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What is
Potomac Horse Fever?

Potomac Horse Fever (PHF) is a seasonal disease, occuring in summer and fall in temperate regions of Canada and the United States. Horses with PHF develop gastrointestinal symptoms and, in serious cases, laminitis. PHF is serious and potentially fatal, and can affect all horses, regardless of age, breed, or sex.

Case of laminitis


17%–36% of horses with PHF will die

Approximately 25% of affected horses die from PHF, and more will be euthanized due to laminitis, a severe complication of PHF. The earlier a horse is treated, the greater its chance of survival.

How is Potomac
Horse Fever contracted?

Infected aquatic insect

Horses only develop PHF if there are infected snails and insects where they are stabled or pastured. If one horse develops PHF, it means that horses in the same geographical area are at risk of contracting PHF.

PHF occurs when horses ingest a specific bacterium. This bacterium lives in flatworms, also known as “flukes.” These infected flukes then move to infect different aquatic insects, such as mayflies, caddisflies, dragonflies, and damselflies. Horses can become infected either by drinking out of water sources infested with these infected aquatic insects, or by eating infected insects in their feed or pasture.

Clinical signs and symptoms of Potomac Horse Fever

Different signs and symptoms may indicate your horse is suffering from PHF. You should call your veterinarian if your horse seems to be suffering from one or more of the following symptoms:


Fever is a sign that your horse is fighting an infection. Fever is a sign of other diseases as well, so check for other signs.

Decreased appetite

Unexplained loss of appetite can indicate a serious problem.


A quick way to check for dehydration is the “skin-pinch test.” Pinch a fold of skin on the point of the shoulder. If the fold remains more than two seconds, it’s a sign of dehydration.

Stocking Up

Do you notice generalized swelling in your horse’s legs, specifically below the knees and hocks?


Is your horse’s normally keen interest fading out? Like humans, horses can feel depressed, and it can be a sign of pain or discomfort from an illness or injury. Pay attention to changes in your horse’s normal behaviour.

Diarrhea (can be severe)

Easily identifiable, diarrhea can also lead to dehydration.


Signs of colic include changes in behaviour that indicate abdominal pain, such as restlessness and pawing at the ground, stretching out, turning their head toward their flank, and wanting to lie down or roll.


A severe and painful inflammatory condition of the hoof, laminitis causes horses to develop an awkward gait and stance. Check for early signs of laminitis, such as a bounding digital pulse and hooves that are warm to the touch.

Abortion in Pregnant Mares

Abortion may even occur several months after clinical disease in pregnant mares.

Treatment and prevention of Potomac Horse Fever

PHF is typically treated with a course of antibiotics. Horses with PHF are also often administered electrolytes and/or IV fluids to treat dehydration, and non-steroidal anti-inflammatories for pain relief.

Vaccination protects your horse against PHF

Regular vaccination aids in preventing PHF. Since PHF is a risk-based vaccine, ask your veterinarian if your location is at risk. The frequency of vaccination against PHF varies from every 3–4 months to yearly, depending on the incidence rate of PHF in your geographical location.

In addition to vaccination, other practices that may help to reduce the occurrence of PHF include:

Eliminating standing water and access to natural water sources
Reducing exposure to insects by covering feed and regularly cleaning out feed, water buckets, and troughs
Learn more about how Potomavac can help

protect your horse against Potomac Horse Fever.

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