Potomac horse fever has been a problem for horses in the region for many years. Dr. Lyle Bischoff, owner of Powell Veterinary Services, has seen a couple recent cases of the potentially fatal disease.

Jennifer Fernandez, who lives south of Powell, lost one horse, Lillie, to the disease in 2016. It started out as a mild case of colic. 

“Her symptoms were mild, but she passed away within three days of getting them,” Fernandez said. 

Fernandez was new to the area and unaware of the risk of the disease. With so many people now moving to Wyoming, she hopes more horse owners from other states will become aware of the risk. 

Kenneth Mitchell, who lives in the same area as Fernandez, had a couple horses contract the disease this year. Like Fernandez’s horse, Mitchell’s horse started out collicky. 

“He’d take a mouth full of hay and then spit it out,” Mitchell said. 

The horse also had other symptoms, such as blisters around his muzzle. Mitchell isolated the horse on some dry land and he recovered within a couple weeks.  

“I think he was pouting because he couldn’t be around his buddies,” Mitchell said. 

The second horse that contracted the disease had more mild symptoms; he recovered within a week. 

In horses, the disease initially has mild symptoms. However, more severe symptoms, such as acute laminitis, can occur. 

Scott Schell, entomologist with the University of Wyoming, said there’s 5-30% mortality. Additionally, mares that contract the disease 100 to 160 days into their pregnancy and recover can still abort 190 to 250 days into their pregnancy.

Route of infection 

Potomac horse fever (PHF) got its name from the Potomac River, where the route infection was first identified. It was first found in the Big Horn Basin in 2002.

Bischoff was treating horses in Cowley and Lovell for severe diarrhea. In a September 2002 memo from the Wyoming State Veterinary Laboratory Department of Veterinary Sciences, Bischoff said he didn’t think of PHF at first, since it’s mainly found in horses that live around water. However, testing of blood and fecal samples from two sick horses confirmed PHF.

Researchers are still trying to understand the lifecycle of the disease, but it’s now believed the main route is through the accidental consumption of insects. There’s some steps before it reaches the insects, however. 

Schell said Potomac horse fever is a spillover disease. It comes from a bacteria that has to live in another organism’s cells. 

It originates in a liver parasite in bats and birds, which become infected when they eat insects. This parasite emits eggs in the bile duct of the host, and then the eggs come through the animals’ feces into the land and water.

Potomac horse fever then takes another step before infecting horses, as snails eat the deposited droppings, along with the parasite eggs. 

“It’s kind of a parasite of a parasite,” Schell said. 

The snails secrete the parasite into the water, where it gets into aquatic insects, such as mayflies and caddisflies. The horses then either drink the insects in their water or eat them off forage. 

“The only thing we’ve figured out is the horse is going to have to accidentally consume caddisflies or mayflies,” Schell said. 

Caddisflies, stoneflies, damselflies and dragonflies can all be infected with a larval stage of the parasite that is itself infected with the bacteria.

After one to three weeks of contracting the disease, the horses will stop eating, appear depressed, get a high fever and have very watery diarrhea. 

Bischoff said their tails will be caked with it. 

“If you see that, you need to get them treatment,” he said. 

Cases have also been seen in Uinta County.

Protecting your horses

State veterinarian Hallie Hasel said “prevention is the number one key” to protecting horses from the disease.

The animals are at highest risk of getting sick from PHF during the late summer and fall. 

“Try not to let them graze along river beds during” that time, Hasel said.

Bischoff said anywhere you have water could be an area of high risk, such as standing puddles, ponds and swampy areas. 

Schell said where there are electric lights around the horses, there’s a risk as well. 

“These aquatic insects, when they’re in the adult stage and they’re flying around, they can be attracted to lights, such as in barns,” he said. “And when they’re flying over water troughs … and land into the water the horses drink, they get accidentally ingested.”

Fernandez, who lost her horse in 2016, said she changes her horses’ water regularly and doesn’t let them graze in those high-risk months.

When a horse shows signs of the disease, it’s important to quarantine him or her, and contact a vet. Horses cannot spread the disease among themselves, but isolating them when they show the symptoms helps narrow down the diagnosis.

“If you recognize your horse may have this and contact the vet, they can start treatment earlier. It’s a better prognosis for your animal,” Schell said. 

If untreated, the mortality rate is about 30%, Schell said. If it survives untreated, there’s still a risk it will develop bad laminitis. The horse will then founder and possibly become crippled. 

Vets now have a definitive test that can determine if a horse has the disease. And early treatment can help.

Bischoff said he provides the horses with an oxytetracycline antibiotic and supportive care, such as an anti-inflammatory drug “just to make them feel better.” 

The horses usually respond to the treatment in a few days. If not, it suggests secondary problems such as edema in the belly. 

There is a vaccine available, but there are many different strains of the disease. So the vaccination is no guarantee. 

“It’s more of a peace of mind thing,” Bischoff said. 

He recommends two doses of the vaccine the first year, and then annual booster shots. Sometimes immunity might only last three or four months, so additional boosters may be necessary.

For those who don’t live around water, Hasel recommends vaccination if a horse is traveling to an area where the disease is found. 

Besides restricting their grazing and making sure their water is fresh, Fernandez has been vaccinating her horses ever since they had a bout several years ago. She said she hasn’t had any contract the disease since. 

“Knock on wood,” Fernandez said.