WHEN OUTBREAKS HAPPEN

electron micrograph of an influenza virus particle

Worldwide equine flu outbreaks are monitored by the Expert Surveillance Panel on Equine Influenza Vaccine Composition, which is supported by the World Organization for Animal Health (which uses its historic French acronym, OIE). Virus samples are collected during each outbreak and analyzed—with a primary goal of helping to keep vaccines up to date.

“The panel meets every year in February or March to review the situation for the previous year,” says Thomas Chambers, PhD, of the Maxwell H. Gluck Equine Research Center of the University of Kentucky, who sits on the OIE panel.

One of the panel’s more interesting findings is how vaccination rates seem to influence the frequency and severity of equine influenza outbreaks. “In the United States, there is a certain level of vaccination, and the disease is enzootic, which means it is always around at some low level,” Chambers says. “As a result, we see a few outbreaks at racetracks affecting maybe 20 or so horses, or an outbreak at a veterinary hospital, like we saw in Oregon in October 2015, that affects a relatively small number of horses before they get it under control.”

By comparison, he says, “India has had huge outbreaks involving many thousands of horses, and afterward the disease will disappear for about 10 years or so. Then they’ll have another huge outbreak, followed by another long period of quiet. They don’t routinely vaccinate in India, so we think that when they have a huge outbreak, just about every horse is exposed and develops some immunity. This high level of ‘herd immunity’ apparently stays in place for some years after. But, of course, it eventually wears off and then they experience the next big outbreak.”

So many American horses are vaccinated against EIV that a large outbreak is unlikely, but the sudden appearance of a foreign strain of the virus could cause trouble.

“At some point there will probably be a reintroduction of Florida Clade 2 strain [seen primarily in Europe], which makes it desirable to use vaccines with updated virus strains,” says Chambers.

Current U.S. vaccines probably offer some protection against foreign strains. “All current flu strains circulating in the world, whether Clade 1 or Clade 2, trace their lineage back to the same origins—they are all H3N8 viruses—consisting of American lineage, Florida sub-lineage,” says Bryant Craig, DVM, technical services veterinarian with Merck Animal Health. “If an outbreak of Clade 2 EIV were to occur on U.S. soil, there would likely be some cross-protection from currently vaccinated horses that would lessen the impact of the disease.”

However, says Chambers, “Sooner or later current vaccines will need updating, and the only way we can tell how EIV is changing is by studying diagnostic samples from infected horses.” He urges horse owners to help: “If your horse has a flulike respiratory infection, have your veterinarian take nasal swabs—or ideally nasopharyngeal swabs—and submit them to a veterinary diagnostic laboratory along with a vaccination history including the specific vaccine products and the dates they were given.”