October 31, 2014
Last year had been a pretty good year — as far as porcine epidemic diarrhea virus goes — for the Pipestone System, with only two sow farms testing positive for the disease. Scott Dee, veterinarian with Pipestone (Minn.) Veterinary Services, and director of research, was cautiously optimistic heading into 2014. He credits fellow Pipestone veterinarian Joel Nerem, also director of health and biosecurity for Pipestone Systems, with designing and implementing a porcine reproductive and respiratory syndrome virus-based, system-wide biosecurity protocol using risk factors for PRRS virus, “and it seemed to be working nicely.”
But then the hangover from New Year’s quickly wore off. “Within one week several farms became infected,” Dee says. With tight biosecurity in place, the veterinarian team was busy looking for answers as to what caused the rapid infection in their system. An investigation of many of the infected sites pointed to a common thread: Each of the three breeding herds had an unexpected feed outage that required an “emergency” delivery.
Feed had been deposited into a designated external bin on the site, which then provided feed for a specific portion of the herd. Clinical signs of PEDV started to appear one to two days after the emergency delivery, and only in the animals that had consumed feed from that specific bin.
“This started to make us think about what is the role of feed in the spread,” Dee says. “We didn’t have a biosecurity plan for feed in our system.”
The veterinarians had a summit, and “our leadership told us to use the resources that you have to figure it out and figure it out fast,” Dee says. “This was a new paradigm. With no background information or data relative to PEDV transmission through feed, we had to start from ground zero.” There was no literature or references to learn from, as well as a lack of sampling methods and a lack of infectivity assays.
So, the Pipestone Applied Research team went to the acutely infected farms to see what they could learn, knowing that animal byproducts had already been pulled out of the feed supply in 2013. They started studying the environmental risks on the farm. “Maybe there’s potential for contamination of the feed through the risk of the environmental contamination,” Dee says.
Samples were taken throughout the environment, and PEDV RNA was detected in the dust particles from exhaust fans, the ground directly below the exhaust fans, external concrete pads at critical access points, farm personnel, and in vehicle cabs and tires. Adam Schelkopf, another Pipestone veterinarian, even went to the feed mill and started studying what happens when the truck comes into the feed mill. He swabbed the truck driveway entrance to the feed mill, as well as the exterior of the trucks as they came into the feed mill.
“The most compelling [finding] was when we went into the now-empty suspect feed bins,” Dee says. “We started collecting the feed material that had collected on the inside wall of the bin. We thought this would be a controlled way to sample feed versus a free-catch sample … also a much better way to reduce contamination just from the environment.”
Dee says the PEDV Cycle threshold (Ct) measures were “very strong” from the samples taken from within the suspect bins: 20.25, 22.60 and 19.50, respectively. A Ct less than 38 is considered PEDV-positive.
After seeing these results, the Pipestone veterinary team got together again to propose a cycle of environmental contamination: farm becomes infected; the premises surrounding the buildings become contaminated via virus in exhaust air dust; exterior critical access points become contaminated; delivery vehicle (feed truck) and personnel access farm premises and become contaminated; vehicle travels back to mill carrying virus on tires, undercarriage and cab; truck enters mill and contaminates the floor; droppings from truck and spilled feed are swept into the pit; batches of feed are contaminated and delivered to the farm; the cycle repeats and the environmental load grows.
“This was our hypothesis; based on the diagnostic data and the temporal relationship between delivery and clinical signs, we propose this as a means that feed can become contaminated,” he says.
To determine infectivity of the feed bin samples, Pipestone Applied Research and South Dakota State University (SDSU) collaborated to “not look for blame,” as Dee puts it, but to find answers. “We have to have factual evidence,” Dee says. “We’re not out to target or blame the feed industry. Our only stake in the game is to keep our farms [PEDV-] negative. That’s all we care about.”
The study, conducted at the SDSU Animal Resource Wing Biosafety Level rooms, consisted of three groups of 3-week-old, PEDV-naive pigs.
Treatment group: Pigs consumed feed containing polymerase chain reaction (PCR)-positive samples from the aforementioned three feed bins.
Positive control group: Pigs consumed feed spiked with stock virus.
Negative control group: Pigs consumed feed containing saline placebo.
The pigs were allowed to consume treated feed via natural eating behavior over the duration of this seven-day study. Each day, rectal swabs were collected and clinical signs were observed.
Pigs in the treatment group started to show clinical signs of diarrhea on the fourth day after ingestion of the PCR-positive feed. On that day, the rectal Ct of those pigs was 34.09, while the Ct of the diarrhea was 18.94. Pigs in the positive control group, those consuming the spiked feed, showed clinical signs on Day 2 after ingestion, with a rectal Ct of 26.63 and 32.21 in the vomit samples. Pigs in the negative control group stayed negative throughout the duration of the trial, and Dee expected the positive control group to exhibit clinical signs before those receiving the PCR-positive feed samples. “There’s probably more infectious virus in the lab sample than in the bin samples that were collected in the field,” he says.
Dee says what this study has proven is that PEDV transmission can occur through contaminated feed. “The risk of feed as a vehicle for PEDV introduction into a farm is real,” Dee says. “Both mechanisms — contaminated ingredients and environmental contamination — must be recognized and managed, and we can’t forget all the other risk factors. Can’t focus entirely on feed; we can’t forget about the air, the trucks, the people.”
Dee also says there needs to be a unified effort to combat PEDV on hog farms: “The feed industry is our partner. We need to work together.”
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