By Heather Smith Thomas

Viral Influenza is one of the most common infectious respiratory tract diseases in horses, not only in the U.S but also around the world. Young horses, age one to five, are generally most susceptible. Older horses may have some immunity if they encountered the virus in the past.

INFLUENZA OUTBREAKS

Thomas Chambers, PhD, Reference Laboratory for Equine Influenza, at the Gluck Equine Research Center, University of Kentucky is part of an international expert surveillance panel for equine influenza.

“The panel meets every year to review the situation for the previous year,” Chambers said.

“One of the curious features about outbreaks is that their pattern is different in different parts of the world, and it seems to be associated with vaccination. In the U.S. there is a certain proportion of the equine population that gets vaccinated, and the disease is enzootic—which means it is always around, at a low level,” he said. Horses in the U.S. are likely to be exposed to this disease at some point.

“We generally don’t see huge outbreaks. We see a few outbreaks at racetracks, or at a veterinary hospital, that affect a small number of horses before they get it under control.”

These are small sporadic outbreaks, generally localized, although traveling horses can carry the virus and start another outbreak.

“This is what we’ve seen in the U.S. for a long time. By contrast, some countries have a very different pattern. India, for example has huge outbreaks–many thousands of horses–and afterward the disease disappears for 10 years or so. Then they have another huge outbreak, followed by another long period of quiet,” Chambers said.

“They don’t routinely vaccinate in India, so probably when they have a huge outbreak, just about every horse is exposed, and develops some immunity. This ‘herd immunity’ apparently stays in place for several years after. This seems to keep the lid on the disease for a period of time, even without vaccination.” Herd immunity eventually wears off and then there’s another big outbreak.

“Here in North America we have a few low-level outbreaks–nothing dramatic. It’s only been in the last eight years that vaccines have caught up with recommendations we made in 2010—that vaccines should contain both the Clade 1 and Clade 2 strains. Since the Florida Clade 2 strains circulate elsewhere (primarily in Europe) and because there is international horse traffic, we can’t expect that we only have to worry about Clade 1,” he said.

“Dr. Nicola Pusterla at the University of California at Davis identified some imported horses that were infected with the Clade 2. Fortunately they were quarantined, and the disease was caught before these horses could start any outbreaks in the U.S. But at some point there will probably be re-introduction of Clade 2, so owners and practitioners should use vaccines with updated virus strains.”

PREVENTING INFLUENZA

Chambers suggests testing for the virus if you have a sick horse with fever, cough and nasal discharge (typical signs of respiratory disease). “Try to get the horse swabbed [to test] as soon as possible and get the swab sent to a diagnostic laboratory. Do it as soon as you notice that the horse is sick. The longer you put it off, the poorer the sample will become. A test for the virus won’t be as sensitive.”

It’s also important to keep good vaccination records—the date when each horse is vaccinated, and with what product. “We need the specific product, not just the name of the company. The big manufacturers today market various products, not just the equine flu vaccine, but maybe five different kinds of flu vaccine, in different combinations with other vaccines. One type may be in combination with herpes, or tetanus, or Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE), and some combinations with West Nile. So record not just the date and vaccine company, but also which specific product was used.”

Vaccine is just one tool, since disease prevention is more complex than simply vaccinating. Biosecurity is also crucial, to keep disease out of a farm.

“Some biosecurity procedures are simple, such as just washing your hands. It’s also helpful to quarantine new arrivals until you are sure they are not infected. Quarantine can be your first defense against introduction of disease. Other practices like not using the same grooming tools, water buckets, etc. with multiple horses, are also helpful. Viruses, especially influenza, transmit easily from horse to horse and it doesn’t have to be direct nose-to-nose contact. The virus can be passed to another horse on grooming tools, or by someone using an endoscope on multiple horses without disinfecting it in between,” Chambers said.

It is easy to transmit disease, and also easy to kill this virus by disinfecting things between horses.

“Soap and water will kill the influenza virus, as will laundry detergent, bleach, Lysol, etc. It generally doesn’t last a long time in the environment but maybe lasts longer than we originally thought. We used to think it only lasts a few hours, but now we realize it may last a few days, or in some circumstances (shielded from heat and direct sunlight) it might last weeks,” he explains.

“Maybe 90% of it is dead within a few hours, but it could take much longer to kill that last 10%.” If a sick horse coughed into the hay or bedding, and another horse came along the next day or so and noses around in that hay or bedding, there might be risk.

Aliza Simeone, VMD (Director of Biosecurity and Assistant Professor of Clinical Infectious Diseases and Biosecurity, University of Pennsylvania School of Veterinary Medicine, New Bolton Center) says this disease shows up more often in winter, just as it does in other species and in people.

“It is spread by infectious droplets. When sick horses cough, they cough out a lot of the virus and it can also travel in drops of mucus or saliva. It can be transmitted directly to another equine or spread on hands that haven’t been washed, or clothing that’s been ‘snotted’ on by a horse,” she said.

“Simple steps like washing hands or using a good alcohol-based hand sanitizer between horses are effective. Avoid having to put your hands in horses’ mouths. I did some work at Thoroughbred tracks before starting my present job, and was happy to see how much more we are doing identification with microchips so we don’t need hands in mouths all the time to check tattoos,” she said.

“I recommend routine temperature-taking, especially for horses that move around a lot or are in heavy work, or stabled at racetracks or traveling to races. The more often we check temperature (usually once or twice a day is adequate) the sooner we know if something is going on. We may not know what it is, but the start of a fever can enable us to get that horse away from the others and keep a closer eye.”

Incubation period with influenza is short—just a few days—and a horse might be already shedding virus before showing signs or before it tests positive.

TREATMENT AND RECOVERY

Most horses recover with basic supportive care.

“If they have a high fever, your veterinarian may suggest medication to reduce fever and making sure they stay hydrated, and taking them out of work (full rest) to aid recovery,” Simeone said. “The trainer shouldn’t continue working that horse when it is sick, and for several weeks after,” she says. That kind of stress will slow their recovery and won’t benefit their training if they are having trouble breathing.

“There can be cases that are very mild, with just a day or two of extra nasal discharge. In some cases the horses don’t appear sick at all, or mild signs are overlooked, but these horses are still able to spread the virus,” Simeone said.

How long a horse might shed the virus depends on that horse’s own immunity. “Some will shed for a shorter time than others, but to stay within the window of safety we generally give a recovered horse a two-week period before it comes in contact with other horses. The recovered horse may still be coughing even after it is no longer shedding the virus.” This is similar to a person who has recovered from a cold but is still coughing and may still have decreased lung capacity, but that person is no longer contagious.

RESEARCH

“We are always interested in improving vaccines,” Chambers said. “My lab has done work on various new technologies over the years. There isn’t anything right now that will totally replace conventional vaccines; they do work, if they are given on the proper schedule. If you keep horses regularly boosted, the conventional vaccines work, and are simple,” Chambers said.

“Practitioners like the fact that they can just give a shot in the neck, and horses are fairly used to getting shot in the neck. There are alternatives, and we’ve done a lot of work on some of those alternatives (such as intranasal vaccines) and in some respects we think they work better,” he says.

“Future vaccines may be different, however. In the human field there’s been a lot of work on what is called a universal vaccine (against the basic virus itself rather than the various strains). Within a few years we may be looking at a universal influenza vaccine. We need to make sure it is effective, however,” Chambers said.

“If it works, we would not need to worry about updating the virus strains. A universal vaccine induces the horse to make antibodies that will neutralize the virus—but not by attacking the specific strain. The virus has a defense of always changing, and the idea is to not go after those changes, but attack the virus itself. ” he says.

VACCINES

There are several types of vaccines available—inactivated (killed virus) products administered intramuscularly, the modified live virus intranasal vaccine, and the canary pox vector vaccine which is administered intramuscularly.

“I think an influenza virus vaccine administrated by the intranasal route should be very effective because it should mimic the immune response that the actual influenza infection stimulus,” says Chambers.  “In particular it should give good local protection in the nasal mucosa which is the route of entry for infection.”

An intranasal vaccine has some advantages but other vaccine types have their own advantages.  “The conventional vaccines work, if you keep horses regularly boosted,” he says.  It is important to follow label directions regarding administration, boosters, and vaccination schedules.