An encephalitis virus once thought to be inert in horses has been identified in a Brazilian horse exhibiting neurologic signs, researchers in that country say.

The Saint Louis encephalitis virus (SLEV) can cause mild encephalitis in humans and some animal species, but horses have never before shown clinical signs despite frequent infection in North and South America, said Renato de Lima Santos, DVM, MS, PhD, professor at the Federal University of Minas Gerais, in Belo Horizonte, Brazil. However, a 12-year-old male horse from southeastern Brazil recently developed clinical signs of neurologic disease, which the researchers believe were caused by SLEV.

“This case is very important because, to our knowledge, it is the first SLEV infection in a horse that is associated with neurologic disease,” said Santos. “The horse is known to be a reservoir of this virus, but now we know that the virus can actually cause disease in the equine host.”

The virus is transmitted from one host to another by mosquitos, Santos said. It is considered endemic in the Americas, from Canada to Argentina, and there are no vaccines or effective treatments for the disease in any species.

In their study, Santos and his research team evaluated the brains of 170 horses from southeast Brazil that had shown neurologic signs for unknown reasons. Only one of these horses was positive for SLEV, Santos said. This horse was free of other equine encephalitis pathogens, including the rabies virus, equine herpesvirus-1 and -4, West Nile virus, Eastern equine encephalitis virus, Western equine encephalitis virus, Venezuelan encephalitis virus, and Sarcocystis neur