Hawaii has identified its first outbreak of a deadly pig virus that emerged in the continental United States last year, confounding officials who are uncertain how the disease arrived over thousands of miles of ocean.

The state confirmed Porcine Epidemic Diarrhea virus (PEDv) on a farm on Oahu, the most populous Hawaiian island, on Nov. 20, according to the Hawaii Department of Agriculture.

Farmers and the federal government have been working to contain PEDv since it was first detected in the United States in the spring of 2013. The virus has killed at least 8 million pigs, roughly 10 percent of the U.S. hog population. PEDv was previously found in parts of Asia and Europe. It is unknown how it came to the United States.

Hawaii had toughened import requirements for live pigs in July in a bid to prevent the spread of PEDv, banning infected hogs and requiring tests for PEDv prior to shipping.

State officials do not know how PEDv arrived on their shores and are testing animal feed from the infected farm to try to determine whether it may have transmitted the virus, acting State Veterinarian Isaac Maeda said in a telephone interview Monday.

 
"We live out in the ocean," Maeda said. "A lot of things you see on the continental U.S., we don't see out here."

Chances of determining how PEDv arrived in Hawaii are "not looking very promising," he added.

The outbreak occurred on a farm with about 150 pigs, and about 25 percent died, according to Hawaii's agriculture department. Veterinarians sent samples from the farm to the Kansas State University Veterinary Diagnostic Laboratory, which confirmed the PEDv infection.

"It was surprising because it was a long distance from your traditional swine channels," Tom Burkgren, executive director of the American Association of Swine Veterinarians, said about the outbreak.

The farm did not use feed containing porcine plasma, which has been suspected of spreading PEDv, Maeda said.

Researchers have previously established that PEDv in the continental United States can spread from pig to pig by contact with manure, which contains the virus. It can also be spread from farm to farm on trucks.

Hawaii quarantined the infected farm and stopped the movement of pigs on the west side of Oahu to contain the outbreak.

PEDv is not a threat to humans or food safety, according to the U.S. Department of Agriculture.