Equine Influenza

Equine influenza is a common disease that causes acute respiratory signs. In nations with extensive horse breeding and racing industries, it is currently considered to be the most economically important respiratory disease of horses. The prevalence of equine influenza is 50-60% in most field studies. It recently became possible to identify cases of equine influenza virus as opposed to equine herpesviruses 1 and 4 and equine arteritis virus via a nasal swab using a new diagnostic test. Of the three, the equine influenza virus was determined to be the single most important viral respiratory pathogen. Influenza has a high morbidity (illness) rate of greater than 90% in an outbreak, with a low mortality (death) rate.

The equine influenza virus has learned to evade the horse's immune system by changing its surface proteins so that the immune system does not recognize the virus. This is called antigenic drift.

The equine influenza virus is divided into subtypes A1 and A2. The A1 influenza has not been isolated in a U.S. horse in the last 15 years. The A2 versions of the 1990s have not had very much antigenic drift. However, the A2 equine influenza virus has been mutating into two distinct lineages--the American-like viruses and the Eurasian-like viruses. Equine influenza mutates at a much slower rate compared to human influenza. This means our equine influenza vaccines do not need to be updated as often as the human ones.

Controlling and Preventing Influenza

Here is a rule of thumb that I tell my clients: For every day the infected horse has a fever, he should be out of work for a week. So if a horse has a fever for three days, then he should be rested for three weeks. It's easy to remember, and clients like the rule. This rest period after influenza is very helpful because it takes two to three weeks for the respiratory ciliated epithelial cells (those lining the respiratory tract) to heal.

Vaccination is critical for controlling and preventing equine influenza. There are two types of equine influenza vaccines on the market in the United States: Inactivated and intranasal cold-adapted modified-live. These vaccines need to be administered before the horse has a risk of exposure to equine influenza (for example, vaccination with the second booster seven to 14 days before a competition). These vaccines help prevent infection and reduce disease if the horse gets equine influenza. A good immunization program protects the horse by reducing the amount of virus shed after infection, and lessening the duration and severity of the clinical signs in an infected horse.

Pregnant mares should be vaccinated four to six weeks before they foal using an inactivated equine influenza vaccine to booster their antibody level to pass to the foal via colostrum. I recommend using an up-to-date equine influenza strain (1990s strain) in this booster vaccination. This updated strain given to the mare will help protect the foal for the first six months of life if he suckles enough colostrum in the first 24 hours.

We've come to believe that Mother Nature's system works well-- horses under six months of age rarely get influenza unless their dams are not vaccinated or lack antibodies needed for protection against the specific circulating flu strain. The U.S. equine influenza strains did not mutate much in the 1990s, so vaccination with a strain of a recent '90s inactivated vaccine should have cross protection. We have also discovered that maternal antibodies keep young foals from developing a good antibody response on their own to immunization with influenza vaccines.

It is now recommended that a foal's first set of vaccinations against equine influenza should be after six months of age, with the foal receiving a series of three shots given one month apart. The latest information about maternal antibodies is that they can interfere with foal vaccinations for a prolonged period; therefore influenza vaccination can be delayed until six to nine months of age. Even so, it takes at least three vaccinations with inactivated equine influenza one month apart to properly prime the foal due to maternal antibody interference. An alternative method that seems to protect well is to vaccinate foals with maternal antibodies using the intranasal cold-adapted modified-live intranasal vaccine at seven and 11 months of age.

Adult horses should receive an annual booster with a vaccine using a '90s strain of equine influenza. If a horse is in competition or stabled among many other horses, he might need more frequent vaccinations (the general recommendation is every six months). There have been some great advances in equine influenza research recently, with hopes that one day good management practices and vaccinations will make equine influenza a disease of the past.

About the Author

Robert E. Holland Jr., DVM, PhD

Robert E. Holland Jr., DVM, PhD, is a senior technical service veterinarian for Pfizer Animal Health. He served on the AAEP task force revising equine vaccination guidelines, and his research interests include virology, biomechanics, drug testing, and air flow.

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