Although influenza is endemic in many countries and continually circulates throughout the equine population, it has consistently
been responsible for periodic explosive outbreaks throughout the globe and throughout history, frequently with devastating
consequences. Most of these outbreaks are in populations with lower to no previous immunity (unvaccinated) or are associated
with sufficient antigenic drift that allows the virus to evade vaccine immunity. (Antigenic drift refers to the fact that
influenza virus continually modifies its protein structure and changes itself so as to increase its infectivity. Vaccines
made against one particular strain of virus become less effective against a modified newer strain.)
While the drift of equine influenza is slower than that of human influenza, it's recommended that equine vaccines contain
killed viral antigens from flu isolates obtained within the most recent five years.
Many horse owners have started questioning the frequency of vaccinations, and holistic-oriented websites promote fewer vaccines
at less-frequent intervals. While, in general, the idea of reducing unnecessary vaccinations and the resulting stress to a
horse's immune system is a reasonable goal, history teaches that reducing vaccination for equine influenza is likely to result
in serious problems. Because of the particular characteristics of the virus and the historical lessons learned from past epidemics,
current recommendations are to vaccinate horses for flu every six months. The FEI has required this vaccination frequency
since 2005, and influenza is the only vaccine specifically required by that organization. Yet, with all that has been learned
and all the precautions and regulations in place, problems have still cropped up.
In 1992, an influenza outbreak occurred among horses stabled at the Royal Hong Kong Jockey Club. This outbreak was important
because Hong Kong requires all racehorses to be vaccinated against equine influenza and because there had been no cases of
influenza reported since 1972. Still, this virus broke through what had been considered good quarantine protocol and vaccine
protection. Nearly 1,500 horses were affected (37 percent of all horses within the outbreak zone). The most seriously affected
were from countries where the disease did not naturally occur. Researchers investigating this outbreak concluded that while
vaccination did not prevent influenza, the disease in vaccinated horses was less severe, and the signs lasted a shorter period
then when horses were unvaccinated.
Flu-like signs were discovered in 16 horses stabled in Sydney, Australia, in August 2007. Much of the country eventually was
involved, and the agricultural minister called the outbreak, "the biggest risk ever faced by the Australian Thoroughbred industry."
It is thought the virus was brought into the country by a stallion shipped there from Japan for the breeding season. Australia
leads the world in the number of racetracks and race clubs, and its nearly $7.7 billion racing industry employs 204,000 people.
This robust section of the nation's economy was put in serious jeopardy by equine influenza virus.
Controlling and preventing further outbreaks
Identification of infected horses and isolation are the best methods of dealing with an outbreak. This "know and don't go"
policy has stimulated many equine groups and organizations to implement or improve their information and warning programs
so horse owners can be made aware of potential outbreaks and avoid travel to certain locales or equine events.
Last June, Merial launched an Equine Outbreak Alert Program aimed at providing just this sort of information to owners, who
could then adjust their show or travel plans to avoid potential problem areas.
The May influenza outbreak involving more than 200 horses within a 20-mile radius in the Normandy region of France should
remind us that equine influenza remains a persistent threat to the global equine community. The affected horses in this area
had all been vaccinated, and no specific cause has been found for the introduction of the disease. Researchers will be looking
at horse movement, weather, antigenic variability and all the other factors that they have learned that impact influenza infection
When it comes to this disease, the most appropriate statement from history may be one from former Chancellor of Germany Konrad
Adenauer: "History is the sum total of things that could have been avoided." Using appropriate influenza vaccines on good,
repeatable schedules; monitoring horse movement; maintaining quarantine measures; and continually learning from what happened
before will not stop influenza outbreaks. But even if we cannot avoid infections, we may be able to reduce their extent and
severity, which is a good start for the future.
Dr. Kenneth Marcella is an equine practitioner in Canton, Ga.
1. Firestone SM, Cogger N, Ward MP, et al. The influence of meteorology on the spread of influenza: survival analysis of an equine
influenza (A/H3N8) outbreak. PLoS ONE 7(4):e5284.