Anaplasmosis in horses, also known as Equine Granulocytic Anaplasmosis (EGA), is a tick-borne disease caused by the bacterium Anaplasma phagocytophilum.

This condition typically affects horses in areas where the tick vectors are present, including various parts of the United States, Europe, Africa, and South America. [1]

Horses affected by anaplasmosis experience symptoms that can include fever, depression, swelling of the limbs, uncoordinated gait, and yellowing of the eyes and gums. [1][2][3][4] A variety of blood tests are available for diagnosing EGA. [1][2][5]

Most horses have a favorable prognosis without the need for treatment. However, in more severe cases, the condition is highly responsive to antibiotic therapy and supportive care. [1]

Equine Granulocytic Anaplasmosis

Equine granulocytic anaplasmosis (EGA), formerly known as equine granulocytic ehrlichiosis is caused by infection with Anaplasma phagocytophilum. This bacterium can infect horses, other domestic animals, and humans. [5]

A. phagocytophilum is part of the Rickettsia group of gram-negative bacteria that are obligate intracellular organisms. [1] This means that these bacteria can only grow and multiply within cells of a host. They require arthropod vectors, such as ticks and mites, for transmission between animals. [6]

The bacterium A. phagocytophilum , was previously classified as Ehrlichia equi. This earlier nomenclature is the source of the alternative name “Equine Granulocytic Ehrlichiosis.”

The term “granulocytic” in this context refers to the type of white blood cells, namely granulocytes, that are primarily affected by the infection. [1][2] Granulocytes, a key component of the immune system, play a crucial role in fighting infections.


EGA was first identified in the United States in 1969 and has since been reported in various countries worldwide. [2][7] The disease is most commonly reported in tick-infested regions, including Canada, the United States, South America, Europe, Asia, and North Africa. However, it has the highest prevalence in the northwestern United States. [1][8]

The actual prevalence of anaplasmosis is challenging to determine as many infected horses are asymptomatic with no outward signs of infection. [2][5]

EGA is a seasonal disease, with the highest incidence in the late fall, winter, and spring. [4][8] This pattern aligns with the activity of infected ticks, which are the primary vectors of the disease.

Climate change poses a risk of increased anaplasmosis exposure in horses. Climate change significantly influences the behavior and distribution of tick populations, as well as extending the duration of tick-active seasons. [5]

Clinical Signs

In the initial stages after infection, horses with EGA commonly exhibit clinical signs such as: [1][2][3]

  • Lethargy
  • Fever
  • Decreased appetite
  • Increased heart rate
  • Increased breathing rate

Younger horses, typically those under 4 years old, tend to show less severe clinical signs. Foals less than a year old often present with only fever, whereas older horses are more likely to experience a more severe progression of the disease. [1][8][9]

Severe Clinical Signs

As the infection progresses and spreads throughout the body, symptoms tend to worsen. This can lead to the emergence of additional clinical signs, including: [1][4]

  • Yellow eyes and gums
  • Signs of dehydration, such as sunken eyes and dry gums
  • Swelling of the limbs and skin around the abdomen
  • Red spots on the gums
  • Unwillingness to move
  • Stumbling, uncoordinated gait

Although horses with anaplasmosis may appear unwilling to move, the disease is not associated with laminitis. [1]

Rarely, EGA can also be associated with symptoms like: [1][9]

Secondary Complications

While complications arising from anaplasmosis are uncommon, they have been reported in horses.


Severe infection with A. phagocytophilum can lead to breakdown of muscle tissue, also called rhabdomyolysis. This results in firm, painful muscles, typically along the horse’s back and upper hindlimbs. [1][10]

Abnormal muscle tissue breakdown can be identified on bloodwork. In horses with rhabdomyolysis, blood tests reveal elevated levels of muscle enzymes, specifically creatine kinase and aspartate aminotransferase. [1]

Neurologic Disease

Occasionally, anaplasmosis can lead to severe neurologic disease in horses. Neurological symptoms can include: [1][3][9][11][12]

  • Head tilt
  • Difficulty eating or swallowing
  • Recumbency
  • Seizures
  • Collapse

The mechanism by which A. phagocytophilum causes neurological issues is poorly understood, but is believed to involve a combination of inflammation of the nerves and damage to blood vessels. [4][5]