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Horse Owners Warned of Anaplasmosis Risk

The tick-borne illness can mimic the symptoms of other diseases, and proper diagnosis is the key to the cure.

By Leslie Potter | November 29, 2016

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There are several different ailments that could cause a horse to have an increased temperature, decreased appetite, and lethargic behavior. A recent Facebook post from the Deerfield Veterinary Clinic in New Hampshire describes an increase in one illness that most horse owners probably wouldn’t think of immediately: Anaplasmosis.


Anaplasmosis is caused by a bacterium called Anaplasma phagocytophilum which is transmitted by ticks. According to the Centers for Disease Control, the black-legged tick and the western black-legged tick are the primary transmitters of this bacterium. In humans, the disease was first discovered in the mid-1990s and reported cases have increased ever since. In humans, it is most common in New York and New England and in the upper Midwest, specifically Minnesota and Wisconsin.

In horses, however, equine anaplasmosis was first reported in California and is still most common in northern California, according to the Merck Veterinary Manual. However, it is found across the United States, on the west coast of Canada, and in parts of Europe.

Although horses and humans (and other animal species) can get anaplasmosis, there is no evidence that it can be transmitted directly from a horse to humans. Ticks bites are likely the only source of infection.

Symptoms of Equine Anaplasmosis

The symptoms of the disease vary from one horse to another and are often different in horses of different ages. According to the Merck Veterinary Manual, most infected horses will have a fever. Young horses may also show depression, swelling in the legs, and a lack of coordination. Adult horses may have all of those symptoms as well as a reluctance to move and jaundice. If an afflicted horse has an existing infection from a different cause, it may become worse.

According to Blue Ridge Equine Clinic, equine anaplasmosis can be diagnosed by examining the blood under a microscope to look for the presence of the bacteria. A titer test can also be used, which shows if a horse has had an immune response to the bacteria.

Horse in Tall Grass

Treatment and Prognosis

Although the disease can be fatal, death is an extremely rare outcome and is more likely in horses that have other simultaneous infections. Some horses with mild cases can recover within a few weeks even without treatment. However, to minimize an affected horses discomfort and reduce the risk of complications, it’s essential to catch the disease early and treat it appropriately. Most cases can be cleared up with the correct antibiotics. Horses with more severe symptoms may be treated with injectable corticosteroids.

There is no vaccine available for equine anaplasmosis, so the best prevention is the same as for other tick-borne ailments: Avoiding tick bites. Depending on where you live and ride, you may not have a way to fully protect your horse from ticks. However, checking for ticks after your horse has been out in the woods or tall grasses will give you the chance to find the little disease-carriers before they’ve had a chance to bite your horse.

As always, your veterinarian will be your best resource for information on the diseases that affect horses in your area and what you can do to prevent and treat them.


Leslie Potter is a writer and photographer based in Lexington, Kentucky. www.lesliepotterphoto.com

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