Potomac Horse Fever

PHF Basics 

Potomac horse fever (PHF) is a potentially deadly bacterial disease caused by Neorickettsia risticii which can affect horses of any breed, age, and use. It is endemic (i.e., it occurs regularly) in the Hudson River Valley and should be on all area horse owners’ radar during peak disease season: mid- to late summer and early fall. Most affected horses recover, but PHF has a reported mortality rate of 5% to 30%. 

Named for where it was discovered — near the Potomac River in Maryland — testing has confirmed PHF in nearly every U.S. state, including New York. While the disease occurs in horses around the state, in 2022, the Cornell University Animal Health Diagnostic Center reported they’d identified more cases in Albany, Dutchess, Saratoga, and Tompkins counties since 2019.

Cases are most common in equids grazing near water sources like lakes, ponds, and streams as Potomac horse fever transmission centers around aquatic creatures. Flukes containing N. risticii infect mayflies, caddis flies, damselflies, and snails. Horses inadvertently ingest the infected insects, which allows the bacteria access to the equine digestive tract. (A word of warning: Infected insects attracted to barn and stall lights can still inadvertently wind up in feed and water sources, even if horses don’t reside near water bodies.) 

While PHF is not contagious amongst horses, equids residing near infected horses could be at increased risk of disease since infected insects could remain in the area.  

Clinical Signs 

The most common Potomac horse fever clinical signs often follow a pattern as they develop: 

• Depression and loss of appetite; 

• High fever (at least 101.5°F); 

• Decreased abdominal sounds; and 

• Moderate to severe diarrhea, often alongside mild colic. 

More severe clinical signs can include dehydration, sepsis, and laminitis. Potomac horse fever in pregnant mares can also cause abortion. 

Diagnosis 

Since clinical signs can also occur with other — sometimes contagious — conditions, diagnostic testing can both confirm PHF and rule out other diseases. We generally use a PCR assay to detect exposure to N. risticii in a blood or fecal sample. Bacterial culture from a fecal sample remains the gold-standard, however it takes significantly longer to produce results. 

Treatment 

Treatment involves: 

• Eliminate N. risticii by administering tetracycline antibiotics; and  

• Managing clinical signs with supportive care, including anti-inflammatories, intravenous fluids, and anti-diarrheal medications. 

If cases are severe or deemed at risk for laminitis development, veterinarians can apply ice boots as a prophylactic measure.  

While many cases can be managed on the farm, severe cases might require hospitalization to ensure access to intensive care. 

Prevention 

Potomac horse fever prevention centers on vaccination. For adult horses here in the Hudson River Valley, we recommend an annual three-dose series with a monovalent vaccine (one that protects against PHF only rather than more than one disease); we administer the first dose in early spring (before horses can be exposed to N. risticii) and continue through the first hard frost. Foals can receive their first vaccination at three months of age. 

Additionally, reduce horses’ exposure to insects by: 

• Limiting horses’ access to water bodies during peak PHF season (late spring through early fall here in New York); 

• Turning off barn lights at night to avoid attracting insects; and 

• Keeping horses’ food covered to prevent contamination. 

Please contact us to schedule an appointment if your horse needs a PHF vaccination or booster.