include:

- fever

- severe headache

- muscle aches (myalgia)

- chills and shaking, similar to the symptoms of influenza

- nausea

- vomiting

- loss of appetite

- unintentional weight loss

- abdominal pain

- cough

- diarrhea,

- aching joints

- sensitivity to light

- weakness

- fatigue

- change in mental status (extreme confusion, memory loss, inability to comprehend environment- interaction, reading, etc.)

- temporary loss of basic motor skills

Symptoms may be minor, as evidenced by surveillance studies in high-risk areas. Gastrointestinal tract symptoms occur in less than half of patients and a skin rash is seen in less than 10% of patients. It is also characterized by a low number of platelets, a low number of white blood cells, and elevated serum transaminase levels in the majority of infected patients. Even though people of any age can get HGA, it is usually more severe in the aging or immune-compromised. Some severe complications may include respiratory failure, kidney failure, and secondary infections.

Human granulocytic anaplasmosis (HGA) is a tick-borne, infectious disease caused by "Anaplasma phagocytophilum", an obligate intracellular bacterium that is typically transmitted to humans by ticks of the "Ixodes ricinus" species complex, including "Ixodes scapularis" and "Ixodes pacificus" in North America. These ticks also transmit Lyme disease and other tick borne diseases.

The bacteria infect white blood cells called neutrophils, causing changes in gene expression that prolong the life of these otherwise short-lived cells.