Month: August 2023

Possibly the only thing more unsettling than being a horse owner experiencing an infectious disease outbreak on their farm is to be a veterinarian who experiences one.

That was the case for Dr. Trisha Dowling, professor of veterinary clinical pharmacology at the Western College of Veterinary Medicine in Saskatchewan, Canada. In 2008, Dowling took a horse and a pony off her farm and trailered them to a local lesson barn to use their indoor arena. Neither the horse nor pony encountered any other horses while they were there.

A week later, one of Dowling's horses colicked with unusual symptoms; then the pony fell ill. At first, Dowling thought the pony had contracted rabies and tried to manage the disease unsuccessfully. Two weeks after that, another horse fell ill and this time, she tested the horse for equine herpesvirus (EHV).

“Eventually, six of my horses developed neurological signs,” recalled Dowling. “All survived but one mare remained slightly ataxic for the rest of her life. My youngest horse only developed a fever and a unilateral uveitis. One of my neurological horses developed bilateral uveitis.”

There are a few 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 into equine herpes myeloencephalopathy (EHM), which is characterized by neurological symptoms and can quickly turn intense. If a horse develops advanced neurological signs, the disease can prove fatal.

EHV can easily be spread through nasal secretions, and it's unknown how long it can live on shared surfaces or hands.

EHV-1 is a major concern at 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 spontaneously begin shedding it through their nasal secretions if they're under stress. It may also be true that a small percent of the population could be “super spreaders” who can carry and shed huge amounts of the virus, sometimes without appearing to be ill themselves. This makes the virus very hard to control.

Dowling contacted Dr. Lara Maxwell, professor of physiological sciences at Oklahoma State University. Maxwell has made a study of pharmacology of many equine drugs, as well as their effectiveness against specific diseases. Part of Maxwell's research has dealt with the use of the antiviral valaciclovir against EHV-1, and she suggested Dowling give it a try.

Valaciclovir is the second generation in a closely-related group of drugs that have been tried for EHV-1; acyclovir was its predecessor but had poor bioavailability and had to be given by injection. The oral valaciclovir is easier for laypeople to give, and much more of the drug got to the horse.

“One of my endurance horses was highly rated and I was hoping to make the Canadian Equestrian team for the 2010 World Equestrian Games,” said Dowling. “Dr. Maxwell recommended the treatment with valacyclovir as soon as possible. Basically, I bought all the available supply of the drug in Saskatoon ($4,000 worth) and treated her. She was the only one of my horses that did not develop any signs of EHV even though she tested positive. We did end up as alternates for the team in 2010 – oh, well. But it was worth the cost to me to protect the most valuable of my horses against the clinical signs of EHV.”

Dr. Nicola Pusterla, professor of medicine and epidemiology at the University of California-Davis, said that we now know valaciclovir can be a great weapon against an EHV-1 outbreak like Dowling's – if it's given in the right context. Studies have since shown that if you give it to a horse who's already exhibiting the neurological signs classic of EHM, you probably won't do much to change the outcome of the case. It's also a good protective option if it's given to a horse before they're exposed to EHV-1, but most of the time, horse owners don't know they're taking their horse into a facility where another animal is shedding the virus.

Where it is useful, according to Pusterla, is treating horses who may be showing the earliest signs of illness but for whom neurological symptoms haven't shown up yet – those with fevers and mild lethargy.

“We know that valaciclovir is very potent and repeatedly shown to work best in the early stages of disease,” said Pusterla.

He also thinks it's a good way to slow an outbreak.

“Experimental studies have shown that by the time a horse has a neurological deficit, the use of valaciclovir will not alter the outcome,” he said. “Where I see one of the huge benefits in treating an EHM horse with valaciclovir is the rapid decline of viral shedding and viremia … putting them on a drug which rapidly, in 24 or 48 hours, will significantly reduce the amount of viral shedding and nasal secretion is a tremendous benefit in reducing or preventing environmental contamination or transmission to other horses.”

Pusterla does have concerns that valaciclovir and the third generation of the drug, ganiciclovir, can be overused by panicked horsemen who want to give it to outwardly healthy horses as a way of protecting them. It works for this, for as long as the drug is being actively given, but Pusterla worries it will ultimately create viral resistance to the drug, similarly to the way scientists have noted concerns about a growing bacterial resistance to antimicrobials.

The evolution of veterinary understanding of valaciclovir's use against EHV-1 wouldn't have been possible without years of academic research from Maxwell and her colleagues. Some of those key studies were funded by the Grayson-Jockey Club Research Foundation, which funds research that benefits horses of all breeds. Since its foundation in 1940, the organization has given more than $40 million to underwrite over 426 projects at 45 universities. Grayson-supported research has changed the way owners, farm managers, trainers, and riders of all disciplines manage horses in times of wellness, illness, or injury. Find out more about its current projects here.

In Dowling's case, the drug functioned as it was supposed to – and despite having horses who had lasting deficits from EHM, she hasn't had any more problems on her farm.

“I tested my horses after 'stressful events' such as a colic surgery and after 100-mile endurance rides,” she said. “I was never able to demonstrate any shedding in any of my horses. I brought in naïve horses and they did not become infected. The mare who remained ataxic gave me two nice fillies in the next couple of year.

“We have not had an outbreak around here like the 2008 one (over 100 horses at the [index] barn tested positive and three were euthanized),” Dowling said. “The cost of the drug is what would stop most of the horse owners around here. But if the horse is valuable enough, it seems like a good idea to try to protect them from what can be devastating consequences of EHV infection.”

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