Potomac fever, formally called Potomac horse fever (PHF), is an acute and potentially fatal intestinal disease in horses caused by a bacterium called Neorickettsia risticii. First identified near the Potomac River in Maryland in the 1970s, it has since been documented across North America and beyond. The disease is seasonal, tied to freshwater environments, and marked by fever, severe diarrhea, and a risk of a painful hoof condition called laminitis.
What Causes Potomac Fever
The bacterium behind Potomac fever, Neorickettsia risticii, is an obligate intracellular organism, meaning it can only survive and reproduce inside living cells. It belongs to the Anaplasmataceae family but sits on its own distinct genetic branch, which is why it was eventually reclassified from its earlier name, Ehrlichia risticii. The disease is sometimes still called equine monocytic ehrlichiosis.
What makes this pathogen unusual is its lifecycle. It doesn’t spread directly from horse to horse. Instead, it lives as a symbiont inside parasitic flatworms called trematodes (a type of fluke). These trematodes cycle through freshwater snails as their first host. The snails release larval stages of the trematode into the water, where they infect aquatic insects like caddisflies, mayflies, and damselflies. Horses become infected when they accidentally swallow these insects, whether by grazing near water, drinking from streams, or eating hay or feed that dead insects have landed on. Barn lights near waterways can attract swarms of these insects at night, increasing the risk.
Once the bacterium reaches a horse’s gut, it invades and multiplies in the cells lining the colon. From there it enters the bloodstream and infects immune cells, specifically monocytes, macrophages, and mast cells. This spread through the immune system is what makes the disease systemic rather than just a localized gut infection.
Where and When It Occurs
Potomac fever is strongly linked to freshwater environments. Proximity to lakes, rivers, and streams is a recognized risk factor because these habitats support the snails and aquatic insects that carry the trematode. The disease peaks during warmer months when insect populations are highest, typically from late spring through early fall. Regional patterns vary: detection rates have been higher in the Midwest during fall compared to other areas, and annual variation suggests that local environmental conditions, such as rainfall and temperature, influence how many vectors are present in a given year.
Though it was first recognized in the eastern United States, Potomac fever has been confirmed in horses across the country, in parts of Canada, and in South America, including Brazil. Any property near flowing or standing freshwater carries some level of risk during the warmer months.
Symptoms and Timeline
The hallmark signs of Potomac fever are depression, loss of appetite, fever, dehydration, and watery diarrhea. In a study of confirmed cases in Ontario, diarrhea was the primary reason owners sought veterinary attention in 63% of horses, while fever prompted the call in 30%. A smaller number, about 7%, were first seen for colic-like abdominal pain. The median time between the first noticeable symptoms and a veterinary exam was two days, though some horses were seen within six hours and others not for up to a week.
Not every horse follows the same pattern. Some develop a biphasic fever, where body temperature spikes, dips back to normal, then rises again before diarrhea begins. Others may show only mild, intermittent loose stool. In pregnant mares, the infection can trigger abortion, sometimes weeks after the initial illness has resolved. Laminitis, a painful inflammatory condition of the tissue connecting the hoof wall to the underlying bone, is one of the most serious complications. It can develop during or shortly after the acute illness and, in severe cases, can be career-ending or fatal on its own.
How It’s Diagnosed
Diagnosing Potomac fever based on symptoms alone is difficult because fever and diarrhea in horses have a long list of possible causes, from salmonella to clostridial infections to sand colic. A veterinarian will typically consider the season, the horse’s proximity to water, and the clinical picture together. The most reliable confirmation comes from a PCR test on blood or feces, which detects the bacterium’s genetic material directly. Blood antibody tests can support the diagnosis but are less definitive on their own because a single positive result may reflect past exposure rather than active infection. Paired blood samples taken 10 to 14 days apart showing a rising antibody level are more informative.
Treatment and Recovery
Potomac fever responds to antibiotic therapy, specifically intravenous oxytetracycline, which is given every 12 to 24 hours for up to five days. This antibiotic has poor oral absorption in horses, so it must be administered through a vein. The earlier treatment begins, the better the outcome. Horses that receive antibiotics before severe diarrhea develops tend to recover faster and with fewer complications.
Supportive care is just as important as the antibiotic itself. Horses with significant diarrhea lose large volumes of fluid and electrolytes, so intravenous fluid therapy is commonly needed. Anti-inflammatory medications help manage fever and reduce the risk of laminitis. Horses at high risk for laminitis may also receive icing of the hooves as a preventive measure. Most horses that are treated promptly recover within a week or two, though some take longer to fully regain weight and energy. Horses that develop laminitis face a much longer and more uncertain recovery.
Prevention Strategies
A vaccine for Potomac fever exists, but its effectiveness is inconsistent. The bacterium has multiple strains, and the vaccine may not protect against the strain circulating in a given area or year. Many veterinarians still recommend vaccination for horses in high-risk areas as one layer of protection, but it should not be relied on as the sole preventive measure.
Environmental management makes a meaningful difference. Turning off or relocating barn lights during peak insect season reduces the number of aquatic insects drawn to feeding and housing areas. Keeping feed covered and using fans in stalls can also help. If your property borders a stream or pond, limiting pasture access closest to the water’s edge during summer evenings, when insect activity peaks, lowers the chance of accidental ingestion. Cleaning water troughs frequently to remove drowned insects is another simple step.
Horses that have recovered from Potomac fever are not permanently immune and can be reinfected in subsequent seasons, so these management practices remain important year after year.