There have been numerous outbreaks of the deadly neurologic form of equine herpesvirus type 1 (EHV-1) recently. There seems to be no rhyme or reason, and nothing that indicates it springs from a single focal point and has spread, like West Nile virus did. A few cases here, a single horse there, an outbreak at a racetrack or barn. It’s scary. Herpesviruses have been around, well, we really don’t know how long. They live in us, our horses, and many other species. That’s called latent infection. The virus doesn’t cause clinical signs, but it’s there, waiting. Then, when some stress hits–such as transportation, racing, training, shows, illness, a reduction in the immune system, corticosteroid use, or even new herd dynamics from horses coming or going–it becomes active and begins to cause problems for its host horse. Then the host horse can spread that active virus through respiratory or nasal secretions into the environment. In his talk at the World Equine Veterinary Association, Laurent Couetil, DVM, of Purdue University, said the virus "may remain infective in the environment for up to 14 days and on a horse’s hair coat for up to 42 days. Infected horses may shed EHV-1 in nasal secretions for as long as 14 days."

Okay, let’s stop right there for a moment. A horse either is exposed or has a latent infection that becomes active. The incubation period for EHV-1 is six to 10 days before the horse shows clinical signs. The horse can shed that virus (and research from the University of Kentucky’s Gluck Equine Research Center points out that the mutated neurologic form of EHV-1 sheds in very high quantities), the virus can live 14 days in the environment, and 42 days on the hair coat?

Did you say 42 days?

So why are we having quarantines end three weeks after the last clinical sign? That’s only 21 days–half the time the virus still could be alive and well and being carried around to other horses