USDA said its Animal and Plant Health Inspection Service, or APHIS, conducted an investigation after a shipment of swine from Canada showed signs of illness, according to a USDA press release issued early yesterday. The tests were negative, so the animals have been released for slaughter.

"Preliminary test results, the last of which were received this morning, are negative for foreign animal diseases of concern," the USDA said. "Based on information currently available, USDA animal health experts believe the illness is likely caused by viruses already known to exist in the United States, enterovirus or circovirus, which are not foreign animal diseases. The swine tested positive for both these viruses, neither of which poses a threat to human health."