October 31, 2014
When it comes to the risk of infecting a herd with porcine epidemic diarrhea virus (PEDV) via feed, there is a lot that is still unknown, says Mike Tokach, a member of the Kansas State University (KSU) Applied Swine Nutrition Team. When speaking to a sizable audience at the recent Allen D. Leman Swine Veterinary Conference in St. Paul, MN, Tokach notes, “We can break things down to three areas: the things we think we know about feed and PEDV, things we still need to understand to manage feed risks, and what options producers are really left with at the end of the day.”
The link between PEDV and feed ingredients was initially suspected based on observations made in Canada during PEDV outbreaks in early 2014, explains Pedro Urriola, research assistant professor in the Department of Animal Science at the University of Minnesota. “In Canada there were outbreaks of PEDV in multiple nursery farms that sourced weaning pigs from multiple [PEDV]-negative sow farms, thus indicating a PEDV source other than vertical transmission,” he says. “All these farms shared the same source of pre-starter and starter diets, and that sparked a recall of feed.”
The feed company and farm veterinarians involved with the Canadian farms performed polymerase chain reaction (PCR) tests on 55 truck deliveries, with negative test results. These results ruled out PEDV transmission from contaminated trucks. Urriola says the investigation continued with testing of 76 samples of nursery-pig diets and six samples of spray-dried porcine plasma (SDPP). Samples of feed (three out of 76 samples) and SDPP (five of six samples), tested positive by PCR. The PCR test confirms presence of the virus, but not whether or not the virus is capable of infecting a susceptible host.
“Therefore, the Canadian Food Inspection Agency [CFIA] tested virus infectivity by feeding susceptible pigs with the diets and SDPP under investigation,” Urriola says. The study demonstrated that the porcine blood plasma in question contained PEDV capable of causing disease in pigs. However, the study could not demonstrate that the feed pellets containing the blood plasma were capable of causing disease.
Urriola says the Canadian experiences led to questions about testing feed ingredients. The University of Minnesota is currently conducting three National Pork Board-sponsored research projects to measure virus inactivation conditions, develop feed-specific assays and investigate the risk of PEDV transmission in porcine feed ingredients.
U.S. Feed Suspicions
Tokach says epidemiological evidence in the United States has also suggested that some PEDV breaks were associated with feed as the route of entry for the disease. Several farms sourcing weaned pigs from different [PEDV]-negative sow farms also broke with PEDV without explanation or any other obvious route of transmission. Investigations by veterinarians and testing of samples led to suspicions that contaminated feed may have been the culprit. Recent research has demonstrated that putting PEDV into feed can cause an outbreak of the disease. This level of infectivity is different than that seen with many other viruses that have plagued the pork industry in the past. “We have a much different situation with this virus than what we have dealt with with other viruses over time,” Tokach explains. He notes that neither pseudorabies or the porcine reproductive and respiratory syndrome (PRRS) virus were capable of being spread in feed.
Measuring PEDV Infectivity in Feed
Tokach says research has shown that feed can have a high Cycle threshold (Ct) count and still infect pigs with PEDV. Many laboratories currently use a Ct of 37 as the cutoff for declaring a sample PEDV-negative. “We have found the infective minimum Ct can be above the cutoff point for some labs; for example, some researchers have discovered that a Ct count of 37 can actually be infectious,” Tokach says. “That is very troubling, and a big concern for us going forward. That is one of the things we need to better understand.” (For an explanation about Ct counts and PEDV testing terminology, see sidebar on page 34).
Tokach says that almost all U.S. porcine ingredients have a positive PCR test for PEDV, but points out that a positive PCR does not always mean that the virus is infectious. The difficulty is that the only way to verify whether feed or a feed ingredient with a positive PCR is infectious or not is by conducting a bioassay study in which the suspect sample is fed to live pigs. “We need to develop a low-cost test for infectivity so we can test as many samples as possible, and test feed before it goes to a production site, instead of having to find out after we have fed it,” he says.
He points out that if the PED virus is placed into a feed mixer, it can become evenly distributed, much like a feed ingredient. “It is possible to spread PEDV throughout the whole feed mix during normal processing procedures,” he says.
While additional work is being done to research ingredients as possible disease vectors, and on prevention strategies such as feed-mill audits, Tokach says some basic biosecurity steps can also help when it comes to the feed manufacturing and delivery process. Traffic going from and coming to feed mills is a major concern when it comes to spreading PEDV, especially in hog-dense regions of the country. “Traffic coming back from finishing or other pork production sites just has to be considered positive,” he says. “A transport vehicle that has been to a positive site can test positive for PEDV. We are going to have to move toward managing feed mills like a food plant, with a one-way flow. We are going to have to think about how we organize every step, from ingredient receiving and storage to outgoing flows in feed-mill setups.”
Pork checkoff-funded research is currently underway to evaluate the heat and temperature conditions at which PEDV is inactivated in feed. Feed-mill contamination and ingredient handling to minimize PEDV risk are also areas in which research is currently being conducted.
In the meantime, Tokach says pork producer-response and management approaches to prevent possible PEDV infection in their herds via feed can be divided into four main areas: the “ostrich approach,” purposeful use, “not in my feed,” and a “not here, not there, not anywhere” strategy.
Tokach deems the riskiest approach to PEDV the “ostrich approach,” meaning that some producers and feed mills are saying they don’t believe that feed is a possible source of PEDV infection. Instead of trying to be proactive, people taking this approach go on like nothing has changed. “This approach may have the lowest up-front cost,” Tokach says. “And I say up-front cost because if you are a mill that starts infecting farms, or if your farm becomes infected, it does become a high-cost approach.”
Tokach says many production systems are choosing to continue to use porcine products in their operations in a controlled and purposeful manner. These farms may take the products out of the diet on sow farms, or out of their multiplier nurseries, but still use the products in order to take advantage of benefits in the commercial nursery or in the finishers. “These farms go through a process of auditing suppliers and limiting the sources of ingredients,” he says. “A good example of how this auditing process works would be with meat and bonemeal. We know that renderers in the U.S. have done a great job of improving their processes — but even within those suppliers, some are much better than others.”
For the purposeful-use approach, Tokach suggests producers would visit the sites where they are obtaining porcine byproducts to help determine what plants might be the best candidates for providing those ingredients. “Determine which products are highest and lowest risks, and put together a list of approved suppliers after you have conducted the audits. Be aware that this can be a big challenge, and it may be difficult to truly assess risk.” Tokach emphasizes that producers have to make their own decisions about the type of risk they are willing to incur. The purposeful-use approach may or may not require that products test PCR-negative. This is considered a low-cost option that does provide some level of protection.
Not in My Feed
Some producers are choosing to continue to source their feed ingredients from mills that handle porcine products, but are asking that the products are not used in the farm’s specific feed ingredients. “Taking this approach requires attention to sequencing of mixing, load-out and delivery processes,” Tokach says.
Not Here, Not There, Not Anywhere
Some producers are choosing to source feed from mills that handle no porcine products. Tokach says this may not be an option for producers who live in areas with fewer mills to choose from. “The ingredient risk may be reduced, but we are seeing some poultry meals and bakery meals coming back as positive for PEDV. Sometimes this is due to cross-contamination, and sometimes ingredients may not all be what we are expecting them to be,” he says.
In reality, all four approaches to sourcing feed and ingredients may exist within the same production system, Tokach says. “Producers must determine their level of risk aversion to determine the best feeding strategy for their production system. Our knowledge has increased greatly in the last nine months since we first saw feed as a possible vector for this disease, but we still have a lot to learn. At the end of the day, we still need to feed the pigs.”
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