Potomac Horse Fever (PHF): Symptoms, Treatment & Vaccination

horse at the watering hole

Potomac Horse Fever (PHF) is a disease that affects horses and can cause serious health problems. It is caused by the bacterium Neorickettsia risticii, which infects the horse through the ingestion of contaminated food or water. PHF can be fatal if not treated promptly, so it’s important to be aware of the signs and symptoms and to seek treatment if they are present.


The disease was first identified in the United States in the 1980s when a cluster of horses on Maryland’s Potomac River showed clinical signs. The bacteria responsible for causing PHF was isolated and named after the river where the outbreak occurred. 


Potomac Horse Fever (PHF) is generally characterized by an acute onset of fever as high as 107º, depression, loss of appetite, and colic. Other commonly seen symptoms include anorexia, lethargy, diarrhea, abdominal pain and distension, increased heart rate and respiration rate, mild to moderate ataxia or lameness due to muscle pain, ventral edema and laminitis. On rare occasions, horses may exhibit neurological signs such as facial paralysis, hyperesthesia, or mild vocalization.


Potomac Horse Fever (PHF) is typically diagnosed by examining the horse’s clinical symptoms and, in some cases, performing laboratory tests. Blood analysis can help determine if antibodies are present, indicating a PHF infection. Other tests may include intestinal fluid samples, fecal swabbing, or urine testing. In cases where the horse has died, a necropsy (post-mortem examination) can also be performed to look for evidence of the disease.

If you suspect your horse has PHF, it is important to contact your veterinarian immediately. Early diagnosis and treatment are critical for the best possible outcome.


The primary treatment for PHF is supportive care. This includes antibiotics to prevent secondary bacterial infections and fluids to support hydration. Tetracycline antimicrobials may also be used in more severe cases. Anti-inflammatory drugs can help reduce the risk of laminitis and colic, which are both common complications of PHF. If the horse has diarrhea, electrolytes and other dietary supplements may be necessary to prevent dehydration.


The best way to prevent Potomac Horse Fever (PHF) is through vaccination. Vaccination with a single dose of an appropriate commercial vaccine can provide protection for up to six months, and consecutive annual vaccinations are recommended. Horses that have previously been infected with PHF should also be vaccinated annually as they may not develop lasting immunity from the first infection.

In addition to vaccination, other preventative measures can help reduce the risk of PHF. Horses should be kept away from standing water sources, such as streams and ponds, where carriers may live. Stable management practices that include cleaning stalls frequently and avoiding overcrowding can also reduce the spread of disease in horses.


How long does it take for Potomac Horse Fever to heal?

Typically, Potomac Horse Fever resolves itself within two weeks with proper medical treatment. However, complications can arise if the horse’s immune system is compromised due to other illnesses or syndromes. If this is the case, it may take longer for the disease to resolve.

Which drug is most effective against Potomac Horse Fever?

The most common drug used to treat Potomac Horse Fever is oxytetracycline and the oral combination of erythromycin and rifampin. This antibiotic helps to reduce the symptoms and severity of the disease, as well as prevent any further complications from arising.

Can a horse get Potomac fever twice?

Yes, it is possible for a horse to get Potomac Horse Fever more than once. However, this is typically only seen in horses that have not been properly vaccinated against the disease. If a horse has been vaccinated, it will typically have a much milder case of the disease and recover more quickly.

Is Potomac fever fatal?

Potomac horse fever (PHF) is a serious illness that can be fatal in horses. The severity of the disease and whether it is fatal depends on many factors, including the health status of the horse, how early diagnosis and treatment are initiated, and whether or not the horse has been vaccinated for PHF.

How long does the Potomac Horse Fever vaccine last?

The duration of the Potomac Horse Fever vaccine depends on the specific vaccine your horse received. Generally speaking, most vaccines last for up to six months. However, some vaccinations may offer protection for 9 months.


Potomac Horse Fever (PHF)

Potomac Horse Fever

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