Vigilance and Vaccination

By D. Craig Barnett, DVM

Unless your horses live in Iceland, Australia or New Zealand, or are completely isolated from other horses, equine influenza will eventually attempt to invade your barn. Highly contagious and swiftly progressive, the equine influenza wirus usually rides in on one infected horse and then gallops rapidly through the stables in a matter of hours or days, leaving you with a barn full of feverish, coughing, miserable animals----and exacting a high cost in terms of both time an money.

Simply put, equine influenza is one of the most common infectious respiratory diseases to strike the horse. Except as noted, it is seen in virtually all breeds, in either sex, and at all ages (with youngsters ages 2 through 5 being most susceptible in almost every country around the world. Today's horses are highly mobile, on both a national and international scale, which contributes to the spread of this disease. Breeding farms, training facilities, horse shows, race tracks, and other venues where high numbers of animals come and go and are densely housed run a greater risk of infection in the population. However, because equine influenza does not constantly circulate, but is instead introduced sporadically  and then is eliminated with relative speed by the equine immune response  tow weapons deal quite effectively with this culprit: vigilance (as in the quarantine of new horses, the scrupulous cleaning of tack and equipment, etc.) and vaccinations (when given often enough and before exposure).

History and Change

The equine influenza virus is divided into two major subtypes: A1 and A2. because subtype A1 has not been seen in horses in the United States for at least the past 15 years, we are left to deal with subtype A2, which, in turn, has mutated into two distinct lineages: American-like viruses and Eurasian-like viruses.

To further complicate matters, the virus constantly changes its surface proteins, creating an ever-evolving "disguise" in an effort to evade and "fool" a horse's immune system, thereby allowing the virus to pass into the animal unrecognized. Known scientifically as antigenic drift, this naturally-occurring phenomenon from time to time may inhibit vaccines from eliciting adequate protective immunity, as it reduces the degree and duration of protection conferred either by previous infection or by vaccination. The good news is that, unlike human influenza viruses, the antigenic drift in the equine virus is much slower. Therefore, equine influenza vaccines have not had to be updated nearly as often during the last decade in order to remain effective.

Clinical Signs & Treatment

Horse owners and managers should watch for the following clinical signs of equine influenza, especially when animals are housed in close quarters and/or are transported often, such as to horse shows, racetracks or breeding barns:

·Fever (from 102.5 to 106.5 degree F)

·Frequent dry cough

·Lethargy

·Myalgia (muscle weakness)

·Serous nasal discharge

·Anorexia ("going off feed")

·Dehydration, and

·Predisposition to secondary bacterial infections.

Symptoms may vary in severity within an exposed population based on each individual animal's general health, the dose of virus the horse, and the level of immunity in its system. Always consult your veterinarian promptly if a horse in you care exhibits one or more of these clinical signs. It is usually beneficial to try to definitively diagnose the presence of an equine influenza infection. In so doing, certain measures  such as isolation of sick horses and/or vaccination to boost immunity  might be promptly and properly administered. A diagnostic test currently marketed for human use can be used by your veterinarian on your horse right in the stall and provides results within 15 minutes.

Although the virus itself will not respond to antibiotics, and needs to run its course until the horse's immune system succeeds in killing it off, symptoms can be treated with other therapies, bringing some comfort and relief to the horse as it recovers. Nonsteroidal anti-inflammatory drugs, such as phenylbutazone ("bute") or flunixin meglumine (Banamine), might be administered. In addition, antibiotics will have some effect on any secondary infections that occur in relation to the virus.

Finally, a rest period of at least two to three weeks  preferably in quarantine  gives the horse a chance to fully recover its health and energy, and for the ciliated respiratory epithelial cells in the affected upper respiratory tract to regenerate and heal. A good rule of thumb is one week of rest for every day of fever. Exercise should not be resumed for at least one to two weeks after clinical signs have completely abated. Exercise without appropriate rest may lead to secondary infections or complications such as pleuropneumonia. Of course, this down time greatly affects the business bottom line, as not only is recovery time lost, but additional weeks of training  especially in the case of competitive show and race horses  may be needed before the horse regains its previous form.

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