Potomac Horse Fever in Horses: Diarrhea, Colic, and Laminitis Risk

Potomac horse fever, often shortened to PHF, is an acute infectious disease of horses caused by Neorickettsia risticii. It most often affects the intestinal tract, leading to fever, depression, reduced appetite, colic signs, and diarrhea. In some horses, the most devastating complication is laminitis, which can become severe very quickly.

This disease is considered non-contagious from horse to horse. That matters for barn management, because a sick horse does not usually infect stablemates directly the way some respiratory diseases do. Even so, PHF is still an emergency because the horse can become dehydrated, endotoxemic, and painful in a short period of time.

Cases are most often seen in spring, summer, and early fall, especially in areas near waterways. Horses of any breed or age can be affected. Some pregnant mares may also abort months after infection, so broodmares with fever or diarrhea during PHF season deserve prompt veterinary attention.

The good news is that many horses improve when treatment starts early. The challenge is that early signs can look vague at first, so fast recognition by the pet parent and quick involvement from your vet make a real difference.

Potomac horse fever is caused by the bacterium Neorickettsia risticii. Research over time has shown a complex life cycle involving freshwater snails, trematodes, and aquatic insects. Horses are thought to become infected mainly by accidentally eating infected aquatic insects or ingesting contaminated material in feed, forage, or water.

Insects linked with transmission include mayflies, caddisflies, damselflies, dragonflies, and stoneflies. This helps explain why cases are often associated with farms near creeks, rivers, irrigation ditches, ponds, and other water sources. Barn lights can attract these insects at night, which may increase exposure around feed tubs, water buckets, and stall fronts.

The incubation period is usually about 10 to 18 days after exposure. PHF is not considered contagious between horses, so isolation is not usually needed for direct transmission control. Still, your vet may recommend practical biosecurity steps while sorting out other causes of fever and diarrhea, since diseases like salmonellosis can look similar.

Vaccination can be part of prevention, but it does not provide complete protection in the field. That means a vaccinated horse can still get PHF, although vaccination may reduce severity in some cases. Prevention usually works best when vaccination is paired with environmental steps to reduce insect exposure.

Your vet will start with the history, season, geography, and physical exam findings. A horse with fever, depression, reduced gut sounds, colic, or diarrhea during warmer months, especially near waterways, raises concern for PHF. Because several serious intestinal diseases can look similar, diagnosis usually includes testing rather than relying on signs alone.

The most useful confirmatory test is typically PCR, performed on EDTA whole blood and feces. Testing both samples can improve the chance of detection because the organism may not be present in blood and manure at the same time. If possible, samples are best collected before oxytetracycline is started, since treatment can reduce the amount of detectable bacterial DNA and lead to a false-negative result.

Bloodwork may show changes such as low white blood cells early on, then later a high white blood cell count. Your vet may also assess hydration, protein loss, electrolyte changes, and inflammation. In pregnant mares, PHF may be considered if there is fever, diarrhea, or later abortion.

Serology can sometimes support the picture, but it has important limits. Antibody tests may be hard to interpret because of false positives, prior exposure, or vaccination. For that reason, your vet will usually weigh lab results together with the horse's signs and response to treatment.

Prevention focuses on reducing exposure and building a practical vaccine plan with your vet. Because PHF is linked to aquatic insects, horses living near streams, rivers, ponds, irrigation ditches, and wet pasture areas may have higher risk. Turning off or reducing barn lights at night can help because lights attract mayflies and caddisflies. It also helps to keep feed and water as clean as possible and avoid letting insects collect in buckets, troughs, or feed tubs.

Vaccination is available, but it should be viewed as one layer of protection, not a guarantee. The currently available vaccine is a killed product labeled as an aid in prevention. Field protection can be incomplete and short-lived, so horses in endemic areas may need a schedule tailored by your vet, often timed before peak summer and fall exposure. In some higher-risk areas, your vet may discuss more frequent seasonal boosters.

If PHF has occurred on your farm or in your region before, stay especially alert in warm months. Take temperatures promptly when a horse seems dull, off feed, or mildly colicky. Early recognition often matters as much as prevention.

For pregnant mares and horses with a history of laminitis, prevention planning deserves extra attention. Ask your vet whether your farm's geography, insect pressure, and local case history justify a more aggressive vaccination and monitoring strategy.