Potomac Horse Fever
By Heather Smith Thomas
Potomac Horse Fever occurs mainly during summer and fall. It was first documented in the Potomac River Valley of Maryland in l979 but has been diagnosed in nearly every state, every Canadian province, Europe and South America.
Dr. Katherine Wilson (Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, Virginia) says the bacteria infect a certain type of fluke that parasitizes aquatic snails and slugs. At first people thought the disease was transmitted to horses via biting insects, since this pathogen belongs to the same family of bacteria that cause tick fever. Then scientists discovered snails were involved.
“For many years we thought horses had to eat the snails to get the bacteria. Then these bacteria were also found in aquatic insects like Mayflies and caddis flies. So now we think horses are infected by ingesting the insects,” says Wilson.
“These aquatic insects can be attracted to lights around the barn and fall into water troughs or feed,” she says. There is usually water nearby, but the horses don’t have to be out in a pasture with the water and snails.
“Once a horse is infected, it may take one to three weeks to show signs of disease. The classic situation is a horse going off feed, with diarrhea and fever. This is usually a biphasic fever; the horses have fever, then their temperature comes back to normal, and then they develop another fever. Then most of them develop profuse diarrhea and become severely dehydrated,” says Wilson.
“About 30 to 40% develop laminitis. Most fatalities result from not being treated or because of laminitis,” she says.
“There are wide ranges of clinical signs, however. Some horses are just off feed, with fever, and may have decreased fecal output, acting a little colicky—and never develop diarrhea. Bloodwork will show low white blood cell count, and if we test for PHF they test positive,” she says.
“The best way to diagnose PHF is to identify the bacteria in a sample of blood or feces. This is done with a PCR (polymerese chain reaction) test which detects bacterial DNA. But if we suspect the horse might have PHF we start treating immediately, before we get test results,” she says.
“Diagnosing PHF can be confusing. Sometimes these horses look like mild colic or an impaction because they are dull and not passing manure. Fever would be a clue, however, because we don’t see fever in impaction. If we run blood work, their white cell count is low, and that’s not typical of impaction colic, either.” In these instances most veterinarians would start treatment, in case it is PHF.
“It is best to treat as quickly as possible, before they get secondary complications from dehydration. This can make the prognosis worse and treatment more prolonged,” says Wilson.
The antibiotic commonly used is oxytetracycline intravenously. “In a mild case this can be done on the farm, but if the horse has diarrhea and is dehydrated it needs IV fluid and other supportive care—and laminitis prevention,” she says.
Most of them recover and don’t develop laminitis. “Fatalities range from 5 to 30%. There are different strains of the bacteria, which may be a factor in differences in severity of clinical signs,” says Wilson.
“Usually if a horse recovers from PHF, it is immune for up to 2 years. There is a vaccine available, but it doesn’t have very long duration of immunity—perhaps 2 months–and only protects against one strain. In areas with high prevalence of the disease, we recommend vaccinating twice—if it’s not cost prohibitive for a farm with many horses. We suggest vaccinating in late spring, before summer fly season, and again mid summer, to give protection through fall,” says Wilson.
“The vaccine may not prevent PHF, but might reduce severity of clinical signs. The vaccinated horse may have a mild case rather than severe life-threatening illness.”
Other ways the disease might be prevented is to reduce exposure to aquatic insects. “One well-known farm was able to reduce incidence of PHF just by turning off their outside lights at night,” says Wilson.
