The Future of Equine Flu

The face of equine influenza is evolving, and so must our understanding of its behavior and the vaccines that protect against it. Updating flu vaccines, transmission of flu between horses and dogs, and flu mutation were featured topics in a recent Equine Influenza Workshop held Nov. 3-6 in Miami, Fla. An international group of scientists, vaccine manufacturers, and government officials chaired by Jenny Mumford, PhD, director of science at the U.K.'s Animal Health Trust in Newmarket, shared research notes and discussed handling flu as it changes.


Antigenic map of human influenza A (H3N2) virus strains from 1968 to 2003. The relative position of strains (colored shapes) and antisera (uncolored open shapes) show how the strains have changed over time; the letters refer to the isolation location (HK=Hong Kong, etc.). This model could help scientists stay one step ahead of the next influenza strain to emerge and mutate, and it could eventually be adapted to predict equine influenza strains.

Tom Chambers, PhD, heads an international influenza reference lab at the University of Kentucky's Gluck Equine Research Center. He said that in 2004, the Expert Surveillance Panel (a group of leading equine flu researchers from the Office International des Epizooties and World Health Organization, WHO) recommended an update in the strains to be used in equine vaccines—the first recommended update since 1995 ( The Panel reviewed the rationale for the update at the workshop.

"First in 2003, in Newmarket, there was a large outbreak of equine influenza where some of the horses that contracted it had been vaccinated with vaccines that met the old standard," Chambers explained. "That's a really a surefire sign that it's time to update the vaccines."

Additionally, a 2003 South African equine flu outbreak showed the need for updates. "Those horses (in South Africa) were not vaccinated, but when the panel analyzed the strain, we could see it cross-reacted poorly with the previous strains (that vaccines are designed to protect against)," he said.

According to Chambers, all of the equine vaccine manufacturers have samples of the South Africa strain or similar strains circulating in U.S. horses, and vaccine updates are in the works.

Janet Daly, PhD, senior virologist at the Trust, said, "Updating vaccine strains is a lengthy and costly process, and it's understandable that the manufacturers would prefer to demonstrate that their current vaccines will cross-protect against the newly emerging strains of flu. Unfortunately, although it is relatively easy to demonstrate protection against new strains under the ideal circumstances of a vaccine trial, out in the field you can only really be sure of protection if the vaccine contains strains that are a good match to the circulating viruses."

Relative to human outbreaks, the WHO global flu program has accumulated flu serological test results for 40 years. Terry Jones, PhD, of the University of Cambridge, gave a novel presentation of that data at the conference that could eventually help equine vaccine strain selection. Chambers explained, "The mass of data gets hard to conceptualize, and Dr. Jones showed some new mathematical models that were developed to help conceptualize that data and possibly give us some predictive power on what the next (human flu) variant is going to be. Maybe down the road when we have enough data, we can apply that (modeling) to equine flu."

The rest of the meeting was devoted to influenza in other species, specifically recent research on the equine influenza strain that jumped from horses to Florida racing Greyhounds in 2004 ( Since the outbreak, influenza has been passed from dog to dog, and pet dogs (non-racing Greyhounds) infected with canine influenza virus have been identified throughout Florida, in 13 states, and in the District of Columbia. Researchers contend the highest risk groups for canine flu include dogs in shelters or other adoption-type facilities, kennels, and pet stores.

The Florida outbreak reports caught Daly's attention. She followed up on an isolated mysterious outbreak in England in which seven foxhounds died after severe respiratory signs, depression, and ataxia in 2002. Daly tested preserved tissues from the foxhounds, reporting her results at the workshop. She concluded the foxhounds also died from an equine influenza virus strain. She stressed that the outbreak was isolated—no other U.K. cases have been found.

Scientists discussed the possibility that dogs might have been infected at some point in the past by eating raw horse meat infected with influenza virus, but there is no evidence to support this theory. Cynda Crawford, DVM, PhD, of the University of Florida, has been studying the Florida canine flu cases. She said no one knows when, where, or how the virus jumped from the horses to dogs, or even what dog population was infected first.

Springing out of the canine flu experience are studies to see if the equine flu (that mutated and adapted in the dog) could be passed back to horses, and if so, if the current equine flu vaccines protect against it. Also, Crawford and others are finding out how dogs respond to equine flu vaccination.

Preliminary results show that an inactivated vaccine produced only a small response in dogs, and scientists want to develop a canine flu vaccine that is effective against the strain.

Flu Mutation

Also at the workshop, scientists revisited human clinical signs of the avian flu virus (H5N1) that has been on the radar screen in Southeast Asia for several months. The strain has also infected tigers that ate raw infected poultry. Chambers explained, "The (equine influenza) virus mutated and adapted in the dog and now it can be transmitted dog-to-dog by the aerosol route. Now a whopping  dose of virus isn't needed anymore for dogs to become infected. That's what we're afraid of happening with the bird flu in Southeast Asia. If the virus can mutate to make itself less lethal perhaps, but more contagious, control of the disease could be difficult.

"Speaking as a virologist, influenza is actually a simple virus, and the way it survives is by a very simple strategy of always changing itself, always mutating," said Chambers. "That's a simple but very effective strategy. Ninety-nine percent of flu is going to die before it can get in to a different species. It's the one mutant virus that causes all the problems. For that reason, we have to continue and improve surveillance, and continue with research in order to fight it."

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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