In a letter to veterinarians dated April 28, Brett Whitehead, Director, Equine and Ag Retail Business, of Intervet Schering-Plough Animal Health, encouraged practitioners to return all PreveNile vaccine. The recall was initiated after an increased incidence in the number of adverse events following vaccination with currently marketed serials of PreveNile.

The company noted in the letter that Intervet Schering-Plough has notified the USDA's Center for Veterinary Biologics about this recall action.

West Nile virus (WNV) is an important neurological disease of horses in North America characterized by a wide array of clinical signs that include fever, ataxia (incoordination), depression, behavioral changes, weakness or partial paralysis, recumbency, and death. The mortality rate is an estimated 33%, and horses that do survive can exhibit residual effects (e.g., gait abnormalities, behavioral changes).

The disease is transmitted primarily by mosquitoes. While prevention can include reducing mosquito numbers and exposure of horses to mosquitoes, the most important means of preventing WNV infections is vaccination. The American Association of Equine Practitioners (AAEP) considers the WNV a core vaccine and assures horse owners and veterinarians that it is "an essential standard of care for all horses in North America"