CHAMPAIGN – In 2002, when crows everywhere died of West Nile Virus, horse owners learned a hard lesson – horses are very susceptible to the disease.
No University of Illinois horses died that year, but vaccination for West Nile became routine. So animal scientists were stunned in September when a new filly who recently had her booster went down in a field.
Kevin Kline, head of the UI horse program, said the diagnosis, after extensive testing, was West Nile. The filly died at the UI's Large Animal Clinic a day after she was found, he said.
"We purchased her this spring, a Standardbred carrying her first foal," Kline said.
Ryan Avenatti, a graduate student in charge of the horse farm, said someone passing his pastured horses called him Sept. 19 to tell him one was lying on the ground away from the herd. He went straight to the field.
"She had no front or rear coordination, but she was very alert and upset," Avenatti said. He contacted Kline and UI veterinarians and they took the mare to the clinic immediately.
"She became almost completely paralyzed, staggering and falling," Kline said. "Vet med doesn't rule out anything, so they considered plant poisoning, West Nile and equine herpes, which requires extensive quarantining. Her symptoms were clearly neurological."
"They pumped her full of anti-inflammatories," he said of supervising veterinarians James Brendemuehl and Tom Goetz. "They tried to save her. She was in a sling, totally paralyzed. By the next day, she'd degenerated pretty significantly."
When it was clear she wouldn't recover, the veterinarians euthanized the filly.
Brendemuehl said West Nile is one of four different types of encephalomyelitis that make horses sick. The others include Eastern, Western and Venezuelan, but typically, Midwestern veterinarians only see Eastern and West Nile, he said.
All are mosquito-borne, and horses are dead-end hosts that can't pass the virus on like birds do, Brendemuehl said. He said other large animals like cattle aren't susceptible.
"It's a crazy little virus that likes to replicate in horses," he said.
Brendemuehl said West Nile attacks neurological tissue, but not all horses die.
Some recover completely, but many others are left with neurological damage.
"But when they come in paralyzed like this filly, the prognosis is very poor," he said.
Kline is thinking closely about his vaccination program. Like most owners, he uses a killed virus and booster because it's easier on horses. But he's thinking about using live virus which works faster.
Uriel Kitron, an epidemiologist at the veterinary college who studies West Nile, said this summer's weather has caused more West Nile activity in the state.
"Weather is very important," Kitron said. "It's getting late in the year, but it's still warm. In hot, dry summers, everything develops faster. Also, mosquitoes do well in storm drains, and in a dry year, you don't get them flushed out by heavy rain. This year, there were some really heavy rains in Chicago, and that had a lot to do with the lack of activity up there."
Brendemuehl's doing some follow-up on the case to find out about the filly's vaccination record. He said killed virus should work well even if it's slower to convey immunity. Live virus only takes one shot and it works in a matter of days, he said.
On Friday, Brendemuehl went to the UI horse farm, which houses about 55 mares, foals and stallions, to vaccinate all the foals. This time, he said, he's using a brand new live vaccine.
"Since we've had a positive there, I'm using this single-dose vaccine because onset of immunity is faster," he said.