MANHATTAN, KANSAS, US — The US Department of Agriculture (USDA) in August 2021 confirmed that African swine fever virus (ASFV) had been diagnosed in the Dominican Republic. This posed a tremendous threat to the swine industry of the United States given the proximity to the mainland.
When ASFV emerged in China in 2018, the virus was able to move rapidly and easily throughout the country due to movement of animals and contaminated fomites, showcasing the ability of ASFV to cause disastrous consequences on a naïve pig population.
An introduction of ASFV into North America is a significant threat because the United States maintains significant trade relationships with countries that have endemic ASFV. Regulatory control of live animals and pork-containing foods substantially reduces this risk, but there is evidence that feed and/or ingredients may be potential vectors of ASFV introduction.
Previous research has been conducted to determine the minimum infectious dose of ASFV in water and feed. Recent research also has shown that ASFV can survive in various feed ingredients during transboundary, transatlantic shipping. Because ASFV can survive in feed during shipping, the United States is rightfully concerned that a contaminated feed or ingredient has the potential to introduce ASFV into the nation’s swine population.
Regardless of its method of entry, there is concern that infection of US pigs may result in contamination of the feed supply chain, and rapid and widespread distribution of the virus like what was seen with porcine epidemic diarrhea virus (PEDV). Field evidence suggests that ASFV can be distributed throughout the feed supply chain, and this has been confirmed with recent research published.
It has been determined that the distribution of ASFV in the feed manufacturing environment is widespread and persists even after manufacturing additional feed batches initially free of ASFV. A similar pattern was observed with PEDV, indicating the extreme importance of preventing ASFV entry into US feed mills as ASFV will remain in the environment for extended periods of time following its initial introduction.
If a feed manufacturing facility becomes contaminated with a virus, there are no current recommendations for the best practices to clean and disinfect these facilities. Research is needed to determine optimal methods for disinfecting feed manufacturing facilities, especially equipment that is not designed to be disinfected. This led the Institute for Feed Education and Research (IFEEDER), Animal Nutrition Association of Canada (ANAC) and United Soybean Board (USB) to join with the Swine Health Information Center (SHIC) to launch a research project at Kansas State University (KSU) that will evaluate several methods for cleaning and disinfecting feed mills following a potential African swine fever (ASF) outbreak.
Although this research is pertinent for ASFV, it is not feasible to conduct ASFV feed-based research with the available facilities in the United States. Previous research has demonstrated Seneca Valley virus 1 (SVV1) to be the most stable virus in feed; therefore, recent studies evaluated disinfection and flushing procedures using SVV1, PEDV, and PRRSV, which are all currently present in the United States.
To reduce the risk of virus transmission in feed, multiple mitigation strategies have been evaluated. Flushing, or running an ingredient through the feed manufacturing equipment, can help remove residual virus particles from the equipment and dilute remaining virus concentrations in subsequent batches. A rice hull flush, with or without chemical additives, reduced the presence of PEDV in both the flush batch and subsequent batches of feed.
Flushing study
A recent study at KSU was conducted to evaluate different physical and chemical flushes in order to reduce the viral presence and infectivity of SVV1, PEDV, and PRRSV when contaminated feed was introduced into a feed manufacturing facility. In this experiment, feed manufacturing equipment was primed with a virus-negative batch of feed (primer batch), followed by feed inoculated with equal quantities of SVV1, PEDV, and PRRSV (positive batch), followed by a flush treatment, and three subsequent batches of virus-negative feed (Sequence 1, 2, and 3). During each batch, feed would be mixed for 5 minutes in a 50-pound mixer and discharged at a rate of 10 pounds per minute into a feed bin. Feed would then be poured into the hopper of a bucket elevator with 74 buckets at an equal discharge rate (10 pounds/minute) and conveyed through a downspout into a fresh feed bin. The application and composition of each flush treatment were as follows:
- Ground corn: 50 pounds of ground corn (500 microns) was manufactured as previously described.
- Liquid formaldehyde: A liquid applicator pump and spray nozzle was connected to the 50-pound mixer. A pre-measured 0.8-pound aliquot of 10% liquid formaldehyde was dispensed onto 50 pounds of ground corn. Once dispensed, the ground corn plus liquid formaldehyde solution was allowed to mix for 5 minutes and manufactured as previously described.
- Liquid formaldehyde plus physical abrasive: A 25% abrasive mixture of 12.5 pounds of calcium limestone and 37.5 pounds of ground corn was added to the mixer. A pre-measured 0.8-pound aliquot of 10% liquid formaldehyde was dispensed onto the abrasive mixture. Once dispensed, the abrasive mixture plus liquid formaldehyde solution was allowed to mix for 5 minutes and manufactured as previously described.
- Double flush — physical abrasive followed by liquid formaldehyde: The first flush, a 25% abrasive mixture of 12.5-pound calcium limestone and 37.5 pounds of ground corn, was manufactured as previously described. Following the physical abrasive flush, a pre-measured 0.8-pound aliquot of 10% liquid formaldehyde was dispensed onto 50 pounds of ground corn. Once dispensed, the ground corn plus liquid formaldehyde solution was allowed to mix for 5 minutes and manufactured as previously described.
- Dry formaldehyde: 50 pounds of a 4% dry formaldehyde was added to the mixer and manufactured as previously described.
One important thing to note is the safety precautions that need to be considered for these treatments. The current approved application of liquid formaldehyde is 6.5 pounds/ton or 0.3% inclusion in the diet. The inclusion used in the liquid formaldehyde treatments was five times the recommended levels at 32.5 pounds/ton or 1.6% inclusion in the diet. For the researchers’ safety, full face respirators with disposable cartridges were used once either liquid or dry formaldehyde products were implemented. Environmental formaldehyde levels were monitored throughout feed manufacturing of subsequent batches with a formaldehyde meter.
It was concluded that flush batches between the purposefully inoculated batch and subsequent virus-free batches of feed were able to reduce viral RNA regardless of treatment type. Chemical mitigants reduced viral RNA, specifically SVV1 RNA, more quickly than the non-chemical ground corn treatment. However, viral RNA was still present in feed and within the environment after the final batch. The flush treatments were able to reduce SVV1 infectivity of the subsequent feed and dust samples when given to pigs in a bioassay, but non-detectable PRRSV RNA was still infectious when consumed by pigs.
Flushing feed manufacturing systems following a pathogen introduction can reduce the initial contamination but cannot completely eliminate the risk of recontamination in both the feed and the environment. Detailed data from this project will be released in the 2023 Kansas State University Swine Day.
Pelleting as a thermal mitigant
In addition to flushing procedures, researchers at KSU wanted to evaluate the efficacy of pelleting as a thermal mitigant for PEDV, PRRSV, and SVV1 in contaminated feed. Pelleting is a form of thermal processing and is considered a point-in-time mitigation as it is capable of inactivating viruses during processing but does not protect the feed from cross-contamination.
The effect of pelleting on PEDV infectivity has been previously investigated and considered variable. At low conditioning temperatures (<130°F) pelleted feed was still able to cause infection in a swine bioassay while feed conditioned at greater than 130°F failed to cause clinical signs of infection even though viral RNA was detected in the initial inoculum. However, dry heating complete feed at temperatures between 248°F to 293°F took up to 25 minutes for viral inactivation compared to the relatively short conditioning time of 30 seconds used in pelleting.
Pelleting experiments rarely have been conducted with other swine viruses due to the specialized equipment and space needed for these experiments. Viruses endemic in commercial swine production, such as PRRSV and SVV1, have been evaluated for viral mitigation at room temperatures, but feed contaminated with these viruses have not undergone pelleting parameters. Therefore, data was extracted from the previous experiment conducted at KSU to determine the objective of evaluating the efficacy of pelleting as a thermal mitigant for PEDV, PRRSV, and SVV1 in contaminated feed.
During this experiment, feed manufacturing equipment was primed with a virus-negative batch of feed (primer batch), followed by feed inoculated with equal quantities of SVV1, PEDV, and PRRSV (positive batch). During each batch, feed would be mixed for 5 minutes and discharged into a feed bin. Feed would then be poured into the hopper of a bucket elevator and conveyed through a downspout into a fresh feed bin. All feed would be pelleted in a CL-5 Laboratory Pellet Mill reaching a conditioning temperature of 180°F with a 30-second retention time. Feed would be dispensed into six trays each holding approximately 8.8 pounds of pelleted feed per tray. As trays were filled, they would be placed in a double door pellet cooler for 10 minutes before being sampled and discarded into double lined autoclave totes.
Trays 1-3 would contain pellets that had not been conditioned to 180°F, while trays 4-6 would all contain pellets that had reached the conditioning temperature of 180°F. Tray 4 was considered the low-temperature tray, as it shared circulating air in the pellet cooler with trays that had not reached the goal conditioning temperature. Tray 6 was considered the high-temperature tray as it was placed in the pellet cooler only with trays that had met or exceeded the conditioning temperature of 180°F.
It was concluded from the experiment that the use of pelleting reduced the quantity of detectable RNA in the feed and environmental surfaces within the feed mill. However, SVV1 and PEDV RNA remained detectable in post-thermal processing samples, but these samples were considered non-infectious when pigs were inoculated during a swine bioassay.
Pelleting at 180°F did not sufficiently reduce the risk of PRRSV infection as pigs showed clinical signs of PRRSV infection during the same bioassay. Furthermore, the accumulation of viral RNA throughout the feed manufacturing facility may further increase the risk of recontamination. Pelleting is a suitable technique for reducing the risk of contamination provided care is taken to avoid contact between the pre-thermal and post-thermally processed feed and sufficient temperatures are utilized.
Chad Paulk is an assistant professor of feed science and management in the Department of Grain Science and Industry at Kansas State University. He may be reached at [email protected]. Also contributing to this article were Cassandra Jones, Jason Woodworth and Jordan Gebhardt.
