Disease Du Jour: Equine Influenza

"Vets and horse owners should "consider that equine influenza can spread 50 feet upwind and 100 feed downwind,” stated Dr. Tom Chambers.
closeup of horse nose
Does this horse have equine influenza? Horses can have and spread equine influenza even if they don’t appear sick. Getty Images

How far can a horse spread the equine influenza virus?

“Literature in the 1960s—when flu was first studied—estimated about 30 feet,” said Tom Chambers, PhD, a professor at the University of Kentucky Gluck Equine Research Center. “Now we think it is 50-100 feet depending on wind direction. That [distance] was a hopping debate during the Australian equine influenza outbreak.”

“Could it spread 1,000 yards?” Chambers posed. “I’m skeptical. And the father it goes, the more diluted it becomes. Even if you can detect it with PCR it might not be an infectious dose.

“My advice is that consider equine influenza can spread 50 feet upwind and 100 feed downwind,” he stated.

In this episode of the Disease Du Jour podcast, Chambers helps us better understand the equine influenza virus and how it affects horses.

Chambers, who has been at the Gluck Center for 30 years and who studied avian and human influenza viruses for 10 years before that, has been in the forefront of equine influenza vaccine research, including that done for Merck’s Flu Avert I.N.

Chambers heads the OIE Reference Laboratory for Equine Influenza at the University of Kentucky. Many of the virus strains in current equine influenza vaccines came from Dr. Chambers’ laboratory.

Editor’s note: The need to fight animal diseases at global level led to the creation of the Office International des Epizooties (OIE) through an international agreement signed on January 25, 1924. In May 2003, the organization became known as the World Organisation for Animal Health, but it kept its historical acronym of OIE until May of 2022, when it adopted the acronym WOAH.

The Equine Disease Communication Center (EDCC) has reported equine influenza cases so far this year in Colorado, Florida, Kansas, Maryland, New York, Oregon, Rhode Island, South Carolina, Washington and Wyoming. And that included the outbreak at the BLM wild horse facility in Colorado, where as of May 17 a total of 144 horses had died since the equine influenza outbreak began on April 23. Chambers shared these other interesting statistics about equine influenza:

  • 2018-2019 in Europe went from 50 cases per year to 4,000
  • Australia 2007 spent $1 billion to control an influenza outbreak

While herpesvirus is on everyone’s mind, equine influenza outbreaks are more common than herpesvirus outbreaks.

In this podcast we began by discussing how veterinarians can re-engage their clients to mitigate the risk of equine influenza.

Influenza Today

Chambers’ starts by talking about the Merck Respiratory Biosurveillance Program that has been collecting and disseminating information for many years about equine influenza and other respiratory illnesses. “Kudos to them!” stated Chambers.

He said that in 2021, there were equine influenza outbreaks in more than 30 states with over 100 isolates taken.

“This is a disease that moves around,” cautioned Chambers. “A horse in Florida in February might be in Kentucky in March or Maryland in May.”

He also warned that equine influenza can be mild enough that no one notices, or it can be severe, like what happened in Colorado. “It can be inapparent to lethal,” he stated.

Chambers said that what happens when a horse is exposed to influenza includes multiple factors, including the vaccination status of the horse.

The Colorado outbreak was a perfect example of this, he said. The West Douglas herd was reported to be hard to handle to get vaccinations or swabs, he added. “Some were not vaccinated, some had a single dose, and some had 2 doses but only had the second one a few days before the outbreak,” said Chambers. “In the rest of the facility, there were signs of disease but no fatalities.”

The European outbreak was different. Viruses change over time. Horses had been vaccinated, so there was a lower severity of disease. However, Chambers explained, in Europe the Florida Clade 1 “showed up” when previously Clade 2 was enzootic and it “overtook” the vaccine.

Australia was completely different in that it was a flu-free country prior to the outbreak, so no horses had been vaccinated in the face of spreading influenza virus. Flu spread like wildfire in this naïve population.

“The moral of the story in Australia is that the virus can be transmitted,” said Chambers. “It ‘escaped’ from the quarantine facility designed to stop that.”

Proper and Early Diagnosis

Chambers emphasized the importance for equine veterinarians to get a proper diagnosis with a sick horse.

“Get a nasal/pharyngeal swab early in the disease to a diagnostic lab,” he stressed. “Within 24 hours you will have a result of flu, strep, herpes or whatever. That is useful in how to treat the horse and manage an outbreak.”

Starting a Vaccination Program

Chambers advised that veterinarians help horse owners develop a vaccination program based on AAEP guidelines and that reflects the state of knowledge with the state of vaccines.

He said you need to start a vaccination program when a horse is a foal, then “stick to the schedule.” The second point is that the veterinarian needs to know what the horse is used for, will it be shipped, is it a performance horse, will it compete as a 2-year-old?

“What are the risks posed if the horse is shipped to different surroundings?” posed Chambers. “Consider that with your vaccination schedule.”

Chambers stated that he is “not one who thinks that you need to vaccinate every 3-4 months. You reach a point where the antibodies you are trying to produce get ‘sucked up’ by the vaccine.

“There is a happy medium between too often and not often enough,” stated Chambers. “Find that happy medium. Six months might not be often enough, and three months might be too often. Veterinarians need to have a strategy with risk prevention.”

Chambers advised that from three weeks to one month ahead of travel for a horse is the perfect timing for vaccination.

He also said mares need to be vaccinated in order to produce protective colostrum.


While this isn’t a “sexy” topic, it is critical to reduce the risk of disease spread.

“Biosecurity is different for the traveling horse,” noted Chambers. “The best way to keep diseases out of your farm is to take steps. Quarantine new arrivals and understand fomites that can transmit virus and transfer it to a horse. Think about flies going from Horse A to Horse B, and Horse A just came back from a show or event.

“Quarantine for two weeks,” he stressed.

He advised that those using commercial or even personal transportation for their horses understand that the van can become the fomite.

“Wash the trailer with soap and water, which kills influenza,” he said. “Bleach is even better. Veterinarians and horse owners should be mindful of the dangers or risks that a newly arriving horse poses. Make sure that it hasn’t picked up something on the van.”

Regular monitoring of the horse’s health is something owners should do,” stressed Chambers. “Today the horse feels great, and tomorrow it is sick. Don’t ignore the horse out in the paddock. Check its health and temperature on a daily basis. If there are other risk factors, check them morning and evening.

“Any sign of snots or being off feed, quarantine,” he said. “Many things can go wrong, so it’s not just the flu you are looking for. There are a lot of bacterial infections where you can use antibiotics.”

What’s New?

Chambers aid there are several projects that are ongoing in his lab at the Gluck Center. One they are just starting is on the respiratory tract microbiome of the juvenile horse.

“We don’t know a lot about the microbiome of the foal and juvenile horse,” noted Chambers. “We don’t know the normal microbiome of the foal and young horse.”

The respiratory tract of the horse is open to the outside, so it can be colonized by bacteria, fungus and microbes,” he noted. “Foals can pick these things up from their surroundings.”

Chambers said they do know that the respiratory microbiome changes when a foal gets influenza. “The epithelial lining is killed and sloughs off with flu,” he noted.

That lining is a defense mechanism with cilia “beating” to push a thin layer of mucous containing foreign particles up to the pharynx, where it is swallowed. “Once it is swallowed, usually microbes are inactivated in the stomach,” noted Chambers.

“When the flu kills those cells, you get opportunistic infections like Strep zoo,” said Chambers. “These are agents that can be found in many horses, but often do not cause disease. But when the flu opens the gateway to the lower respiratory tract, that changes the microbiome.

“We want to know the starting point. What is healthy? posed Chambers.”

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