Veterinarians in Central Kentucky have confirmed nearly 30 cases of Potomac horse fever (PHF) in area horses so far in 2017. At press time, six have been diagnosed since the beginning of August.
The University of Kentucky Veterinary Diagnostic Laboratory’s (UKVDL) Equine Disease Maps indicate four cases were diagnosed in May, nine in June, and nine more in July, for a total of 28 cases at press time. Most of the affected horses have been from Fayette County, with other cases being confirmed in horses from Woodford, Bourbon, Harrison, Scott, and Taylor counties.
In addition to performing PCR diagnostic testing on samples from live horses, Jacqueline Smith, PhD, MSc, BSc, Dipl. AVES, UKVDL epidemiologist and founder of the UKVDL Disease Mapping Initiative, said the laboratory has conducted two necropsies confirming PHF.
“Typically June, July, and August are the months for the most cases of PHF, yet it can be seen throughout the spring, summer, and fall,” Smith said. “We average seven cases per month during the summer for PCR testing and average one to two per summer month in necropsy.”
Bryan Waldridge, DVM, MS, Dipl. ABVP, ACVIM, of Park Equine Hospital at Woodford, in Versailles, Kentucky, who has treated many PHF cases over the years, agrees, and said he expects additional cases to be confirmed throughout August.
Keeping this in mind, area owners are encouraged to watch their horses closely for signs of disease and alert veterinarians of suspicious signs as soon as possible. Smith said PHF’s fatality rate ranges from 5–30%, and horses treated early in the course of disease might have a better chance of surviving.
Potomac horse fever is caused by Neorickettsia risticii, an organism found in some flukes (a wormlike parasite) that infect aquatic snails and insects (such as caddisflies and mayflies). Horses can be exposed by inadvertently ingesting aquatic insects infected with flukes carrying the bacteria or by drinking flukes directly from rivers or streams. However, even horses residing far from water bodies aren’t out of PHF’s reach.
“I’ve seen this in racehorses, and there was a report of Potomac at a Minnesota show barn, as well,” Waldridge said of horses contracting PHF without residing near natural water sources. “Pole lights (near barns) or big lights on the barn will attract the flies. The flies can then get into horses’ water buckets, feed troughs, and hay, and the horses can consume them that way. There’s, for sure, a role in having lights on your barn.
“I’ve seen (PHF) on farms where the horses never drink water that doesn’t come out of a spigot, and the farm I’m thinking of had the worst Potomac I’ve ever seen,” he added.
Also complicating diagnosis is the fact that PHF has been known to yield highly variable clinical signs. Smith said signs owners should watch for include acute onset fever, depression, decreased appetite, mild coliclike signs, watery diarrhea, edema (fluid swelling), and, later in the course of disease, acute laminitis.
However, not all affected horses develop all these signs.
“The signs are so vague,” Waldridge said. “The two you hear about most are fever and going off feed—those are usually the first things you see.”
However, he said he’s already had a case this year in which the horse never developed a fever. And in the past, he’s had cases that have never had diarrhea.
To Vaccinate or Not?
Walk into most barns, and you’ll hear owners “wishing” for some ailments over others when their horses aren’t healthy:
“Pray for an abscess!”
“I hope she’s just a little dehydrated and not colicking badly.” “That leg’s just a little stocked up from standing in the stall … right?”
One thing you won’t hear them hoping for is Potomac horse fever (PHF). In fact, many owners go to great lengths to protect their horses from contracting this potentially deadly disease. One option is vaccination with one of the inactivated whole-cell vaccines on the market.
“Although vaccination has been reported to protect 78% of experimentally infected ponies, it has been marginally protective in the field,” said Jacqueline Smith, PhD, MSc, BSc, Dipl. AVES, University of Kentucky Veterinary Diagnostic Laboratory’s (UKVDL) epidemiologist and founder of the UKVDL Disease Mapping Initiative. “Vaccine failure has been attributed to antigenic and genomic heterogeneity among the more than 14 different strains isolated from naturally occurring cases.”
In other words, the different virus strains are all unique, and protection against one strain won’t necessarily afford protection against another.
“Furthermore, vaccine failure may also be due to lack of antibody protection at the site of exposure, because the natural route of transmission has been determined to be oral ingestion of the agent,” she added.
While some veterinarians recommend vaccinating, Bryan Waldridge, DVM, MS, Dipl. ABVP, ACVIM, of Park Equine Hospital at Woodford, in Versailles, Kentucky, said it’s not one of his go-to products.
“I don’t routinely recommend it,” he said. “I’ve seen horses on farms where the owners vaccinate, and those horses still get sick.”
Like Smith, he said it comes down the bacterial strain. If vaccinated horses are exposed to the specific strain in the vaccine, effective protection is more likely. If they’re exposed to a different strain, it’s less likely to afford protection against the disease, Waldridge said.
Still, some veterinarians recommend the vaccine for at-risk horses. Discuss your horse’s risk level and whether vaccination is appropriate with your veterinarian.
“It definitely appears to be a strain-related disease,” he said. “How lucky or unlucky you get and how sick the horse gets in the end depends on which strain ends up infecting the horse.”
Should a horse contract PHF, treatment centers around intravenous oxytetracycline administration for several days.
“If given early in the clinical course of the disease, a response to treatment is usually seen within 12 hours,” Smith said. “This is associated with a drop in rectal temperature, followed by an improvement in demeanor, appetite, and borborygmal (gut) sounds.
“In animals that exhibit signs of enterocolitis (inflammation of the small intestine and colon), fluids and non-steroidal anti-inflammatory drugs should be administered,” she added. “Laminitis is more common than in other causes of enterocolitis and, if it develops, is usually severe and often refractory to treatment.”
Waldridge also cautions that just because a horse’s clinical signs appear to resolve initially, the situation can still take a rapid downturn.
“The horse can be back to almost being normal,” he said. “Within 24 hours, you might as well have been pouring distilled water in their left ear. You never know. That’s just the thing with Potomac … you just hope you’ve got a wimpy strain that you’re going to get with a dose or two of oxytet.
“The bad ones, it’s a six-lane highway to hell treating them. It just seems like no matter what you do you’re not getting anywhere.”
While PHF can, indeed, be challenging to prevent, owners can take steps to reduce horses’ risk of consuming infected insects and snails.
“Try not to put pole lights over feed and water troughs,” Waldridge recommended. “Put lights somewhere you don’t have stalls or feeding implements.”
He also said that, although horses can be exposed to the causative agent in the absence of a natural water source, keeping horses away from ponds, streams, and other water bodies could help reduce their risk of inadvertently ingesting contaminated insects and snails.
Finally, “Keep water troughs and buckets free and clear of bugs,” he said.
Chances are, Central Kentucky veterinarians will still see their share of PHF cases this year. But Waldridge said they’ll face each one head-on and learn from the ones that don’t end with a healthy horse.
“It’s my most-hated disease,” he said. “Every year I swear I’m not going to let it get the best of me. There’s always at least one horse a year that just tortures you—you’ve done everything right, and then it still gets you.”