As international horse transport becomes more widespread, some owners might wonder if the same equine influenza vaccines designed to protect horses from common domestic strains of the disease will also shield them from foreign strains they might encounter in other countries or even at large domestic horse shows. Researchers in Japan have determined through a recent study that most strains of equine influenza are "antigenically similar" to each other, meaning current vaccines should be able to protect against foreign strains as effectively as they do against domestic strains.

According to Thomas Chambers, PhD, head of the Office International des Epizooties (OIE) International Reference Laboratory for Equine Influenza–located at the University of Kentucky’s Gluck Equine Research Center–has been studying and identifying different "clades," or types, of the equine influenza virus for the past 15 years. Each of these clades used to circulate within specific geographic regions; however the various clades are no longer bound to these regions, most likely due to the international movement of horses.

Virus mutation further complicates the issue of protecting horses against various flu strains: "Compared to the viruses that cause flu in other species, equine influenza viruses are notorious for mutating (a process called antigenic drift) in an attempt to evade horses’ immune systems," explained Hiroshi Kida, DVM, PhD, the director of the Research Center for Zoonosis Control at Japan’s Hokkaido University. "It is in this way that new outbreaks of equine influenza (can) occur, despite immunization."