Equine herpes virus 1, the virus that has sickened horses at Pimlico, seems to be infecting more animals than in the past and causing more serious symptoms, experts say.
"It appears that it is more common," said Klaus Osterrieder, a veterinarian and virologist at Cornell University College of Veterinary Medicine. "And it appears that it is nastier."
The virus, known as EHV-1, causes a wide range of symptoms. It often produces respiratory problems and fever, and it can cause pregnant mares to abort their fetuses. The most severe version attacks the nervous system and can lead to paralysis. The horse that was destroyed Friday at Pimlico suffered from the neurological form and couldn't even sit up.
The virus is a relative of the human herpes virus family, which can cause a variety of symptoms, including cold sores, chickenpox and, in rare cases, neurological problems.
EHV-1 is actually carried by 70 percent to 80 percent of all horses. In most, it lies dormant and causes no symptoms. But for reasons that remain unclear, the virus becomes activated in some horses, at which point the animals show symptoms.
Like many researchers, Osterrieder, who specializes in EHV-1, suspects stress triggers viral activation. "These horses are extreme athletes," he said. "And they are under a lot of stress."
Once a horse comes down with activated EHV-1, it can infect other horses with relative ease. The virus can be transmitted by air and can survive in a wide range of temperatures. It can live on buckets, grooming equipment and human hands. (Although humans can carry it from horse to horse, they are not susceptible to the virus.) EHV-1 is, however, vulnerable to sunlight and detergent.
In the past, EHV-1 outbreaks at tracks or stables have not been unusual. "It circulates constantly," said Rusty Ford, equine program manager for the Kentucky state veterinarian's office. A track in Kentucky, Turfway Park, is now experiencing an outbreak of EHV-1. So far, two horses there have been destroyed after exhibiting neurological symptoms.
What worries veterinarians, horse owners and researchers is that outbreaks seem to be occurring more often and inflicting neurological damage on a larger proportion of those infected.
Some scientists suspect that a particularly destructive strain of EHV-1 might be behind the recent outbreaks. Several studies have uncovered a genetically distinct version of the virus that seems more likely to cause neurological damage.
Others think that increased travel by horses plays a role in the apparent rise in cases. "There are no geographic limitations now. You can go to England or to Dubai like you're going around the corner," said David Zipf, chief veterinarian for the Maryland Racing Commission.
There is no cure for EHV-1. Horses that become sick typically receive antibiotics, which can reduce fever, and cortisone injections, which can minimize nerve damage. But these drugs do not attack the virus itself. In the past two years, some veterinarians have tried acyclovir, a human antiviral medicine, which seems to help some stricken animals.
Horses that are not carriers can take a vaccine to prevent infection. But some veterinarians and horse owners worry that the vaccine might put horses at risk of contracting the virus.
In the absence of a cure, quarantine is the most effective strategy once an outbreak occurs.