The present project was initiated to assess if feed testing positive for PEDV with RT-PCR was infectious to pigs. This study was part of a larger investigation into an outbreak of PEDV in a commercial swine production system. The feed used in the present study was from a farm that experienced its first PEDV outbreak in January 2014. Feed used during the outbreak tested positive for PEDV with an RT-PCR assay; however, RT-PCR only detects the presence of viral RNA and does not mean viable and infectious virus was present in the sample. In the on-farm case, the attending veterinarian froze aliquots of the suspect feed as a wet mash in an attempt to preserve viable virus. PEDV is extremely difficult to culture outside of a pig model; therefore, a bioassay was used to assess infectivity of the feed fed during the outbreak. The suspect feed was fed to PEDV susceptible pigs for one week, during which time the pigs, feed, and environment were repeatedly tested for PEDV.
While we were not able to confirm the feed as a source of PEDV in the present case, the sensitivity of the bioassay may have been limited by the small number of pigs (n = 10) and the amount of feed the weanling age (from 10 to 20 days of age) pigs could consume during the trial period. Additionally, there was a 28 day lag from the time the feed was manufactured to the time of the initiation of the bioassay, a time period that may have decreased the viability of any virus that may have been present in the feed at the time of delivery to the farm. Nonetheless, the growing number of case reports implicating feed as a potential PEDV source indicates systematic, active surveillance of emergent porcine coronaviruses in feed and feed components is needed to overcome the inherent limitations of retrospective investigations.