Peak Potomac horse fever (PHF) season is July through September, and the disease can cause affected horses devastating disease signs. Our Church’s Mobile Veterinary Service team presents information about this condition and explains how you can reduce your horse’s risk.

Potomac horse fever basics

PHF was first identified in 1979 along the Potomac river in Montgomery county, Maryland. The disease may also have existed in northern California since the 1970s. PHF has now been reported in most U.S. states, five Canadian provinces, South America, Europe, and India. The disease is caused by a gram-negative bacteria, Neorickettsia risticii. More recently N. findlayensis has been isolated from PHF cases and may also be capable of causing disease. 

risticii’s lifestyle is complex, involving trematode species that infect an intermediate host. N. risticii infects trematodes (i.e., virgulate cercariae), and these organisms parasitize freshwater snails. When water temperatures are warm, the snails release the infected cercariae, which can then develop into metacercariae in a second intermediate host, including certain aquatic insects (e.g., caddisflies, mayflies, damselflies, dragonflies, and stoneflies). Transmission to horses can occur through several routes, including:

  • Exposure to N. risticii-infected cercariae in the water.
  • Ingesting metacercariae in aquatic insects when they graze.
  • Consuming adult insects trapped on the water surface.
  • Consuming adult insects attracted by stable lights that accumulate in feed and water. 

Clinical cases most commonly occur in horses near rivers, lakes, or other aquatic habitats. Disease risk increases for horses who graze pastures that border waterways such as freshwater rivers, streams, ponds, and irrigation ditches. 

Potomac horse fever signs

When a horse ingests an intermediate host, the cercariae or metacercariae that parasitize the host are resistant to stomach acid, allowing them to survive and release N. risticii. The bacteria invade the horse’s white blood cells (WBCs), particularly in the gastrointestinal (GI) tract lining, causing inflammation and damaging tissues that can lead to a toxin release. The incubation period varies between one to three weeks, and signs include:

  • Fever — For the first few days, fevers tend to be transient, but as the infection progresses, they become more persistent, typically 103 to 106 degrees. These horses usually exhibit obvious dullness and inappetence.
  • GI stasis — PHF reduces gut motility, causing ileus.
  • Diarrhea — Within 24 to 48 hours, watery diarrhea occurs, varying from mild to profuse output.
  • Colic — Affected horses often exhibit colic signs such as pawing, looking at their flank, and rolling.
  • Limb swelling — An affected horse’s lower limbs may become edematous.
  • Laminitis — Laminitis occurs in 15% to 25% of cases. Horses who have an extremely high fever or experience toxemia have a high laminitis risk.
  • Abortion — Affected pregnant mares may spontaneously lose their fetus.

Clinical disease typically has a 5-to-10-day duration, and mortality rates vary from 5% to 30%. Long-term complications associated with PHF are most commonly related to laminitis.

Potomac horse fever diagnosis

PHF diagnosis is usually based on typical signs that occur during the summer and early fall—although the pathogen can survive year-round in favorable conditions—combined with fecal and blood tests. To determine whether your horse has Potomac horse fever, our Church’s Mobile Veterinary Service team may perform these specific tests:

  • Complete blood count (CBC) — A CBC evaluates your horse’s overall health and hydration status. For most horses that have contracted PHF, their CBC initially exhibits a decreased WBC count followed in a few days by a significant WBC increase.
  • Biochemistry profile — Our team uses a biochemistry profile to evaluate your horse’s electrolyte status and organ health.
  • Polymerase chain reaction (PCR) assay — A PCR test is a sensitive and rapid diagnostic screening that can confirm PHF infection. However, a false-negative result can occur, because the organism doesn’t remain in the bloodstream for long. 
  • Blood or feces culture — Cultures may be necessary to rule out other potential causes of illness signs or confirm PHF infection. 

Potomac horse fever treatment 

If our Church’s Mobile Veterinary Service team provides medical intervention early in the disease course, a horse with PHF can often be treated successfully. PHF treatment includes:

  • Antibiotics — PHF is typically susceptible to a particular antibiotic class, and a positive treatment response often occurs within 12 hours. The antibiotic treatment course is usually about five days.
  • Fluids — In many cases, intravenous (IV) fluids are necessary to replace fluid deficits and correct electrolyte imbalances brought on by diarrhea and fever.
  • Nonsteroidal anti-inflammatories (NSAIDs) — Our team administers NSAIDs to help decrease GI inflammation and alleviate colic-associated pain.
  • Ice boots — Because laminitis is a common secondary PHF complication, we may recommend icing your horse’s feet as a prophylactic measure.

Potomac horse fever prevention

Potomac horse fever is a dangerous condition. To reduce your horse’s PHF risk, follow these tips:

  • Vaccinate your horse — PHF vaccines are available, and while they may not prevent your horse from contracting PHF, they can significantly minimize the disease severity and increase treatment success. After receiving an initial PHF vaccine series, horses should be vaccinated annually before peak PHF season. We recommend that horses be vaccinated in the spring and boosted in the fall to maximize their protection. 
  • Prevent waterway grazing — During peak PHF season, prevent your horse from grazing near fresh water sources such as creeks, ponds, and irrigation ditches.
  • Clean water buckets — Clean water troughs and buckets often.
  • Avoid attracting insects — Use warm-colored light-emitting diode (LED) lights in and around your barn so as not to attract insects.

Contact our Church’s Mobile Veterinary Service team to schedule your horse’s PHF vaccine booster and ensure their protection is up-to-date during peak PHF season.