Three school horses at the Columbia Horse Center in Laurel, Md., died last week, prompting veterinarians with the Maryland State Department of Agriculture to investigate the possibility of a virus in the barn.

When the first horse, Aviva, died on Wednesday, March 23, Nanci Steveson, general manager for the horse center, said they initially thought the mare died of a stroke. Steveson said the horse was more than 20 years old and had a history of health problems. The next day, though, Steveson said she found Bailar, another school horse in his 20s, lying down in his stall. When she couldn