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Key takeaways

Anaplasmosis is an infectious, tick-borne disease that affects horses caused by the bacteria Anaplasma phagocytophilum.

  • Bacteria is transmitted to horses through the bite of an infected tick
  • Bacteria invades the white blood cells and alters the cells’ function, impacting the horse’s ability to fight off other infections
  • Symptoms of anaplasmosis include fever, yellow gums, lethargy, and uncoordinated movement
  • Bloodwork is used to diagnose the condition
  • Treatment involves antibiotics to target the bacteria and anti-inflammatories to reduce swelling
  • Prognosis is excellent if treatment is started early, with most symptoms improving within the first 12-24 hours
  • If left untreated, prognosis is more guarded. Most animals recover within two weeks, but rare cases of death have been recorded
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A closer look: Anaplasmosis in Horses

Most cases of anaplasmosis occur during tick season, running from late autumn to spring. It is most prevalent in northern California, but cases have been detected in most U.S. states as well as rare cases in South America, Europe, and Africa.

Symptoms of anaplasmosis vary depending on the animal's age. Young horses under 3 years old often have mild symptoms. In some cases, they may only develop fever. Older horses often display more severe symptoms, including icterus and swelling of the limbs.

Geriatric horses generally display the most severe symptoms. Another factor that increases the severity of symptoms is the presence of underlying medical conditions, especially ongoing infections.

Risk factors

In the majority of cases the prognosis for anaplasmosis is excellent. It is easily treatable, especially in its early stages. Even without treatment, most horses recover after a two week period.

The severity of the symptoms varies, with older horses displaying more severe clinical signs. These horses are most likely to require veterinary care to recover from the infection. Some rare fatalities have occurred due to anaplasmosis, generally linked to secondary infections.

Anaplasmosis is neither contagious nor zoonotic, but both humans and horses can be exposed to the bacteria via tick bite. If a horse is diagnosed with anaplasmosis, it is a warning sign of the presence of infected ticks in the area. The tick species that transmits anaplasmosis is the same tick that can carry Lyme disease.

Possible causes

Anaplasmosis is a tick-borne condition caused by the bacteria Anaplasma phagocytophilum, and is transmitted to horses via the bite of infected ticks. Ticks acquire the bacteria by feeding on another infected animal.

Main symptoms

Testing and diagnosis

The typical diagnostic work-up for anaplasmosis includes:

  • Physical examination
  • Blood tests to detect the presence of antibodies or confirm the presence of the bacteria
  • Blood smear to visualize the bacteria directly

Steps to Recovery

Once identified, anaplasmosis is easily treatable and has an excellent prognosis.

Typical treatment for anaplasmosis includes:

  • Tick removal
  • Oral and IV Antibiotics
  • Anti-inflammatory drugs such as corticosteroids

Symptoms usually improve within 12-24 hours from treatment.

In most cases, the prognosis for anaplasmosis is excellent. If detected and treated in its early stages, the majority of animals improve within 12-24 hours from the start of treatment.

If not treated, most animals recover after 2 to 3 weeks of the onset of symptoms, but inflammation, edema, and ataxia can lead to secondary complications. On rare occasions, anaplasmosis is fatal.


Anaplasmosis is not directly contagious between horses, as it must be transmitted by a tick. Preventing ticks from feeding on horses through topical tick repellents and reducing horse exposure to ticks are the best methods for preventing anaplasmosis.

Is Anaplasmosis in Horses common?

Anaplasmosis is common in areas where the disease is considered endemic. It is most prevalent during tick season (late autumn to spring). Horses living in northern California have the greatest probability of being infected.

Typical Treatment

  • Antibiotics
  • Anti-inflammatory drugs


Levi Smith, DVM Student (Class of 2020) - Writing for Purdue University College of Veterinary Medicine
John E. Madigan , DVM, MS - Writing for Merck Veterinary Manual
Janet E. Foley , DVM, PhD - Writing for Merck Veterinary Manual
Lauren Hughes, DVM - Writing for New England Equine Medical & Surgical Center

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