Understanding Potomac Horse Fever and Your Horse’s Risk

Updated October 8, 2024 | Reviewed by: Liz Schatz, DVM

What is Potomac Horse Fever?

Potomac Horse Fever (PHF) is a disease caused by the bacteria Neorickettsia risticii. It is also known as Ditch Fever, Shasta River Crud, Equine Monocytic Ehrlichiosis, or Equine Ehrlichial Colitis.

The disease was first identified in 1979 near the Potomac River outside of Washington, D.C., but it has since been documented in horses of all ages, breeds, and genders living across the United States and Canada. Potomac Horse Fever primarily affects the gastrointestinal system, with severe cases causing, diarrhea, colic, endotoxemia, laminitis and death.

A mostly seasonal disease, infections typically occur in the warmer months from late spring to early fall. Cases have shown that horses seem to not stay infected long term.

How is Potomac Horse Fever transmitted?

Potomac Horse Fever is transmitted through a complex cycle that involves a horse ingesting an organism that is infected with the N. risticii bacteria. The cycle involves certain aquatic insects (such as caddisflies, mayflies, damselflies, and dragonflies) as well as snails being infected by flukes (parasitic flatworms that’re also known as trematodes) which contain the bacteria.

These insects and snails are often found near lakes, streams, creeks, and other freshwater sources. Horses can accidentally consume N. risticii when infected organisms lay or die on their pasture, hay, or water troughs.

Is Potomac Horse Fever contagious?

There is no evidence of Potomac Horse Fever being contagious between horses, or to people. However, horses at the same barn may be exposed to the same source of infection, and therefore could be more at risk. In rare cases, infected mares can transmit the disease to their fetuses through the placenta, which may lead to abortion.

Clinical Signs and Symptoms of Potomac Horse Fever

According to the Equine Disease Communication Center (EDCC), symptoms of Potomac Horse Fever are variable, but commonly include:

• Fever (up to 107° F)
• Diarrhea (ranging from mild to severe)
• Loss of appetite
• Depression or lethargy
• Laminitis, which can occur with or without diarrhea
• Colic and decreased gastrointestinal sounds
• Swelling in the limbs, ventral body, or prepuce in geldings and stallions
• Abortion in pregnant mares

If your horse has any of these signs, contact your veterinarian as soon as you can. Isolating your horse and following biosecurity protocols can be a smart preemptive move since signs of PHF are similar to those of many infectious diseases.

Diagnosing a Horse with Potomac Fever

Once your veterinarian arrives, they will take a history from you on when the signs first appeared, if any treatment has been given, and your horse’s vaccination status among other important information. They will also perform a physical examination of your horse.

Veterinarians can diagnose Potomac Horse Fever using PCR testing (polymerase chain reaction) which detects genetic material of the bacteria in the horse’s blood, manure, or fetal tissue. Since diarrhea is a common symptom, and that can be similar to signs of other infections, your veterinarian may test for other pathogens too, like salmonella and clostridium.

Treatment and Recovery

Potomac Horse Fever is treated by veterinarians primarily with prescription antibiotics, like oxytetracycline and doxycycline. Many horses that have a fever or diarrhea will improve within 72 hours of treatment.

In more severe cases, supportive care, like intravenous fluids and anti-inflammatories, may be necessary. Horses that develop laminitis will require additional care, whether on the farm or at a clinic, such as icing the hooves, and their prognosis becomes more guarded.

How to Prevent Potomac Horse Fever

Preventing Potomac Horse Fever involves reducing a horse’s exposure to the bacteria-carrying insects. Strategies horse owners can implement at your barn include:

• Vaccinating horses: Stay up to date with the Potomac Horse Fever vaccine. Although the vaccine cannot guarantee 100% protection, it can reduce the severity of the disease. Veterinarians typically recommend vaccines for PHF if you live in an area with an increased risk and they can help you schedule these before peak times of the year.
• Turning off barn lights at night: Avoid attracting insects inside your horse’s stall in the evening by turning off stable lights. This reduces the chance of insects being in contact with your horse’s hay or water and feed buckets.
• Manage pastures: Minimize your horse's exposure to bodies of water, like lakes, ponds, rivers, and streams during warmer months of the year. Horses that graze or drink in areas where these insects thrive (within 5 miles) are more at risk of becoming infected. Consider sectioning off pastures that have access to natural water with fencing to keep your horses away.
• Cleaning and covering feed buckets: Stay on a regular schedule of cleaning your horse’s water buckets and ensuring fresh water is always available. If you’re making up feed buckets, cover them with a cloth to reduce the chances of infected insects getting in the meals.
• Install screens or panel curtains: Screen panels, like Kool Kurtains, can be installed on barn entry ways, run-in sheds, stall windows, and hay storage buildings to keep the bugs out while allowing for airflow. Plus, the mesh screens will shield your horse from most the sun’s rays while you save on insecticides.
• Stay informed on outbreaks: Be in the know when it comes to documented cases of Potomac Horse Fever, as once there is a confirmed case, there is a higher likelihood of more cases. The EDCC can keep you up to date with disease alerts for your geographic area, and your veterinarian and local agricultural extension agents are great informational resources as well.

Evidence-Based Resources

1. “Potomac Horse Fever: Disease Factsheet.” Equine Disease Communication Center, 2024, www.equinediseasecc.org/handler/managedfilehandler.ashx?id=24&ext=pdf.
2. “AAEP Infectious Disease Guidelines: Potomac Horse Fever.” American Association of Equine Practitioners, 2017, aaep.org/wp-content/uploads/2024/02/Potomac_Horse_Fever.pdf.
3. Stewart, Allison J. “Potomac horse fever.” Merck Veterinary Manual, 5 Oct. 2022, www.merckvetmanual.com/digestive-system/infectious-diarrheal-diseases-in-horses/potomac-horse-fever.