Feed batch sequencing to decrease the risk of porcine epidemic diarrhea virus (PEDV) cross-contamination during feed manufacturing

Feed has been identified as a vector of transmission for porcine epidemic diarrhea virus (PEDV). The objective of this study was to determine if feed batch sequencing methods could minimize PEDV cross-contamination. Porcine epidemic diarrhea virus-free swine feed was manufactured to represent the negative control. A 50 kg feed batch was mixed in a pilot scale feed mill for 5 min, sampled, then discharged for 10 min into a bucket elevator and sampled again upon exit. Next, a pathogenic PEDV isolate was used to inoculate 49.5 kg of PEDV-free feed to form the positive control. The positive control was mixed, conveyed and sampled similar to the negative control. Subsequently, 4 sequence batches (sequence 1 to 4) were formed by adding a 50 kg batch of PEDV-negative feed to the mixer after the prior batch was mixed and conveyed; all sequences were mixed, conveyed, and sampled similar to the negative and positive control batches. None of the equipment was cleaned between batches within a replicate. This entire process was replicated 3 times with cleaning the feed mill between replicates. Feed was then analyzed for PEDV RNA by real-time reverse transcriptase semiquantitative polymerase chain reaction (rRT-PCR) as measured by cycle threshold (Ct) and for infectivity by bioassay. Sequence 1 feed had higher (P ˂ 0.05) rRT-PCR Ct values than the positive batch and sequence 2 feed had higher (P ˂ 0.05) Ct values than sequence 1, regardless of sampled location. Feed sampled from the mixer from sequence 2, 3, and 4 was rRT-PCR negative whereas feed sampled from the bucket elevator was rRT-PCR negative from sequence 3 and 4. Bioassay was conducted using 66 mixed sex 10-d-old pigs confirmed negative for PEDV allocated to 22 different rooms. Pigs were initially 10-d old. Control pigs remained PEDV negative for the study. All pigs from the mixer positive batch (9/9) and bucket elevator positive batch (3/3) were rRT-PCR positive on fecal swabs by the end of the study. One replicate of pigs from mixer sequence 1 was rRT-PCR positive (3/3) by 7 dpi. One replicate of mixer pigs from sequence 2 was rRT-PCR positive (3/3) by 7 dpi although no detectable PEDV RNA was found in the feed. The results demonstrate sequenced batches had reduced quantities of PEDV RNA although sequenced feed without detectible PEDV RNA by rRT-PCR can be infectious. Therefore, a sequencing protocol can reduce but not eliminate the risk of producing infectious PEDV carryover from the first sequenced batch of feed.