Potomac Horse Fever

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Potomac Horse Fever (PHF)

Potomac Horse Fever (PHF) is a warm-season disease of horses caused by Neorickettsia risticii, a bacterium that infects the lining of the small and large intestines. First identified in 1979 along the Potomac River in Maryland, PHF can lead to serious illness and, in some cases, life-threatening complications.

How Horses Get PHF

Freshwater insects such as mayflies, dragonflies, damselflies, and aquatic snails can carry N. risticii. Horses contract the disease by ingesting contaminated hay, grain, pasture, or water containing dead infected insects.

PHF is not contagious between horses.
Most cases occur between late spring and early fall, with July, August, and September being the highest-risk months.
Cases are more common in regions near rivers, ponds, or other water sources — including Michigan.

Common Signs and Symptoms

Clinical signs vary by horse and may range from mild to severe. Watch for:

Mild depression or lethargy
Decreased appetite (anorexia)
Mild signs of colic
Fever above 102°F
Dehydration
Watery diarrhea (may appear within 24–48 hours)
Laminitis (founder)

Some horses may only show mild symptoms, while others develop more serious GI issues such as abdominal distension and severe colic. In mares, abortion has also been reported weeks after infection.

Diagnosis

PHF is typically diagnosed based on:

Clinical signs and symptoms
Season and geographic location
Bloodwork to assess hydration, organ function, and white blood cell count
Confirmation through PCR testing or cell culture of blood or feces

Treatment

Early veterinary intervention is critical. Treatment typically includes:

Oxytetracycline, an antibiotic given IV for 3–5 days
NSAIDs like Banamine to control fever and reduce risk of endotoxemia
IV fluids and hospitalization in more severe cases

➡️ With timely treatment, the recovery rate is over 70%.

Prevention & Farm Management

Because PHF is spread through the environment and not from horse to horse, environmental control is key:

Tips to reduce risk:

Turn off barn lights at night to limit insect attraction
Remove standing water where insects breed
Keep feed and water sources covered and clean
Clean up dead insects regularly around stalls and feed areas

Vaccination

There are multiple killed vaccines available for PHF, but effectiveness can vary:

Over 50 different strains of N. risticii exist, while vaccines only cover one
Protection is strongest for about three months
Research suggests the vaccine reduces disease severity, even if it doesn’t prevent all cases