Woven bags used to export pig feed are the most likely source of coronaviruses that have killed millions of U.S. pigs, a recent report states.
Contamination in those bulk containers could explain how the viruses entered the U.S.—likely from China—in March or early April 2013 and spread within weeks to farms across the country, according to the Department of Agriculture report. The reusable woven-fabric bags are used to ship materials such as pig feeds, feed ingredients, soybeans, or sand, usually with 1,000- to 3,000-pound capacities.
“Contamination of products in an origin country is largely out of government regulatory control and likely outside the realm of industry management,” the report states. “Inspections at entry ports are vital, but unable to identify products containing miniscule amounts of contagious virus.”
Of the three coronaviruses that entered the U.S. in 2013, two viruses that cause porcine epidemic diarrhea have killed about 8 million pigs—almost all neonatal pigs—on U.S. farms. The other virus, a swine deltacoronavirus, has been associated with similar clinical signs but a lower number of deaths. It also has spread to far fewer farms.
“Further study is necessary to identify cleaning and disinfection procedures that might be appropriate, but the answer could be as simple as not reusing the bags or, yet to be determined, disinfection procedures such as dry heat prior to reusing the containers,” the report states.
Farm investigations and an early case-control study had implicated feed or feed delivery in the outbreak, the report states. But the cases lacked common manufacturers, products, or ingredients.
Later investigation indicated the viruses most likely spread through contaminated bags to distribution companies that serve networks of feed mills, through which the contaminated bags or their contents would contaminate pig rations destined for farms, the report states.
USDA Animal and Plant Health Inspection Service tests indicated the coronaviruses could remain viable for five weeks at room temperature in tote bag fabric.
Dave Warner, a spokesman for the National Pork Producers Council, said fewer PED outbreaks have occurred as farms have added biosecurity measures, such as increased truck and barn washing and use of heat to dry the trailers used to transport swine. And sows in barns where infections occurred have some immunity, he said.