By Marissa Settatree 
BELLEVILLE – With spring coming to an end and summer beginning, horse owners in the Quinte area are going to have to start vaccinating their horses for Potomac horse fever. 
The Quinte area is one of the geographic locations in Ontario for the illness, which can be fatal if not treated.
According to the Ontario Ministry of Agriculture’s  website, approximately 30 per cent of horses infected with Potomac horse fever die from the disease. It can be prevented with the annual vaccination, but new horse owners or those new to the area may not be aware of the geographic-specific illness.
Potomac horse fever was originally described in 1979 as a disease that infected horses by the Potomac River on the Atlantic coast of the United States, and that’s how it got its name. Since then, the disease has been found in other areas, including near Lake Simcoe and the Quinte region.
Some of the symptoms of PHF are fever, mild to severe diarrhea and laminitis, which is a disruption of blood flow to the horse’s feet, as well as occasionally depression. The disease doesn’t infect humans and is not contagious to other horses.
Horses can contract PHF by eating grass that a woodland snail has travelled across. According to the American Association of Equestrian Practitioners ‘ website, horses can also get Potomac from accidentally ingesting insects such as dragonflies, mayflies, caddisflies and stoneflies that carry the disease from freshwater sources containing freshwater snails.
Ron Herron, a veterinarian at Bay of Quinte Veterinary Service on Highway 62 in Belleville, said that his practice sees around 40 to 50 cases of Potomac every year despite the extensive vaccinations given to horses annually.
According to Herron, the Potomac vaccination is “one of the core essential ones in our area” alongside rabies and tetanus shots.
While the geographic area that has Potomac horse fever stretches from Brighton to Kemptville,  Herron said that there is a certain region stretching from Carrying Place to Stirling that tends to be quite bad.
There are no precautions that horse owners can take to prevent PHF aside from the vaccine, which is for one of the six types of Potomac. The Quinte area is prone to all six types, but there is only a vaccine for one type. This vaccine, however, is still necessary to get and will keep any case of PHF less severe.
“The vaccination doesn’t prevent infection. It just lessens the severity,” Herron said. But he added that horses should still get vaccinated because it can enhance cross-prevention between strains, meaning that if a horse gets a mix of two types of PHF, it can be protected a little more with the vaccine.
The vaccine lasts between six and nine months, but with the winter season keeping PHF at bay because of the lack of grass and insects, horses only need to be vaccinated once a year.
There are still a lot of unknowns about Potomac horse fever, including pinpointing its origins.
“We don’t know what carries it. We know the woodland snail is an intermediate host that probably gets infected from some insect,” Herron said.
Horses that are successfully treated for PHF tend to have a “two-to-three-year natural immunity if they have a natural infection,” according to Herron.
He also said that a horse can be infected with the disease for a while before the owner will even notice.
Herron explained that Potomac is a disease that comes in five stages. The horse will typically contract PHF between May and July where it goes through the first phase. The second phase comes about 60 days later. This is when the disease can get dangerous if the horse hasn’t been vaccinated or if it doesn’t have an immune response.
Essentially, a horse could be infected with PHF for a while before it begins to show the symptoms.
“They are actually infected for far longer than we realize,” Herron said.