The Horse

DEC 2018

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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26 December 2018 The Horse | TheHorse.com medicine clinician at Park Equine Hospital at Woodford, in Versailles, Kentucky Frequency The University of Kentucky reported 26 cases from Jan. 1 to Aug. 27, 2018, which is comparable to most years. Etiology The causative agent is Neorickettsia risticii, a bacterium found in some fluke parasites that infect aquatic snails and larvae of aquatic insects such as mayflies. Horses can ingest the infected insects (through grazing or eating hay and un- covered feeds) or possibly drink the flukes directly from streams and rivers. Potomac horse fever becomes catastrophic when the horse also gets laminitis—inflammation of the sensitive tissues connecting the coffin bone to the hoof wall in the foot. Severe to life- threatening laminitis occurs in about 40% of PHF cases, and it can occur anywhere in the disease's course. It's a side effect you must deal with long-term, says Wal- dridge. If the horse is infected by a strain likely to cause laminitis, there's little vet- erinarians can do to prevent it, he says. Prevention Decide, in consultation with your veterinarian and depending on your particular situation, whether to vaccinate your horse with the risk-based PHF vac- cine. The downside is it's made from one strain from one horse so, depending on what strain your horse gets exposed to, it might or might not be effective, says Wal- dridge. Other preventive steps include: ■ If you're in an endemic area, monitor- ing your horse's appetite and tempera- ture closely; ■ Preventing horses from drinking from natural water sources—fence off ponds and streams; ■ Only offering drinking water from a well or tap; and ■ Being aware of lights in your horse's environment. Insects are drawn to lights and can fall into feed or water buckets and be ingested by your horse. Clinical signs Loss of appetite, fever as high as 104-105 degrees F, diarrhea, and laminitis. Waldridge says he's seen horses with all combinations of these clinical signs. Potomac horse fever tends to be a mid- to late-summer disease, though it began affecting horses in late spring in Kentucky this year, possibly attributable to abundant rainfall in April and record- breaking heat in May. Case horse A 3-year-old Thoroughbred colt from a well-managed breeding farm. The horse had been vaccinated against PHF; he never drank water that wasn't from the tap; and he had no exposure to natural water sources. Still, he got sick and suffered from the worst PHF signs Waldridge had ever seen. It turned out that a light mounted outside the barn near his stall attracted bugs carrying the bacterium. Waldridge's evaluation and diagnosis We first take bloodwork, which indicated a low white blood cell count that goes along with gut disease. We also looked at serum chemistry (to assess liver and kidney function). Definitive tests are polymerase chain reaction to determine the presence of N. risticii DNA in blood and in manure. Treatment The IV antibiotic oxytetra- cycline is our first line of defense, and it's pretty inexpensive. We might also use oral doxycycline. The sooner treatment starts, the better your horse's prognosis. Resolution depends on the strain to some degree. The really bad strains, like the case horse's, cause animals to just get sick and die. But for strains that respond, usually by the end of 24 hours of treat- ment, the fever comes down and the horse goes back on his feed. Then in two to three days his manure starts to firm back up. About 60% of horses respond to treatment. If they do respond and don't develop laminitis, there's no lasting effect. Outcome This horse foundered and was euthanized. Take-Home Message While some diseases have cut-and-dried diagnoses, others can cause vague clinical signs or ones that can be confused with other disease's symptoms. It's imperative to work with your veterinarian to arrive at a speedy and accurate diagnosis. "If you don't pursue the appropriate diagnostics, you may be unintentionally mis-treating a disease," says Gilsenan. h Dire Diagnoses Insects carrying the causative agent of Potomac horse fever are drawn to lights and can fall into feed and be ingested by your horse. ISTOCK.COM

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