GRAND COUNTY, Colo. — In a stark reminder of the persistent and lethal threat posed by vector-borne diseases, a 24-year-old Quarter Horse mare in Grand County, Colorado, has been euthanized following a confirmed diagnosis of West Nile virus (WNV). The case, officially reported on September 30 through the Equine Disease Communication Center (EDCC) Health Watch, underscores a troubling medical reality: even senior equines with a…