The deadly piglet virus that killed millions of U.S. pigs over the past two years may have entered the country on large bags typically used to transport feed and other bulk products, the Agriculture Department said.

The report, released on Wednesday and dated Sept. 24, said the agency does not have definitive proof of how Porcine Epidemic Diarrhea virus (PEDv) initially arrived in the United States. The virus was first identified in the country in the spring of 2013.

The most likely scenario was that the virus came from the use of Flexible Intermediate Bulk Containers - also known as FIBCs or "tote bags", according to the report by the U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS).

APHIS looked at 17 possible scenarios, including whether the virus had been accidentally or intentionally brought into the country. Some of these 17 scenarios, according to the report, now are considered to be plausible explanations.

"The most probable route of dissemination is in the context of recycled food or feed products through distribution companies who generally service a large network of feed mill customers across the Midwest and beyond," according to the report.

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The outbreak of PEDv killed roughly 10 percent of the U.S. hog population between 2013 and 2014. It pushed U.S. pork prices to historic highs and its economic cost to the United States could be as much as $1.8 billion, according to some agriculture economists.

Though the number of new PEDv cases has slowed since its peak in 2014, fears remain that the virus may return. Earlier this year, U.S. researchers identified a new strain, a sign the virus will keep mutating.

CONTAMINATED BAGS

Before the outbreak, there were no federal regulations over the re-use of FIBC bags for importing products, according to the report. Such bags were sold online and frequently used by animal feed mills without being cleaned or disinfected.

The bags could have been contaminated in an "origin country's transport trucks, by exposure to irrigation or flood waters containing organic fertilizer like pig manure," the report said. Other possibilities include wood shavings, grains, fertilizer, compost, animal parts, or bulk rendered products.

Where such contamination may have originated is not known.

It is also not known what could have been inside contaminated bags. The possibilities include vitamin and mineral premixes, livestock vaccines or antibiotics approved for animal feed -- the majority of which are either produced in China, or use ingredients or reagents from China, the report said.

"Some products such as antibiotics are diluted in China with rice hulls. One consultant had seen rice hulls being dried on a roadside where they are likely driven over by contaminated pig trucks," according to the report.

Veterinary researchers tracking the outbreak found in 2013 that there was some indication the PEDv strain seen in the U.S. was 99.5 percent similar in genetic make-up to one identified in the Anhui Province of China.

The big bags are used to haul rice hulls, livestock medication and other feedstuff from small manufacturing plants to large mixing facilities in Asia, according to the report.

Early in the outbreak, federal and state investigators -- as well as an early case-control study -- thought the source of the outbreak could be linked to feed or feed delivery systems. However, investigators could find no common feed manufacturers, products, or ingredients in the initially infected herds.