Equine influenza can cause misery on both a large and small scale. The virus (EIV) responsible for this respiratory infection spreads so easily that if just one horse comes down with it, you can quickly have a barnful of feverish, coughing, snotty-nosed horses on your hands. And although most will recover with no lasting effects, the recuperation period—a minimum of three weeks—can put a serious crimp in training and riding schedules.

Younger horses, aged 1 to 5, are most susceptible to influenza; foals will have temporary immunity if their dams were vaccinated, and older horses are more likely to have been vaccinated or to have developed some immunity from prior exposures to the virus. However, that might be changing. A recent— surveillance study of equine influenza in 2,605 horses, mules and donkeys around the United States between 2010 and 2013 showed that the disease appeared in “a higher-than-expected proportion” of horses in older age groups.

“With EIV being a traditionally young horse disease, it was somewhat surprising to see an increase in the numbers of older horses being diagnosed,” says Bryant Craig, DVM, technical services veterinarian with Merck Animal Health, which conducts the surveillance study in coordination with the University of California–Davis. “The equine influenza virus is constantly undergoing change. The latest changes to occur seemed to have made the older horses more susceptible to the disease. This study is still ongoing, and we are seeing the trend continue.”

Change is the only constant with equine influenza—the virus mutates readily, and the disease has potential to evolve over time. You can no longer just assume that your mature horse isn’t susceptible to the illness, and vaccination alone may not guarantee he won’t fall ill, especially if his boosters aren’t kept up to date. Understanding EIV, and how it spreads, is the key to protecting your horse’s health.

How the influenza virus works

EIV is one of many closely related viruses in the influenza family (Orthomyxoviridae). “This is the same family as all the other influenza viruses, including the ones that infect humans,” says Katie Wilson, DVM, MS, DACVIM, of Virginia–Maryland College of Veterinary Medicine in Blacksburg, Virginia. “Within that family are many different subtypes.”

The particular virus that affects horses is called equine influenza A type 2 (A/equine 2), subtype H3N8. Most influenza viruses are designated with this H-N format, with varying numbers: The H and the N refer to two different types of glycoprotein found on the surface of the virus: Hemagglutinin binds to receptors on the surface of the host’s respiratory tract and enables the virus to enter the cells. Neuraminidase enables the flu virus to exit the host cells to infect more cells. Both are antigens, molecules that stimulate an immune response in the host.

Currently, there are 18 known hemagglutinins and 11 neuraminidases, which can be combined in many ways. The numbers in the virus designation refer to the specific type of these antigens that are present. “Any combination of these makes each individual virus in each host species different,” Wilson says. For example, the infamous Spanish flu of 1918 was caused by H1N1. The “bird flu” outbreak of 2003 was caused by H5N1. With a few exceptions, most individual flu viruses can infect only one species of animal—a healthy person cannot catch influenza from a horse, at least not with the current varieties of equine influenza virus.

EIV, like its relatives in the influenza family, has two characteristics that make it difficult to control. For starters, it spreads quickly from host to host via airborne droplets. “The incubation period is very short—just a couple of days,” says Mark Crisman, DVM, MS, DACVIM, senior veterinarian for Zoetis, who also teaches at Virginia–Maryland College of Veterinary Medicine. “We’ve seen outbreaks sweep through a stable or barn, and in a teaching hospital, where within 48 hours every horse in the barn was coughing.”

The second notable characteristic of influenza viruses is their ability to change constantly. Over time, as the virus spreads from host to host, it undergoes small genetic changes—a process known as antigenic drift. At first, the changes will not be significant enough to affect the host’s immune response, but eventually so many may accumulate that the virus essentially become a new entity, one that requires the immune system to “start over” and make new antibodies.

In addition, flu viruses can change more abruptly: In a so-called antigenic shift different hemagglutinins and neuraminidases combine in novel ways in a virus. For example, pigs can be infected with both human and avian flu strains as well as the swine influenza viruses. If a pig is carrying more than one species of the virus at the same time, those viral genes may mix into a unique new form.

Both antigenic shift and antigenic drift can produce viruses that “jump species” into new populations with no immunity. EIV jumped to dogs in 2004, when an outbreak was reported in racing Greyhounds in Florida; H3N8 is now considered an endemic form of canine influenza as well. Earlier this year, a different canine influenza virus (H3N2, which arrived in the United States from Asia in 2015) spread among cats at an animal shelter in Northwest Indiana.

However, the equine influenza virus changes more slowly than do human influenza viruses. “Human influenza viruses mutate rapidly, and human vaccines need to be updated annually,” says Rob Keene, DVM, equine technical manager for Boehringer Ingelheim Vetmedica, Inc. “With different population pressures equine influenza changes more slowly because virus-horse interactions are less frequent than virus-human interactions.”

Human influenza viruses have higher mutation rates, Keene explains, because modern life—air travel and population density—makes it easier for the virus to pass quickly through many individuals. To survive, the virus must rapidly mutate to avoid the host’s natural immunity. Similar factors are at work with poultry and swine, which are usually kept on farms with populations numbering in the thousands or tens of thousands. In contrast, horses are generally kept in much smaller groups that are farther apart, so the equine virus takes much longer to travel among potential hosts.

The slower pace of change in the equine influenza virus means that the equine vaccines do not need to be updated every year. However, anti-genic drift in H3N8 has divided the virus into different subtypes around the world. “About 15 years ago there was a shift between the European and the American strains,” says Crisman. “These all originated from what we call the ‘Florida strain,’ and they broke into two separate groups that are termed Clade 1 and Clade 2. The Clade 2 strain predominately circulates in Europe, and the Clade 1 predominately circulates in the United States.”

Vaccines: The first defense

“Vaccination is still the most practical and economical means of EIV protection,” says Craig. The American Association of Equine Practitioners lists equine influenza as a risk-based vaccine, meaning that veterinarians may recommend it based on your horse’s risk of exposure to the disease.

Generally, horses in the high-risk category frequently travel to shows or other venues where they encounter large numbers of other horses. “Our surveillance program reinforces the knowledge that travel is a significant risk factor for respiratory disease in the horse,” says Craig, “highlighting the fact that those horses who spend time trailering need an optimal vaccination program to stay healthy.”

But even horses who leave home only occasionally have some risk of exposure. When evaluating your horse’s situation your veterinarian will consider several questions, including, “What is the level of training the horse is in? Training can cause stress on the horse and compromise his immune system. How frequently does the horse travel? And where does the horse travel?” says Meg Green, DVM, manager of equine and large animal veterinary services with Merial, Inc.

Currently, three general types of influenza vaccine are available:

• Inactivated (killed virus), given as an intramuscular injection. If a horse has never been vaccinated, he needs a two- or three-dose series of this formula to confer full immunity, the first two at an interval of three to four weeks, followed by an interval of three to six months before the third. After that, the horse will need boosters at six-month or one-year intervals, depending on his degree of risk.

“In the killed vaccine, the whole virus must resemble the live form of the virus enough to stimulate the immune response, but the vaccine cannot cause infection, and the vaccinated horse cannot shed the virus,” says Green. “The drawback is that this type of vaccine stimulates only humoral immunity, which allows for circulating antibodies within the blood, and makes for a slower immune response.”

On the other hand, says Craig, “These vaccines provide good protection for horses with moderate exposure to the disease and are found in many combination vaccine products.”

• Modified live virus (MLV), administered intranasally. These vaccines contain a live virus that has been altered so that it can replicate enough to stimulate an immune response but cannot cause disease. A single dose confers full immunity for at least six months.

“These induce both humoral and cell-mediated immune responses,” says Green. “Cell-mediated immunity allows for a cell that is infected with a virus to be destroyed before the virus can rupture that cell. As a result, it reduces the amount of virus that can infect other cells, and this limits the amount of infection and reduces shedding in the environment.” 

Administering this vaccine intranasally also means that it induces immunity at the same site where the virus enters the body—in the pharynx and nasal passages. “This type of vaccine provides the best immune response because it mimics a wild-type infection, so all aspects of the immune system are stimulated,” says Craig. “Its primary site of action is in the upper respiratory tract where the virus gains entry, stopping the disease where it starts its infectious process. This vaccine would be recommended for horses with greater exposure, such as boarding stables, racetracks, performance horses or those traveling frequently.”

• Recombinant (canary pox vector), given as an intramuscular injection. These vaccines splice together (“recombine”) portions of DNA from the EIV into the DNA of a harmless canary pox vector. “Recombinant vaccines allow for the safety profile of a killed vaccine while eliciting the same immune response of a modified live vaccine as it stimulates both the humoral and cell-mediated immunity,” says Green. This formula requires a two-dose series to confer immunity in an unvaccinated horse, followed by boosters every six months.

Any of these products are effective when used as directed. “I think that an intranasal vaccine has some inherent advantages but other vaccine types have their own advantages. The conventional vaccines do work, if you keep the horses regularly boosted,” says Thomas Chambers, PhD, of the University of Kentucky’s Maxwell H. Gluck Equine Research Center, who emphasizes that following up with appropriate boosters is critical the first time a horse is vaccinated: “If you don’t give the follow-up booster shots, you rapidly lose the immunity. There is a lot of data showing this. You need to give the priming dose, and then a few weeks later the booster, which will stimulate the immune protection to an effective level, and then keep up the regular boosters.”

One precaution Keene recommends is to choose a vaccine with the strains that are representative of the viruses that are currently circulating. “Current recommendations from the Expert Surveillance Panel on Equine Influenza are to use a vaccine containing Clade 1 influenza strains [for example, Ohio/03], for those horses within the United States,” he says. “If your horse travels or is exposed to horses traveling internationally, the vaccine should also include a Clade 2 strain [for example, Richmond/07].” The EIV strains contained in an influenza vaccine can be found online or within the product insert, Keene says.

It’s important to remember, however, that a vaccinated horse can still fall ill—if it’s been longer than six months since his last booster, for example, or if he’s exposed to an unusually large quantity of the virus. Still, the vaccination will confer some protection. “A vaccinated horse has many advantages over a nonvaccinated horse if he were to contract the disease,” says Craig. “The first would be a milder, shorter course of disease in the affected animal. He will also shed lower amounts of virus, which will lead to decreased spread of the disease to other horses and improve the overall health of the herd.”

Biosecurity: Part of the prevention strategy

Vaccination is an important preventive measure for equine influenza, but it is only a first step. “Vaccination is helpful, but it simply puts a protective barrier around the horse for a period of time; it is not the greatest limiting step for disease prevention,” says Crisman. “I can’t emphasize enough the importance of biosecurity—keeping new arrivals separate from the main herd for two weeks, quarantine of sick horses, etc.—but this is the hardest thing to get people to do.”

The basic principle of biosecurity is to keep healthy horses separated from those who either are already sick or could potentially be exposed to the virus. The specific steps you need to take depend on your horse’s lifestyle, but here are some basic guidelines.

• Keep frequent travelers separate from “homebodies.” Horses who frequent shows and clinics are more likely to be exposed to viruses they could bring home. It’s especially important to separate the travelers from pregnant mares and foals.

• Quarantine new horses before introducing them to the resident herd. Ideally, you’d have a separate turnout area to keep the new guy apart for a minimum of two weeks to ensure he’s not incubating any illnesses. If you have only one turnout area, you may be able to use temporary fencing to cordon off a smaller area for one horse. During this time, check the temperature of the new horse twice daily as you monitor him for coughing or other early signs of illness.

• Keep separate equipment for each horse in your care. Don’t share water buckets or other tools among horses. “The virus is spread most readily with nose-to-nose contact but can also be transmitted on objects,” says Wilson.

• Practice good hygiene. Wash your hands with soap after handling and grooming each horse in your care. Consider mounting dispensers of hand sanitizers at convenient locations in your barn. “The thing to remember about viruses, especially influenza, is that it transmits very easily from horse to horse and it does not have to be direct nose-to-nose contact,” says Chambers. “Horse owners need to be very conscious about how easy it is to spread the virus from a sick horse to another horse nearby. It could be passed from one horse to another on hands, clothing or equipment.”

If a horse at your barn becomes ill, double down on precautions to keep him separated from the others. Most pleasure horse owners don’t have a separate barn they can use to quarantine the sick animal, but you can take some measures to prevent contact with healthy horses. For one, move the ailing horse to an outer stall, away from the main activity of the barn, with fans to direct airflow out a door rather than toward the other stalls. Ideally, you’d designate one caretaker to tend to the sick horse without approaching or touching any others; if that’s not possible, take care of all the other horses first before approaching the sick one.

After the horse recovers, you’ll want to disinfect his feed and water buckets as well as grooming tools and other equipment by scrubbing them in a bleach solution. “Soap and water will kill this virus, as will laundry detergent, bleach, Lysol, etc.,” Chambers says. “It generally doesn’t last long in the environment but we are starting to realize that it may last longer than we originally thought. We used to think it might last a few hours, but now we realize it may last a few days, or in some circumstances—shielded from heat and direct sunlight—it might last weeks. It may be that 90 percent of it is dead within a few hours, but it could take much longer to kill that last 10 percent.”

Even with the best of precautions, your horse is likely to be exposed to equine influenza at some point in his life. But with appropriate vaccination and biosecurity precautions, you can prevent the disease entirely or minimize the severity of his illness.

This article first appeared in EQUUS issue #465,