Fighting Equine Influenza

A recent study may change the industry’s way of thinking.

by DEBRA GINSBURG

You are a responsible horse owner. Your broodmares and young stock receive all the necessary inoculations and are kept up to date on their vaccines. Then the flu bug hits your farm and, one by one, your young horses in training all come down with the disease. You are perplexed. Surely, all the shots they received as foals had to protect them from getting equine influenza. What went wrong?

Unraveling the effectiveness of equine flu vaccines is not as complicated as it first may appear. Like its human counterpart, equine influenza viruses mutate quite rapidly and not all vaccines are totally effective against every strain of flu that is out there. People are recommended that they receive their annual flu shot in September so that their immune systems have sufficient time to build up adequate antibodies against the disease when flu season hits in December. The same case could be made for horses.

As often as not, the virus undergoes a mutation so that the strain of flu you’re vaccinated against in September is not the same disease that hits you during the Christmas holiday season. You lie in your bed, sniffling, coughing, sneezing, shivering, sweating and cursing under your breath how that shot did not protect you from the flu and vow never to get another flu vaccine as long as you live!

"Flu vaccines may not be perfect, but they are the most inexpensive method we have of preventing serious illness," said North San Diego area veterinarian Paul McClellan. "I’m a very firm advocate for having horses vaccinated, even through their adult lives."

It has been accepted practice through the years to start inoculating foals against influenza as early as 60 days of age, but a study conducted by the University of Kentucky Equine Research Center may soon change that way of thinking.

Researchers at the University of Kentucky discovered that most young foals never get the flu because they received large doses of maternal antibodies through their mothers’ colostrum shortly after birth. These maternal antibodies are the newborn’s first line of defense against viruses and they begin to wane as the foal starts building up its own immune system, usually at about six months of age. The University of Kentucky team learned that these maternal antibodies may actually interfere with the vaccination of foals and render commercial vaccines ineffective, especially if they are administered to foals under six months.

Dr. Thomas Chambers, who heads up the influenza unit at the Gluck Research Center, and his graduate student, Dr. Robert Holland, had the assistance of Lexington, Ky., practitioner Dr. H. Steve Conboy, vice-chairman of the University of Kentucky Equine Research Foundation. Together, they collected blood samples of more than 187 foals whose dams were vaccinated against influenza at about 30 days prior to foaling.

The foals were then divided into six groups and vaccinated with the standard two doses of influenza vaccine at two, three, four, five and six months of age. From this sampling, the research team determined that the maternal antibodies prevented the foals from getting a good immune response from the vaccine, even when they were eight months old.

In order to determine if this lack of response was due to an interference of the maternal antibodies or if there was actually a problem with the vaccine, the research team then vaccinated two-to-three month old foals from unvaccinated mares who showed no antibodies for influenza. Surprisingly, these foals exhibited a good antibody response to the vaccine. These findings led them to two conclusions: one, that young foals can respond to inactivated flu vaccines, and, two, that maternal antibodies actually do interfere with a foal’s flu vaccinations.

This study was presented at the American Association of Equine Practitioners’ 1997 annual convention in Phoenix, Ariz. Based upon their findings, the University of Kentucky team is currently working on a new study to determine how to overcome the maternal antibody interference to the influenza vaccine. They are also developing a more effective immunization protocol for equine influenza. This new schedule is comprised of three doses, one each at eight, nine and 10 months of age.

Despite these surprising new findings about equine flu vaccine, many farm managers around California are happy with their current vaccination schedules.

"We are absolutely faithful to our vaccination schedule," said Marianne Dempsey of Longview Stud in Los Alamos. "We know it is not foolproof, but it does cut down on the severity of the disease."

"If our horses do get sick, they will just be down for two or three days," explained Rick Taylor of Golden Eagle Farm in Ramona. "Without the vaccine, the flu can lay them low for 10 days or more."

Ellen Jackson of Victory Rose Thoroughbreds in Vacaville actually has her foals vaccinated a little ahead of schedule—at about 30 days of age—and feels this schedule is very effective.

"Our babies would develop ‘the snots’ between 30 or 45 days," she said. "Giving them a flu vaccine at 30 days would often ward off this condition. They would get a snotty nose for about two or three days maximum. Usually a case of the snots would stay with a foal through the entire summer and makes it more vulnerable to a more serious respiratory virus."

While it is important to having young horses vaccinated against flu, maintaining the health of broodmares is first and foremost in Dempsey’s mind. Pregnant mares at Longview receive a combination influenza/rhinopneumonitis booster in the late fall months as foaling season draws near. This schedule helps to protect their unborn foals in utero. as well as themselves from contracting a disease that could cause them to abort their foals.

"We also need to protect our own horses from any kind of viruses an outside mare might bring to our farm," she said. "If we have a mare coming in to be bred to one of our stallions and the owner doesn’t know her vaccination history, we give her a vaccine and isolate her from other horses when she first enters the farm, just to be safe."

For instance, if a mare has a foal that has died of unknown or undetermined causes, it is best to have an autopsy performed on that foal to ensure the health of the mare. If the foal had been infected with rhinopneumonitis, the disease might have caused flu-like symptoms in the foal but will cause pregnant mares to abort. Therefore, it is vital to find out just why the foal died and discover the kinds of organisms responsible for its death because of what the foal might have passed on to other horses.

Once a horse becomes a yearling, it receives an annual booster against influenza. The new schedule proposed by the University of Kentucky study has the flu vaccine operating at maximum efficiency at a time when a young horse would be entering training and placed in an environment where contagious diseases run rampant.

Jill Fischer of Madera Thoroughbreds and Pat Thompson would generally give their horses a flu/rhino booster about two or three weeks before they leave the farm to go to a training center or racetrack, especially if the animal has been turned out at the farm for some time.

"We’ve had a very good success rate with this program," said Fischer of the booster program used at Madera. "It’s been very effective in keeping our horses from coming down with all the usual stuff when they are at a racetrack or sales facility."

Leigh Ann Howard of Valley Creek Farm always gives her in-training yearlings (18-20 months) a dose of flu/rhino vaccine every 30 to 45 days. Once they turn 2 1/2 years of age, they go on the every 60 day schedule.

"This is often looked upon as overkill," she said, "but I rarely get the dreaded ‘cough’ in my barn. When they do get it, they don’t get very sick and seem to recover quickly."

"There are so many strains of flu out there that you can vaccinate up the ying yang and not catch it all anyway," explained Bonnie Dotson, who manages Solar View Farm in Nuevo. "I’ve found if horses are going to get sick, they’re going to get sick no matter how much you inoculate them."

In addition to their regular series of vaccinations, horses at Solar View also receive Eqstim, a booster for the immune system.

"Sometimes a horse doesn’t look just right," Dotson added. "Its coat may be a little off, but you can’t seem to find what’s wrong. You hit them with this immune booster and it gets them to snap right up. You can even give it to a horse that’s sick to stimulate its immune system to better fight off the virus."

Clydene Boots of Westview Farm near Murrieta gives a flu/rhino booster and an immune booster to all her young horses leaving for sales or training facilities.

"I feel that Eqstim is the most important thing you can do for a horse," she said, "since it makes the animal more able to resist whatever diseases he may encounter. This is especially important, since the stress of such events can leave a horse particularly vulnerable."

Despite debate over the optimum time to start protecting horses against influenza, most farm managers and equine practitioners agree that it is far better to vaccinate than not.

"An influenza epidemic can really shut down a racing meet or horse show in no time because the disease is so virulent and contagious," Dr. McClellan added. "The leading drug companies and research centers are always monitoring new strains of flu and refining their vaccines accordingly. I highly recommend to all my clients to vaccinate their horses on a regular basis. It’s the best kind of prevention they can get for their money."