Potomac Horse Fever in Horses: Symptoms, Diarrhea, and Laminitis Risk
- See your vet immediately if your horse has fever, depression, reduced appetite, soft manure, watery diarrhea, or signs of foot pain.
- Potomac horse fever is caused by *Neorickettsia risticii* and can progress quickly from fever to colitis, dehydration, and laminitis.
- Diarrhea often develops within 24 to 48 hours after the first signs, but not every horse has severe diarrhea early on.
- Laminitis is a major complication and is reported in about 20% to 30% of affected horses, so early hoof protection matters.
- Many horses improve when treatment starts early, but severe cases may need hospitalization, IV fluids, and intensive monitoring.
What Is Potomac Horse Fever in Horses?
Potomac horse fever, often shortened to PHF, is a serious infectious disease that causes inflammation of the intestines in horses. It is caused by the bacterium Neorickettsia risticii and is best known for causing fever, depression, diarrhea, and a meaningful risk of laminitis. In pregnant mares, it has also been associated with abortion.
This disease is most often seen in late spring, summer, and early fall. Cases are linked to exposure to aquatic insects such as mayflies and caddisflies, especially in areas near creeks, rivers, and other waterways. Horses do not usually catch PHF directly from each other, so a sick horse is not generally considered contagious to stablemates.
PHF can start subtly. A horse may seem off feed, quiet, or mildly colicky before diarrhea appears. Because the condition can worsen fast, early veterinary attention gives your horse the best chance of avoiding severe dehydration, endotoxemia, and hoof complications.
Symptoms of Potomac Horse Fever in Horses
- Fever
- Depression or lethargy
- Reduced appetite
- Soft manure or watery diarrhea
- Mild colic signs
- Dehydration
- Decreased gut sounds
- Laminitis or foot soreness
When to worry: immediately. Potomac horse fever can move from a vague feverish horse to a dehydrated horse with severe colitis in a short time. Call your vet the same day for fever, depression, appetite loss, or manure changes during PHF season. If your horse has watery diarrhea, weakness, or any sign of foot pain, treat it as an emergency.
What Causes Potomac Horse Fever in Horses?
Potomac horse fever is caused by Neorickettsia risticii, a bacterium with a complex life cycle involving freshwater snails, flukes, and aquatic insects. Horses are thought to become infected mainly by accidentally eating infected insects, including mayflies and caddisflies, or material contaminated by them.
That is why PHF is often associated with farms near rivers, streams, irrigation ditches, and ponds. Outdoor lights can attract swarms of aquatic insects to barns and feed areas, which may increase exposure. Cases are reported across multiple US states and parts of Canada, so this is not limited to the Potomac region anymore.
Importantly, horses with PHF are not considered a direct infection risk to other horses in the barn. The concern is shared environmental exposure, not horse-to-horse spread. Your vet may still recommend practical biosecurity steps while sorting out the cause of diarrhea, because other infectious colitis diseases can look similar at first.
How Is Potomac Horse Fever in Horses Diagnosed?
Your vet usually starts with history, season, geography, and a physical exam. A horse with fever, depression, reduced gut sounds, diarrhea, and possible exposure to waterways or aquatic insects raises concern for PHF. Because the signs overlap with salmonellosis and other causes of equine colitis, testing matters.
The most useful confirmatory test is typically PCR on blood and or feces to look for Neorickettsia risticii. Some labs also offer antibody testing, but serology can be less helpful for confirming an active case because antibodies may reflect past exposure or vaccination rather than current disease.
Your vet may also recommend a CBC, chemistry panel, fibrinogen or serum amyloid A, electrolyte testing, and sometimes abdominal ultrasound or additional fecal testing. These do not confirm PHF by themselves, but they help assess dehydration, inflammation, protein loss, kidney risk, and how aggressively your horse needs support.
Typical diagnostic cost ranges in the US are about $250 to $900 for exam, farm call, bloodwork, and targeted testing in an uncomplicated field case. If your horse needs repeated bloodwork, hospitalization, or a broader diarrhea workup, the total can rise well above that.
Treatment Options for Potomac Horse Fever in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm call and physical exam
- Basic bloodwork as needed
- Early treatment with IV oxytetracycline when your vet feels it is appropriate
- Oral or enteral fluids if the horse is stable enough
- NSAID use when indicated by your vet
- Close at-home monitoring of temperature, manure, hydration, appetite, and digital pulses
- Early hoof icing or other practical laminitis-prevention steps if feasible
Recommended Standard Treatment
- Exam, CBC, chemistry, electrolytes, and PHF PCR testing
- IV oxytetracycline early in the course when indicated by your vet
- IV fluid therapy for dehydration and electrolyte losses
- Anti-inflammatory and anti-endotoxemia support as directed by your vet
- Frequent reassessment of temperature, hydration, gut sounds, manure output, and hoof comfort
- Prophylactic cryotherapy of the feet to reduce laminitis risk
- Short-term hospitalization or intensive on-farm treatment depending on local resources
Advanced / Critical Care
- Hospitalization in an equine facility
- Continuous or repeated IV fluids and electrolyte correction
- Aggressive anti-endotoxemia and colitis support directed by your vet
- Serial bloodwork and intensive nursing care
- Continuous hoof cryotherapy and advanced laminitis monitoring
- Pain management and mechanical hoof support if laminitis develops
- Additional diagnostics and supportive care for sepsis, protein loss, or complications in pregnant mares
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Potomac Horse Fever in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Based on my horse's signs and season, how likely is Potomac horse fever compared with salmonellosis or another cause of colitis?
- Do you recommend PCR testing on blood, feces, or both for my horse?
- Does my horse need treatment at home, repeated farm visits, or referral to a hospital?
- What signs would mean dehydration or endotoxemia is getting worse?
- How should we monitor for laminitis over the next several days?
- Would hoof icing or other preventive foot support help in my horse's case?
- If my horse was vaccinated for PHF, does that change how you interpret testing or risk?
- What is the expected cost range for the next 24 to 72 hours based on my horse's current condition?
How to Prevent Potomac Horse Fever in Horses
Prevention focuses on lowering exposure and discussing vaccination with your vet. Commercial PHF vaccines are available, but field protection is considered incomplete. In other words, vaccination may help reduce risk in some horses, yet vaccinated horses can still develop disease. Timing matters, so your vet may recommend scheduling vaccination before the highest local seasonal risk.
Environmental management also matters. Practical steps include reducing insect attraction around barns, turning off unnecessary lights at night, keeping feed and water as clean as possible, and limiting access to areas where aquatic insects collect in large numbers. On some farms, bringing horses in during heavy insect hatches may also help.
If your property is near waterways or has a history of PHF cases, ask your vet about a prevention plan tailored to your region. That plan may include vaccination, seasonal monitoring, and a clear action plan for any horse that develops fever or soft manure. Fast recognition is one of the most effective ways to lower the risk of severe colitis and laminitis.