Sixty percent of all respiratory problems are attributed to influenza, an airborne virus that makes its way into the horses system through his nose.

With all the concern over “bird flu” being transmitted to people, influenza is very much in the news these days. And although horses can get influenza, too, the good news is, you’re not likely to “catch it” from your horse. Even so, there are things you’ll want to know about equine influenza.

As a rule, influenza viruses have strict requirements for survival and transmission. Horses, people, birds, etc., are generally only infected by the specific strains that affect that particular species. However, on occasion, viruses do develop the ability to “jump” species, like the bird flu did in Asia.

Interestingly enough, an equine outbreak of severe influenza in China in 1989 came from a strain more commonly found in birds. Twenty percent of the infected horses died. Equine influenza virus has also been found to jump species and infect dogs, causing very severe and potentially fatal pneumonia.

When a species jump happens, the disease tends to be particularly severe. Unfortunately, because the immune system of the new host species has never been exposed to it before, it has no antibodies or specific cellular defenses to recognize and destroy the invader. The virus, in effect, gets a head start of 10 days to two weeks on the immune system.

Until the sophisticated arms of the immune system can get up to speed, the work of fighting the virus falls to the primitive immune system. That means the new host responds with exaggerated inflammatory responses. The body’s extreme reaction makes the animal or person feel very sick and can damage more tissue than a more sophisticated immune reaction would.

Fortunately, it’s a relatively rare occurrence for influenza to jump species. However, influenza viruses within their own species can cause enough problems as it is.

Flu Facts

Influenza spreads by air, so horses kept together in enclosed spaces are more susceptible than those kept outdoors. Foals with no immunity to influenza viruses, and older or sick horses with poor immune responses, are most vulnerable. Lung damage from influenza can take up to 30 days to completely heal. Minimizing stress will help your horse’s immune system fight influenza. If your horse will be traveling or coming into contact with outside horses, consider vaccinating, and discuss the options with your vet.

Catching the Flu
A horse catches the flu by inhaling virus particles that become airborne when an infected horse coughs or sneezes. Therefore, to contract influenza, a horse has to be relatively close to an infected horse.

The need for proximity explains why horses that are not exposed to other horses have little risk of coming down with influenza. Outbreaks tend to occur when horses are brought together in large groups and are in close contact, such as at shows, racetracks and inside boarding stables. The more horses you bring together, the higher the odds that one of them will have influenza and can spread it to the others.

There is no “flu season” per se, and outbreaks can and do occur year-round. However, just like with people, when horses are together in an enclosed area with poor circulation, the amount of virus particles in the air can rise sharply. Barns that are closed up tightly in the winter, horse trailers and vans, and indoor show facilities are particularly high risk environments.

Foals with no immunity to influenza viruses, and older horses or sick horses with poor immune responses, are the most vulnerable to infection. Otherwise, the combination of vaccination and repeated low-level natural exposures keeps most horses free of the disease unless they are in close quarters with a horse producing very large amounts of virus.

There hasn’t been a large-scale influenza outbreak in the United States since the early 1980s, but it could happen at any time. Epidemics tend to occur when the virus mutates in a way that it avoids triggering a “memory” immune response. When this happens, even vaccinated horses are unprotected. Epidemics also occur when imported horses bring influenza virus into a part of the world that had previously been free of the disease. The virus is well established in the United States and Europe, but many other countries are largely free of flu.

Influenza Vaccines

Influenza vaccines are among the few that have actually been tested in challenge studies. In a challenge study, horses are deliberately exposed to the virus after being vaccinated to see if they are protected. Intervet’s intranasal flu vaccine, FluAvert I.N., provides excellent protection against symptoms and the shedding of viruses by infected horses. This vaccine uses a live virus that is capable of entering cells (mimicking what happens in a natural infection), but the virus has been modified so it does not produce disease. Peak protection with this vaccine lasts for six months. But vaccinated horses still have partial protection and much less severe disease at the 12-month mark. All other available vaccines use killed virus, which does not actively infect cells and therefore produces less immune system stimulation.  In a study conducted in Canada (reported at the 2003 American Association of Equine Practitioners meeting) horses were vaccinated for influenza and other diseases using popular combination- and single-disease vaccines from various manufacturers. It found that there was an antibody response to all vaccines, but the response varied between the brands. Of the killed virus, intramuscular vaccines, the Calvenza EIV from Boehringer Ingelheim produced the highest antibody titer and provided the best reduction of symptoms when horses were challenged with live infectious virus four months after the last vaccination. All vaccinated horses had less severe symptoms than challenged, unvaccinated horses and were protected from weight loss. But horses that had received one dose of the intranasal FluAvert vaccine before their intramuscular flu shots showed the best protection of all.

Symptoms
Influenza symptoms are similar to any upper respiratory infection—but worse. Symptoms usually appear within three days of initial infection and begin with a high fever (as high as 106), loss of appetite, and a deep, dry cough.

The virus attacks the ciliated cells lining the respiratory tract. Ciliated cells have many small projections, like hairs, on their surface. Their job is to sweep mucus, dust and surface organisms up out of the lung. When these cells are lost, the lungs have no way to clear themselves.  They become vulnerable to the virus penetrating the tissues, as well as to secondary bacterial infection.

If a secondary infection does not occur, flu symptoms will usually peak about six days after infection, then begin to resolve. In uncomplicated cases, recovery may appear in about 10 days, but the damage to the lungs can take up to 30 days to completely reverse.

Some strains of equine influenza can invade other tissues, producing joint pain, muscle pain, and swelling of the legs and scrotum. The virus may even invade the heart muscle or cause colic.

Working the horse too soon after a bout of influenza can lead to chronic problems with sinus or throat inflammation, lung irritation (including the development of allergies and hypersensitivities), or heart damage. For this reason, it is always wise to rest a horse for a full 30 days following the onset of symptoms to give the body adequate time to heal.

Prevention and Treatment
How ill a horse becomes after exposure to influenza depends on many things, including general health and health of the immune system, whether the immune system has been primed by prior exposure to the strain of influenza (either by vaccination or natural exposure), the dose of virus received, and how aggressive the particular strain of virus is. The best protection is to:

•  Maintain a healthy and well-balanced diet, with adequate high-quality protein, to support the immune system.
•  Keep horses outdoors or in well-ventilated barns or enclosures.
• Avoid exposure to sick horses, and always quarantine new additions.

If your horse is at high risk of exposure, speak with your vet about maintaining a good vaccination program.

But remember, vaccines alone are not 100% effective. You still need to maximize nutrition and minimize stress so that your horse’s immune system stays strong.

If your horse does come down with influenza, you need to involve your vet to set up a treatment program, which includes medications to keep the horse as comfortable as possible. Antibiotics may be indicated in severe cases to protect against, or to treat, secondary bacterial infections.

General nursing care is your job, and is very important. Be sure to:

•  Protect from extremes of temperature, both heat and cold, but keep the horse in a well-ventilated area.
•  Keep dust to a minimum.
•  Use liberal amounts of mentholated ointment, such as Vicks Vapor Rub (or an equivalent generic) on the nostrils and throat.
•  Record how much the horse is drinking and eating, and ask your vet when she/he should be notified about significant reductions in thirst or appetite.
•  Take your horse’s temperature twice a day and record it.
• Report any change in nasal discharge from clear to colored, or in the cough from dry to moist, to your vet.
• Dunk hay to moisten or soak before feeding.

On Alert Worldwide

Following the emergence of a new influenza strain in Europe in the late 1980s and early 1990s, which caused a severe epidemic even among vaccinated horses, efforts to keep track of equine influenza have been considerably stepped up. The International Collating Centre (ICC), based in Newmarket, Suffolk, England, receives quarterly reports of equine influenza about virus activity from around the world. Identifying the virus strains that are circulating makes it possible to alert veterinarians to activity in their areas, including new and potentially dangerous strains. It lets manufacturers know when they need to add new influenza strains to vaccines.

Controlling fever, with medications as directed, helps keep the horse eating, but enough throat pain may remain to put your horse considerably off his feed. The horse is most likely to eat grass. If grazing for at least 16 hours per day in a quiet area isn’t an option, consider offering mashes.

Pelleted “complete” or “senior” feeds can be soaked into a mash. (Look for about 20% fiber in the analysis.). Other good mash ingredients are beet pulp, wheat bran, rice bran, hay pellets and alfalfa meal. Plain oats can be used and add to the appeal.

Experiment with consistencies ranging from mashed potatoes to soup. Add a teaspoon of salt per feeding to encourage the horse to keep drinking.