Porcine epidemic diarrhea virus introduction into the United States: Root cause investigation.

Porcine epidemic diarrhea (PED) was identified in the United States in the spring of 2013, and professionals from many parts of the U.S. swine industry responded rapidly to understand and control the newly emerging disease. In less than two months, the disease had spread to more than 200 herds in thirteen states. Experts from the US Department of Agriculture (USDA) engaged in laboratory diagnostics, analytic support, epidemiology expertise, and data management to facilitate the effort. By 2014, a great deal had been learned about the disease; however, the question of how it entered the United States remained unanswered. In 2014, USDA formed an investigative group to address the question and leverage current knowledge with resources and partnerships not readily available to non-federal investigators. The group formed collaborations with other government and non-government organizations and individuals, and followed many avenues of inquiry; ultimately arriving at a small number of scenarios that describe possible mechanisms for PED introduction. For a scenario to be plausible, it had to explain: contamination of a person or product in the source country, its transit and entry to the United States, rapid dispersal across a wide geographic area, and exposure/infection of pigs. It had to be compatible with findings of swine herd investigations and research studies. Potential products had to have been imported legally during the time prior to the beginning of the epidemic, or delivered to the United States through prohibited channels. Follow-up studies were initiated to gather more evidence for the most plausible scenarios. Of the scenarios, flexible intermediate bulk containers ("feed totes") used to transport bulk feed serving as fomites for movement of PED virus provided the simplest explanation for the accumulated findings of the investigation.