There have been numerous reported cases of equine influenza in the United States in the past year, and a current outbreak of flu in the United Kingdom caused cancellation of racing for several days.
Of concern is whether the current strain of equine influenza virus is new and whether current vaccines are conferring protection against the circulating strain(s).
Tom Chambers, PhD, a professor at the University of Kentucky’s Gluck Equine Research Center who focuses on the study of infectious disease, heads the OIE Reference Laboratory for Equine Influenza at the University of Kentucky.
Chambers, who has been at Gluck for nearly 29 years and who studied avian and human influenza viruses for 10 years before that, has been in the forefront of equine influenza vaccine research. He is currently conducting research on a second-generation modified live virus flu vaccine.
“In a nutshell, there are lots of reports of equine influenza in the last two to three months in the U.S. and in the last week or so in England,” said Chambers on February 8.
Chambers said some of the cases of equine influenza in the U.S. and England involve vaccinated horses.
He said the two questions that are paramount when he hears of vaccinated horses that have been confirmed with equine influenza are, 1) when was the horse vaccinated, and 2) what was the horse vaccinated with?
“Six months out is pushing the expected duration of influenza vaccine immunity for at-risk horses,” said Chambers. But in some of the 2019 influenza horses, “from what I hear second-hand, in England some were vaccinated as little as six weeks prior to becoming ill, and in the States, some were only two to three months out (from vaccination).
“That is definitely cause for concern,” said Chambers.
“That suggests—if it all can be confirmed or outbreaks continue to propagate—that that the vaccines are not working as effectively as they should,” stated Chambers. He added a cautionary note that, “I’m not ready to go there yet without more information.”
“If it should turn out that highly regarded vaccines are breaking down, it is not a good sign., But there are no conclusions yet,” said Chambers. “The reports out of the United Kingdom emphasize that vaccinated horses still show much milder disease than unvaccinated horses, which shows the vaccine still has some protectiveness.”
He said if there is a vaccine breakdown and a new flu strain, then by sequencing the virus they should see some new mutation. An important mutation would cause a change in antigenicity, which is how the virus presents itself to the horse’s immune system. That sequence information is now being gathered.
He said that last year his lab was highly concerned about one mutation that appeared in the equine influenza strains seen in the United States in spring 2017. “But that mutation did not persist, we haven’t seen it since then,” said Chambers. The mutations in the 2018 influenza strains are not the same mutation that they found in 2017.
Those influenza changes highlight the difficulty in keeping up with the genetic mutations of equine influenza.
Chambers said his lab is working on virus isolation right now on the current strain and working with contacts around the world to get a handle on what is changing in the influenza virus. His colleagues in England, at the Animal Health Trust, have been working around the clock to stay on top of the situation. “I understand they are already swabbing horses from 170 different yards, and maybe more before this outbreak is over.”
“Getting the isolate is the tricky part,” admitted Chambers. “It can sometimes take a lot of effort.”
Once the researchers have a current isolate, they can perform antigenicity studies to measure how much different the new strain appears to the immune system, compared to the vaccine strains.
Chambers said that until those antigenic comparisons are done, the international equine influenza expert surveillance panel does not have data to give a conclusive statement as to whether the vaccines need to be updated.
Vaccinating and Boostering
Chambers said whether or not there is a new equine influenza strain circulating, “it certainly is not going to hurt to give an at-risk horse a fresh dose of vaccine.
“When faced with an outbreak situation, you should consider the horse’s risk with travel, training and exposure when determining whether to vaccinate,” said Chambers. “Nothing is foolproof, but if the horse was vaccinated a month ago, it ought to be already at the peak of its immune response. If the last dose was six months or a year ago, then definitely give the horse vaccine. And don’t forget good biosecurity.”
Veterinarians who have potential cases of equine influenza are encouraged to send nasal or nasopharyngeal swab samples to their state’s diagnostic laboratory or directly to Chambers’ lab at the Gluck Center at the University of Kentucky. Information on Guck Center influenza testing can be found at https://vetsci.ca.uky.edu/content/equine-influenza-testing.
Stay tuned for more information on equine influenza as it becomes available.