Focusing on the Flu

Equine influenza can strike a horse regardless of time of year. Competitive horses which attend events where other horses are congregated or horses which live in large boarding barns are particularly at risk. Not every horse which contracts a viral infection will display overt clinical signs, but such a horse might then be a carrier capable of infecting less immune-competent individuals. In this way, a horse which has never left your property could become infected from a non-symptomatic carrier.

Horses at greatest risk of developing influenza are those which are less than three years old. In an outbreak, about 75% of these youngsters get sick. Older horses might have developed some natural immunity over the years that protects them.

Any respiratory inflammation sets up the potential for that horse to develop chronic obstructive pulmonary disease (COPD), also known as heaves. Moldy hay is one means of starting this chronic disease, but damage from respiratory viruses is another common cause of this airway disability.

In a National Animal Health Monitoring System (NAHMS) Equine Study of 1998, it was found that respiratory problems were the third leading cause of days of lost work in the horse population. In general, prevention is the key to good airway health, but even in the best of circumstances, horses do develop respiratory infections.

Clinical Signs

How do you discern if a respiratory problem is brewing? You might notice your horse looking listless. There might be a discharge from his nose or eyes. He might start coughing at the slightest provocation.

You can do a few things to help track down the source of the problem:

Temperature--The first order of business is to obtain a rectal temperature to check for a fever. (Normal rectal temperature in an adult horse ranges between 97-101°F).

Allergens--Determine if something that could stimulate an allergic respiratory condition is present. Are you feeding a new batch of hay, or have recent bales come out of the back of the stack or from an area that might have gotten wet or absorbed moisture from the ground? Shake out a few flakes and see if a hazy dust rises from the pile. Does it have a musty odor like you'd expect with mold? Can you see matted or discolored areas in the hay that might be mold? Is the hay dusty?

Moving from hay, are there strong ammonia fumes in the barn due to urine-soaked bedding? Is there a riding arena adjacent to the stalls where horses would be exposed to airborne particulates and dust? Remember, having compromised lungs increases a horse's susceptibility to viral attack.

Discharge--Does your horse have a nasal discharge? What does it look like? A clear and watery discharge is often considered normal and is not necessarily a problem. A horse with a viral infection might exhibit a colored discharge, if any. An opaque, white discharge often indicates mucus and is related to inflammation in the airways, sometimes associated with an allergic condition. A colored discharge (cream, yellow, or green) usually makes one suspicious of bacterial infection.

Accurate Diagnosis

You won't always get an exact diagnosis for all cases of respiratory infections in a horse. A general diagnosis of "the flu" is often applied to any coughing horse with a fever and nasal discharge. However, a litany of equine respiratory illnesses referred to as "the flu" includes infection with equine herpesvirus (rhinopneumonitis), adenovirus, equine viral arteritis virus, Streptococcal spp. (S. equi, S. zooepidemicus, S. pneumoniae), as well as equine influenza. Historically, testing has not been done to confirm an exact respiratory disease diagnosis because treatment would likely not change, and a virus generally runs its course within a couple of weeks. However, accurate diagnosis can help indicate whether your veterinarian's protocol for a specific disease might need to be changed.

To accurately identify what is causing the problem, it is necessary to run paired serology on blood samples taken two weeks apart. Due to the slow turn-around time on these diagnostic tests, they do not lend themselves to common use. In the past, owners have opted for testing only if the horse's illness is unresponsive to treatment.

However, a stall-side virus detection test for equine influenza is now available to run on nasal swabs prior to administering any treatment. Results from this stall-side test only take 30 minutes to confirm or rule out equine influenza virus as the problem. If there is a large population of horses at risk, it might be best to identify the source of the illness early on so a strategy can be planned to minimize sickness throughout a barn.

Isolation and Control

A sick or coughing horse or a horse with a fever should be isolated from others if the problem is induced by a viral or infectious bacterial infection. Your veterinarian can advise you of an appropriate therapy, as each case is individual in how it should be handled.

Incubation of respiratory viral infections in horses generally takes two to seven days; it takes that long for a horse to start showing clinical signs once exposed. An infected horse will shed virus for at least seven days, although studies have indicated that horses regularly vaccinated for that disease might only shed for a day.

It is possible that if a horse were previously exposed or vaccinated, he might not show clinical signs but still shed virus. This poses a risk to a susceptible population, and hence an epidemic outbreak can occur. It probably is prudent to implement a two-week quarantine on healthy horses returning from interaction with a large population of horses.

Efforts should be made to minimize communal use of buckets, rakes, tack, and equipment. These items serve as vectors for transmission of viral particles. The person tending to the sick horses should care for them last in the chain of chores, then clothing should be changed and careful hygiene measures followed.

Treatment consists of supportive care to control fever and to keep the horse feeling well enough to continue eating and drinking while the virus runs its course. To reduce fever, sponge cool water on the neck and chest if the fever exceeds 103°F, remove the horse from direct sun, and administer non-steroidal anti-inflammatory medications (such as dipyrone, phenylbutazone, or flunixin meglumine).

A horse with a respiratory infection should be allowed at least three weeks for healing before being put back to full work. Severe cases might take even longer. This enables the airways to regain normal function without suffering a relapse.

Clean Air Strategies

Clean air is a definite part of the recipe for tending to a horse with respiratory problems. It is also an excellent strategy for maintaining good airway health. When possible, it is best for a horse to be housed outside where the air is fresh, rather than indoors. Feed hay off the ground rather than in chest-high feeders. Head-down eating enables a horse to clear dirt and dust from his nostrils and airways rather than inhaling irritating particulate matter into the lungs. Shake open flakes of hay and if the hay is at all dusty, soak it thoroughly with water before you feed it. This holds down the dust and spores, keeping them from further irritating the airways.

Immunization Strategies

Athletic horses depend on a huge lung capacity to fuel the muscles with oxygen to maintain exercise. One of the biggest challenges to managing an equine athlete is maintaining respiratory health. Respiratory viruses readily circulate throughout a horse population. To protect against the flu, a minimum program of twice annual boosters of equine influenza vaccine is recommended to ensure good antibody protection. Influenza vaccines are typically given two to four times a year; the frequency depends on risk of exposure. The more stress, the more rigorous the competition schedule, the more travel, the more encounters with other horses, the more frequent should be the vaccination plan.

These vaccines don't guarantee that your horse won't get sick from the flu or contract other respiratory viruses, but a high antibody titer in the bloodstream minimizes the degree of illness your horse experiences if he is exposed. Influenza vaccine imparts relatively cheap protection against the sniffles and coughs that can pull your horse out of work for weeks at a time. In highly trafficked barns and for horses which experience a lot of travel and stress associated with competition, you should vaccinate at least three or four times a year against influenza. Any viral infection of the respiratory tract makes a horse susceptible to bronchopneumonia or COPD. Such secondary problems could have repercussions affecting performance far into a horse's athletic future.

As an organism, the influenza virus is highly adaptable; it mutates in response to environmental and host pressures in a process known as antigenic drift. This phenomenon necessitates the use of up-to-date vaccines that are developed to counteract the current strains of circulating influenza virus. Virus isolates from outbreaks are essential for the updating of vaccines.

Currently some influenza vaccinations are available in an intranasal form, which doesn't seem to cause any adverse reactions. The intranasal format might afford a high level of protective immunity by blocking virus or bacteria at its site of entry, the lining of the respiratory tract. Intramuscular influenza vaccinations aren't thought to be as effective in protecting against influenza infection as we would like, but they do have a good track record of suppressing the degree of infection and the development of clinical signs.

Rules of thumb apply to get the most punch from your vaccines regarding disease protection and the least inconvenience to your training or competition schedule. Ideally, your competition horse should receive vaccine boosters at least two to four weeks prior to travel, competition, or a known risk of exposure. This gives him an opportunity to develop good antibody protection against worrisome disease, while also giving him a chance to recover from transient muscle soreness or fever should such be induced by an intramuscular vaccine.

In the face of an outbreak, it might be worthwhile to vaccinate healthy horses, especially if they are prevented from contacting sick horses. The immune system of horses with a previous vaccination history for influenza should respond with protective antibodies within seven days.

For the equine influenza vaccines given intramuscularly, current recommendations for implementation of the initial vaccine series are to begin a series of three monthly immunizations starting no earlier than nine months of age or else as late as possible in time to complete the series before the weanling goes to sales or training. The primary series is then followed with boosters every three to six months. For the intranasal vaccine, use a primary series of one or two doses followed by boosters every four to six months.

So, the bottom line for minimizing the risk and severity of equine influenza for your horse(s) includes:

  • Manage your horse's environment to minimize dust and allergens;
  • Vaccinate him as needed, depending on his exposure to transient horse populations (consult with your veterinarian);
  • Isolate incoming horses if possible for two weeks, and isolate any sick horses if possible;
  • Observe your horse carefully, noting any changes in behavior, nasal discharge, or respiratory function; and
  • Allow affected horses a full three weeks to recover before returning them to full work.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her recent book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care (available at or by calling 800/582-5604). She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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