Ever heard of Sindbis virus or Middelburg virus? If you haven’t you’re not alone. But you might want to become familiar with them. South African researchers have determined that these “Old World viruses” have been causing considerable neurologic problems in a surprising number of horses—and we’re only just now realizing it.

An “Old World virus,” as opposed to a “New World virus,” is one that originates in the Old World before the Americas were discovered. Old World alphaviruses (which are transmitted via mosquitoes) have been mostly identified on the African continent, said Marietjie Venter, PhD, director of the Global Disease Detection Centre Emerging and Zoonotic Disease Program, in Pretoria, South Africa. New World alphaviruses, such as Western and Eastern equine encephalitis viruses (WEE and EEE), are important causes of severe neurologic disease in horses in the Americas.

Veterinarians have long known that African horse sickness (AHS) is an important cause of neurologic disease in horses on that continent. And not long ago West Nile virus (WNV), which originated in Africa, emerged as a new pathogen in the Western hemisphere and began causing equine neurologic disease. After WNV began infecting American horses, researchers in South Africa confirmed it had previously been missed as another important cause of neurologic signs in horses in Africa.

Still, the cause of neurologic disease in many African horses remained unexplained—until now.

“We started investigating cases of encephalitis