A bacterium called Neorickettsia risticii (formerly called Ehrlichia risticii) causes Potomac horse fever (PHF). The microscopic parasite lives on a type of flatworm called trematodes, also known as flukes, that grow and multiply in fresh water. Horses can become infected by directly ingesting flukes while grazing near or drinking from creeks, lakes or rivers. Or, some insects such as caddisflies, mayflies, damselflies, dragonflies and stoneflies, will, as aquatic larvae, feed on the flukes. When the insects mature, they fly or are carried inland by the wind. Horses then eat the fluke-infested insects – dead or alive – in their feed, pasture, water or even hay.

Some researchers have even speculated there could be a connection with bats and barn swallows, as the bacteria has been found in the intestinal tracts of both species. It is believed the bats and birds eat infected snails or insects and pass on the bacteria via their feces.

“Cases are more likely to occur on farms that are closer to the water, but may occur up to a few miles from a body of water,” said John Baird, Ontario Veterinary College professor emeritus and co-author with Dr. Luis Arroyo of Historical aspects of Potomac horse fever in Ontario (1924-2010). The disease is not contagious and large outbreaks do not necessarily occur on any one farm, he said.

Also known as equine monocytic ehrlichiosis, the name Potomac horse fever was coined when a particularly virulent strain was recognized in the Potomac River Valley in the eastern United States in 1979. Since then, Potomac horse fever has been detected all over the United States, in Europe and South America.

In Canada, it has officially been identified in Ontario, Alberta and Nova Scotia, and, according to Baird, “there is rather strong evidence to suggest the disease was present in Ontario in the late 1870s.”

Areas endemic to Potomac horse fever can be quite specific – more regional than provincial. For example, Ontario’s southwestern and eastern sections tend to see the bulk of cases in that province, with other instances occurring randomly throughout. And in a great example of PHF’s geographical specificity, the disease is endemic in irrigation areas east and south of Calgary, whereas, west of the city it is hardly a concern.

According to Baird, “epidemiologic studies have shown that PHF is markedly seasonal in occurrence.” Cases may occur anytime between May and November. However, 70 per cent occur during July and August.

Attacks White Blood Cells

The disease can incubate for more than two weeks before the onset of symptoms.

The symptoms of Potomac horse fever are similar to a number of other conditions such as the highly contagious bacterial infections salmonella and clostridium. Therefore, taking into account the season, geographic location of the farm and the surrounding environment in combination with clinical signs can help determine if a horse has PHF or another pathogen.

“Clinical signs of PHF vary markedly,” said Baird, explaining that any combination of the signs may be present, with only the rare case showing most of them.

Neorickettsia risticci enters the horse’s white blood cells and multiplies in the in the wall of the horse’s intestinal tract. This causes colitis (inflammation) to varying degrees. At first, the horse may seem depressed with decreased appetite. A fever of 38.9°C to 41.7°C might already be present or will develop.

There may be decreased gut sounds, which Baird said are followed in 24 to 48 hours by “higher-pitched tinkling sounds suggestive of excessive fluid and gas accumulation.”

While diarrhea is common, it does not occur in all cases. “The manure may look normal initially, but within 24 to 48 hours, a moderate to severe diarrhea develops in approximately 60 per cent of affected horses,” Baird said. “In some horses, the diarrhea is transient, but in others it may persist for up to 10 days with the manure ranging from ‘cow-pie’ consistency to profuse, watery in character.”

The horse may also show signs of mild colic along with the diarrhea. Edema (swelling) along the lower abdomen and limbs may occur due to loss of blood protein through the damaged intestinal wall.

When the condition starts to progress, the inflammation allows the bacteria to enter the bloodstream through the wall of the colon, leading to severe toxemia (toxins in the bloodstream) and dehydration. This results in elevated heart and respiratory rates and congested mucous membranes. (A so-called “toxic line” – a thin red or purple band at the gum line just above the teeth is a tell-tale sign of toxemia.)

In 15 to 30 per cent of cases, toxemia results in laminitis, said Baird. Laminitis usually occurs within three days of the initial onset of diarrhea. “Laminitis may progress despite the resolution of other clinical signs,” he noted.

Pregnant mares may abort due to inflammation of the placenta or retained placenta with the aborted fetus showing signs of the infection.

Early Treatment of Potomac Horse Fever is Critical

The course of the disease without treatment is usually five to 10 days, said Baird. Seventeen to 36 per cent of horses will die from PHF itself, while others have to be euthanized due to laminitis or other complications.

The earlier PHF is treated, the more likely a favourable outcome. Therefore, prompt diagnosis is critical. If you suspect PHF, a veterinarian should be called immediately. In some situations, the horse will be transferred to an equine clinic or hospital for treatment.

Most veterinarians will begin treatment right away if they suspect PHF and won’t wait for definitive confirmation. A blood and fecal PCR (polymerase chain reaction) test that detects the DNA of the Neorickettsia risticii in the horse’s white blood cells offers the most accurate and effective diagnosis. PHF results can take up to a week, however. Serum testing for antibodies is rarely used nowadays, as it can yield false positives.

The go-to treatment is the antibiotic oxytetracycline, administered intravenously once a day for three to five days. In mild cases, the oral doxycycline may be given. If started early after the disease’s onset, horses will often respond dramatically to the treatment within 12 hours, with symptoms resolving in three to five days. A quick and positive reaction to oxytetracycline can also prove to be a diagnostic clue.

IV fluids and electrolytes may also be administered if the diarrhea is severe. Non-steroidal anti-inflammatories (often flunixin meglumine – trade name Banamine®) can help counteract toxemia and provide pain relief.

Placing the horse in a stall with deep, soft bedding, packing ice around the hooves, and even applying shoes with supportive pads, may also help thwart the onset of laminitis.

Vaccine: Limited Efficacy

Although a PHF vaccine is available, it doesn’t necessarily prevent a horse from becoming sick because it is made with only one of many recognized strains of PHF. But it can improve the horse’s chance for survival.

“Vaccination has been associated with a poor immune response,” said Baird. “Research has shown that only 50 per cent of the vaccines are protective at six months after vaccination, and protection decreases to 33 per cent after nine months. Vaccination may decrease clinical signs, but it will not prevent infection and illness.”

PHF vaccines are most commonly administered between April and June. The initial dose is two shots, one month apart, followed by an annual booster. When determining whether a horse should be vaccinated, horse owners, along with their veterinarian, should consider the number of incidences of PHF in the area, the horse’s proximity to waterways and whether the horse will be travelling to endemic regions.

Research has shown that horses that have recovered from PHF are resistant to the infection due to natural immunity for up to 20 months. This means the horse will not have to be vaccinated for two years post-illness.

Preventative Measures

Although Potomac horse fever is a difficult disease to prevent, there are a few measures that may lower the disease’s occurrence in areas where the disease is particularly prevalent.

  • Turn off barn lights to avoid attracting flying insects. Or at least keep hay, feed and water away from lights that stay on all night.
  • Eliminate standing water on the farm
  • Prevent access to natural water sources
  • Remove dead insects from water troughs/buckets
  • Use insect traps or other elimination measures
  • Bring horses in at night from July to September
  • Cover feed

If it seems like you are hearing much more about Potomac horse fever recently, Baird assures there is no evidence to indicate instances are on the rise. Instead, he figures “it is more likely that the disease is being increasingly recognized now.”

But recognition and awareness among horse people about PHF is certainly not a bad thing. With early intervention being key to a favourable outcome, calling your veterinarian right away when your horse seems a little depressed and/or off his feed – as inconsequential as the symptoms might seem at the time – could actually save his life.



Angel Mackenzie is well-acquainted with Potomac horse fever. Within a five-year span, two of her mares and a gelding came down with the bacterial gastrointestinal disease. Fortunately for Mackenzie, all three horses made full recoveries, but each was nonetheless, a harrowing experience for the Ottawa-area eventer.

It was in July, five years ago, when Mackenzie noticed her four-year-old Andalusian/Friesian mare, Sonja, was listless, off her feed and had serious diarrhea. What Mackenzie didn’t know at the time was that Sonja had PHF. “Her case had a very sudden onset,” explained Mackenzie. “Like, overnight she suddenly wasn’t her cheerful self. And she loves her food, so we knew something was up.”

The veterinarians at Rideau-St. Lawrence Veterinary Services were on the scene quickly and Sonja was diagnosed. But within 24 hours Sonja developed laminitis. “She had a rotation of her coffin bones that we thought might make her permanently lame,” sadid Mackenzie.

Luckily, Sonja recovered and today she is sound and healthy.

But in June of the following year, just two weeks before she was due to carry Mackenzie in her first horse trials of the season, Sonja’s half-sister, Phaedra, a six-year-old Andalusian/Thoroughbred, stopped eating, seemed more subdued than usual and lost a considerable amount of weight quickly. “She was already a picky eater and is a quieter mare, so it took us two or three days before we realized something was really wrong and called the vet,” said Mackenzie. “The vets didn’t actually ever officially test to see whether it was Potomac, they just assumed based on the symptoms.”

Phaedra, too, was treated and fully recovered. Then last September, Mackenzie’s four-year-old Trakehner/Hannoverian gelding, Viking, seemed, like Sonja and Phaedra had before him, listless and off his food. “Given our previous experience, we were quick to call the vet,” said Mackenzie. Viking was treated immediately with tetracycline and, Mackenzie recounted he was, “pretty much back to himself” the following day.

Mackenzie’s horses are situated in the St. Lawrence River valley where PHF appears to be more prevalent than in most other areas of the province. All three had been vaccinated in March or April of the year they contracted the disease.

Rideau-St. Lawrence vets, Drs. Ben Henderson and John Donovan, who cared for Mackenzie’s horses through the PHF episodes, told her that they believe a local strain of the disease is not prevented by the vaccine. “Although they think that both mares – especially the first one – would have had much worse cases had they not been vaccinated,” she said.

Mackenzie’s advice to other horse owners is to be aware of the signs of PHF, particularly early, more subtle symptoms that might go unnoticed. “Be sure to monitor horses for unusual behavioUr, particularly if they appear lethargic or listless and disinterested in eating,” she said.