Perhaps the only thing more troubling than being a horse owner experiencing an outbreak of infectious disease on their farm is being a veterinarian experiencing it.
Such was the case for Dr. Trisha Dowling, Professor of Veterinary Clinical Pharmacology at Western College of Veterinary Medicine in Saskatchewan, Canada. In 2008, Dowling took a horse and pony from her farm and moved them to a local barn to use their indoor yard. Neither the horse nor the pony encountered any other horses while they were there.
A week later, one of Dowling’s horses developed unusual symptoms. Then the foal fell ill. At first, Dowling thought the pony had contracted rabies and tried unsuccessfully to control the disease. Two weeks later, another horse fell ill, and this time, she tested the horse for equine herpes virus (EHV).
“Eventually, six of my Khuweli showed signs of nervousness,” Dowling recalls. “They all survived but one mare remained slightly immobile for the rest of her life. My youngest horse developed a fever and unilateral uveitis. One of my nervous horses developed bilateral uveitis.
There are several different strains of EHV, but the most common is EHV-1. It often presents with mild respiratory symptoms and fever, but in some horses it can progress to equine myelopathy and encephalopathy (EHM), which is characterized by neurological symptoms and can quickly become severe. If a horse shows advanced neurological signs, the disease can be fatal.
EHV can spread easily through nasal secretions, and it is not known how long it can live on shared surfaces or hands.
EHV-1 is a major concern in large events where horses from different populations may be stabled together. Experts believe that most horses have been exposed to the virus at some point in their lives and may automatically begin to shed it through their nasal secretions if they are under stress. It may also be true that a small percentage of the population can be “super-spreaders” who can carry and shed large amounts of the virus, sometimes without appearing ill. This makes the virus very difficult to control.
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Dowling contacted Dr. Lara Maxwell, professor of physiological sciences at Oklahoma State University. Maxwell studied the pharmacology of many equine medicines, as well as their efficacy against specific diseases. Part of Maxwell’s research dealt with the use of the antiviral valacyclovir against EHV-1, and she suggested Dowling try it.
Valacyclovir is the second generation in a group of closely related drugs that have been tried to treat EHV-1 virus. Acyclovir was its predecessor but had poor bioavailability and had to be given by injection. It is easier for normal people to administer valacyclovir orally, and a much larger amount of the drug gets to the horse.
“One of my endurance horses is highly rated and I was hoping to make the Canadian Equestrian Team for the 2010 World Equestrian Games,” Dowling said. “Dr. Maxwell recommended treatment with valacyclovir ASAP. Basically, I bought every available supply of the drug in Saskatoon ($4,000 value) and got her treated. She was the only one of my horses who had no signs of HCV even though she She tested positive.We ended up as substitutes for the team in 2010 – oh, ok.But it was worth the cost to me to protect my most prized horse from clinical signs of hepatitis C virus.
We now know that valacyclovir can be a great weapon against EHV-1 outbreaks such as Dowling’s disease — if given in the right context, said Dr. Nicola Busterla, a professor of medicine and epidemiology at the University of California-Davis. Studies have since shown that if you give it to a horse that is already showing the classic neurological signs of EHM, you probably won’t do much to change the outcome of the condition. It’s also a good preventive option if it’s given to a horse before they’ve been exposed to EHV-1, but most of the time, horse owners don’t know they’re taking their horse to a facility where another animal is spreading the virus. .
Where it is helpful, according to Busterla, is to treat horses that may be showing the first signs of illness but are not yet showing neurological symptoms – those with fever and mild lethargy.
“We know that valacyclovir is very potent and has been shown time and time again to work better in the early stages of the disease,” said Busterla.
It is also believed to be a good way to slow the spread of the disease.
“Experimental studies have shown that by the time a horse has a neurological deficit, using valacyclovir will not change the outcome,” he said. “I see one of the big benefits of treating EHM horses with valacyclovir is the rapid reduction in virus shedding and viremia… Giving them medication quickly, within 24 or 48 hours, will greatly reduce the amount of virus shedding and viremia. Excretion is an enormous benefit in Reducing or preventing environmental pollution or its transmission to other horses.
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Busterla has concerns that valacyclovir and the third-generation drug, ganciclovir, could be overused by panicked riders who want to give it to apparently healthy horses as a way to protect them. It works for this, as long as the drug is administered effectively, but Busterla worries that it will eventually create viral resistance to the drug, similar to the way scientists have noted concerns about growing bacterial resistance to antimicrobials.
The evolution of veterinary understanding of the use of valacyclovir against EHV-1 would not have been possible without years of academic research by Maxwell and her colleagues. Some of these major studies have been funded by the Grayson-Jockey Club Research Foundation, which funds research benefiting horses of all breeds. Since its founding in 1940, the organization has provided more than $40 million in funding for more than 426 projects at 45 universities. Grayson-backed research has changed the way owners, farm managers, trainers and riders of all disciplines manage horses in times of wellness, illness or injury. Learn more about her current projects here.
In Dowling’s case, the drug worked as it was supposed to – and although there were horses permanently incapacitated by euthanasia, they had no further problems on their farm.
“I tested my horse after ‘stressful events’ like colic surgery and after a 100-mile endurance ride,” she said. “I have never been able to show any shedding in any of my horses. I have brought in naive horses and they have not become infected. The mare that has been in reeling has given me two nice foals in the next two years.
“We’ve never seen an outbreak here like we did in 2008 (more than 100 horses in the barn (Index) tested positive and three were euthanized),” said Dowling. “The cost of the drug is what stops most horse owners here. But if the horse is valuable enough, it seems like a good idea to try to protect it from what can be devastating consequences of HCV infection.