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 neurona (a causative agent of equine protozoal myeloencephalitis). The horse had developed incoordination, depression, and hind limb paralysis, he said, and died within 72 hours of the first clinical signs.

“This is the first reported case in the world of confirmed SLEV infection in a horse that developed clinical neurological signs in the absence of any other detectable infectious agent,” Santos said. “There is no logical explanation why the diagnosis was made in Brazil (since SLEV has a worldwide distribution), although as a mostly tropical country, the climate (warm and humid) favors transmission of arboviral diseases (those transmitted by insects, spiders, and crustaceans) in general, which is likely to impose a higher natural challenge.”

The study, "Isolation of Saint Louis Encephalitis Virus from a Horse with Neurological Disease in Brazil," was published in the open-access journal PLOS Neglected Tropical Diseases