March 2010 Articles

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When horses get the flu

When Horses Get the Flu (Can You?) Story by Eleanor Kellon, VMD Source: MyHorse.com
It's unlikely. But if your horse comes down with influenza, you'll need to know how to care for him to prevent any long-term complications. 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 & infect dogs, causing very severe & 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 & 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 & 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 problem. 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 & are in close contact, such as at shows, racetracks & 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, & outbreaks can & 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 & vans, & indoor show facilities are particularly high risk environments. Foals with no immunity to influenza viruses, & older horses or sick horses with poor immune responses, are the most vulnerable to infection. Otherwise, the combination of vaccination & 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 & Europe, but many other countries are largely free of flu.
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.
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.