Horses contract Potomac horse fever (PHF) by ingesting the bacterium Neorickettsia risticii, which may be present in a number of species that inhabit ponds, rivers and other natural waterways. “We know that a big risk factor is proximity to a water source or river—usually within five miles, but that isn’t always the case,” says Michelle Abraham Linton, BSc, BVMS, DACVIM (LAIM), of the University of Pennsylvania at New Bolton Center. PHF tends to be seasonal, occurring mainly in the hot summer months when aquatic species are most active.
The disease was named after the Potomac River Valley, where it was first recognized in 1979, but it can occur practically anywhere under the right conditions. “If we look at blood tests on horses all across the United States, many of them show evidence that they’ve been exposed,” Linton says. “Incidence and severity varies, possibly because some horses are more susceptible, and it may also depend on how much they ingest.”
• THE VECTOR: N. risticii infects a parasitic fluke (a type of flatworm). At different stages, infected flukes can be carried by freshwater snails and may be free-swimming in the water, where they can be consumed by larvae of aquatic insects like mayflies and caddisflies. When the adult flies later emerge, they carry N. risticii.
Although a horse can become infected with N. risticii by drinking contaminated water or picking up infected snails while grazing, the more common route is by ingesting mayflies, caddisflies and other aquatic insects. That’s why the risk of PHF is greater at barns that are located within a few miles of the natural waterways where N. risticii is present in the local fauna.
Nonetheless, PHF sometimes appears in unexpected places. “Caddisflies can be found in hay,” says William Gilsenan, VMD, DACVIM, of Rood and Riddle Equine Hospital in Lexington, Kentucky. “This is why we sometimes see cases of PHF in horses that are nowhere near a river. If the hay was grown near a river, it is possible that infected flies may be present in the hay. We don’t know how many flies a horse would have to ingest to induce disease. It may vary, with differing pathogenicity among the strains of this bacteria.”
• SIGNS: PHF can produce one or more of three distinct problems: a high fever, watery “pipestream” diarrhea and laminitis. “Scientists have not yet completely figured out how N. risticii causes terrible diarrhea and enterocolitis,” Linton says, “but once the horse ingests it, the pathogen infects a specific type of white blood cell [monocyte] and causes severe inflammation in the lining of the gastrointestinal tract. This leads to diarrhea and often laminitis.”
The first sign of PHF is high fever, which may reach 107 degrees Fahrenheit, accompanied by lack of appetite and depressed behavior. “There is often a two-stage fever,” says Nora Grenager, VMD, DACVIM, of Grenager Equine Consulting in northern Virginia. “The first fever spike might be brief and the owner may miss it because most people aren’t taking their horse’s temperature twice a day. Then up to a week later the horse has a second fever that is more severe.”
Within 24 to 48 hours after the severe fever begins, about 60 percent of horses develop loose stools or diarrhea that may be profuse and watery. At this stage, the horse may also appear colicky and become dehydrated.
However, the greater concern with PHF is laminitis: As many as 40 percent of affected horses develop potentially crippling inflammation of the connective tissues within the hooves. “Horses become dehydrated, lose protein through the inflamed intestinal wall, and develop endotoxemia—that is, when the intestine wall is damaged and inflamed, toxins that are usually contained within the gut can leak out into the bloodstream,” says Linton. The endotoxemia can trigger laminitis.
“Laminitis can be a very early sign, or it can occur even after you get the colitis and diarrhea under control,” says Linton. “The laminitis might occur a week later as the horse seems to be recovering. So even if you don’t see the laminitis early, the horse is not out of the woods yet. I always warn owners that even if their horse is improving, there is potential for laminitis, despite intensive treatment.”
• TREATMENT: The primary treatment for PHF is oxytetracycline, an inexpensive antibiotic that is effective against N. risticii, along with supportive care. “This includes intravenous (IV) fluids and medications to bind up the toxins in the intestine and decrease the diarrhea,” says Linton. “We also give IV medications to decrease the toxins’ effect on the bloodstream and reduce the potential impact of laminitis.”
In addition, icing the feet and lower limbs, along with other steps to pre- vent laminitis, is advisable. “We use anti-inflammatories and anti-endotoxic medications,” says Linton. “The treatment can become very involved so these horses are often hospitalized so we can keep up their fluids. Milder cases can be treated on the farm, but many horses with PHF need to be hospitalized.”
Horses with PHF usually respond quickly after treatment begins. “If the horse had a fever, it usually goes away within 24 hours after we’ve started him on oxytetracycline,” says Linton. “Often the response to treatment is enough to convince us that it is PHF.”
Now that veterinarians know what to look for and how to treat PHF, the survival rates are high—at least 70 percent of affected horses recover. Those that do not are often put down due to complications from laminitis. “The prognosis is pretty good,” says Grenager, “but the ones that get laminitis often have severe laminitis, which becomes a long-term management issue. This can even be a cause for euthanasia.”
Preventive measures against Potomac horse fever:
• Vaccinate. The vaccine against PHF is recommended by the American Association of Equine Practitioners (AAEP) for horses likely to be exposed to the disease. “The vaccine is typically administered twice yearly, in spring and fall, but this schedule varies geographically and is dependent upon recommendations of the veterinarian,” says Gilsenan. “In regions with a long hot season, it might be appropriate to vaccinate more frequently.”
• Reduce exposure to species that carry N. risticii. Turn off barn and outdoor lights that may attract mayflies and caddisflies, which are known to fly for miles to swarm under streetlights and other light sources. Research shows that horses stabled at the ends of aisles, near open doors and night-lights, are at a much higher risk of contracting PHF.
“There was one report of a show barn that was having problems with PHF even though it was miles from a river,” says Grenager. “They had a lot of big safety lights on at night and a number of the horses got PHF. After they turned off the lights at night, the PHF cases dropped off significantly. Many people like to have a security light left on at the barn, but you have to weigh in the risks for PHF.” If security is a concern, consider installing motion detectors on your exterior barn lights, so they will come on only when someone is present.
• Store hay indoors or under a cover.
• Keep horses at some distance from water sources. If your pastures contain natural ponds or streams, con- sider fencing them off; horses grazing too close to the water’s edge may pick up infected insects, or they may drink contaminated water.
• Take special precautions when insect populations are at their high- est. Finally, be aware of when populations of local aquatic insects are likely to peak. In some areas, insects that carry PHF may hatch and emerge by the thousands to mate and die within hours. Your local agricultural extension office may be able to help warn you of impend- ing hatches. At those times, you’ll want to take extra precautions to protect your horse, perhaps by keeping him inside with the lights off.
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