West Nile virus, first identified within the United States in 1999, has since spread across the continental states and infected birds, humans and domestic animals, resulting in many deaths.
Previous studies in mice identified the status of the Oas1b gene as a determining factor for resistance to the infection.
A recent case-control association study showed mutations of the human OAS1 gene was associated with clinical susceptibility to West Nile infection.
"Similar studies in horses, a particularly susceptible species, have been lacking, in part, because of the difficulty in collecting populations sufficiently homogenous in their infection and disease states," the American and Australian researchers said.
"Susceptibility to severe West Nile encephalitis among mammals is naturally variable," they wrote.
"Experimental infections in sheep, calves, pigs, and dogs have shown these domestic species to be poor hosts for, or develop only mild clinical symptoms from West Nile infection, thereby limiting their usefulness for genetic susceptibility/resistance studies.
"Horses however, are particularly susceptible to severe West Nile infection."
Because many horses infected with the virus showed few or no symptoms, while others were severely affected, the species was ideal for the study.
The equine OAS gene cluster most closely resembles the human cluster, they noted.
The researchers undertook a case-control association study to investigate whether, like humans (OAS1) and mice (Oas1b), equine OAS1 played a role in resistance to severe West Nile infection.
They confirmed that mutations in equine OAS1 contributed to susceptibility to the virus.
Their work suggested differences in OAS1 gene expression may determine each horse's ability to resist clinical manifestations associated with West Nile infection.
The study, entitled OAS1 Polymorphisms Are Associated with Susceptibility to West Nile Encephalitis in Horses, has been published in the open-access journal, PLoS ONE.