BRIGANTINE, N. J. (Reuters) - Seals in New Jersey have been dying in unusually high numbers from a mysterious disease that starts with a cough and a runny nose, a leading animal treatment facility said on Wednesday. The Marine Mammal Stranding Center in Brigantine, New Jersey northeast of Atlantic City, reported that one-third of the 31 seals brought to its facility this year have died so far. That is more than double the usual mortality rate of 14 percent. The disease has been found among all four local seal species -- gray, harbor, harp and hooded. But hooded seals, an Arctic species that winters along the U.S. mid-Atlantic Coast, have had the worst of it. Only two of six hooded seals treated at the stranding center have survived. Researchers are trying to determine whether the sick seals brought to the center are suffering from a virus. Some have died just hours after arriving at the facility. The disease responsible for the deaths begins with coughing and nasal discharges and leads to a loss of appetite. Death occurs through respiratory failure. Biologists hope to learn more about the illness from veterinary labs at the University of Pennsylvania, Cornell University and the Armed Forces Institute of Pathology in Maryland, which have all received tissue samples for analysis. Researchers noted that hooded seals have been ranging further south and in larger numbers recently. The New Jersey and Delaware coasts traditionally have marked the southern boundary of their territory. But in recent years, some have turned up in Florida, Puerto Rico and the Virgin Islands.