Addressing the recent Clifford Lecture run by Watt Agri and sponsored by Anitox, leading authority on ASF transmission Dr Megan Niederwerder explained how the virus could access, survive and thrive in feed. To fully understand the role of feed as a transboundary viral disease vector, her story starts long before feed arrives on-farm. For example, it's common practice in some countries to lay cereal grains on roadways in order to dry them, in turn exposing them to all manner of contaminants that could include swine excrement and bodily secretions from passing trucks. Once contaminated, ingredients cross the Atlantic or Pacific Ocean in shipping containers before being transferred via trucks from ports of entry to feed processing facilities. They’re then incorporated into a complete feed diet, before being transported to the farm, where the complete feed diet is fed to pigs.
Dr. Niederwerder’s recent research has focused on expanding our knowledge of the risk of feed and feed ingredients as a route for ASF introduction along with potential mitigation strategies. She and her team have taken a three-part approach to this research:
- Determine ASF’s survival in feed and feed ingredients under a transboundary model.
- Investigate oral dose and transmission through natural feeding behavior.
- Assess tools for mitigating risk of transmission in feed and feed ingredients.
Transoceanic shipment model
First, to understand the stability of ASF in various feeds and feed ingredients, Dr. Niederwerder used a transoceanic shipment model which simulates the environmental conditions that feed ingredients would undergo when being shipped over the course of 30 days. With historical meteorological data, the team matched temperature and relative humidity levels in the experiment’s environmental chamber those typically experienced by ingredients on a transoceanic trip. Dr. Niederwerder examined 12 feeds or feed ingredients as part of this experiment. At the conclusion of the trial, each of the 12 samples maintained a detectable amount of ASF DNA. In nine of the 12, infectious virus was still detected.
Infectious dose and transmission through natural feeding behavior
The second part of Dr. Niederwerder’s study sought to determine the infectious dose and risk of infection for ASF via natural drinking and feeding behavior. To answer these questions, she studied both liquid media and plant-based feed contaminated with various doses of ASF. Importantly, the experimental design allowed these piglets to consume ASF naturally. Five days post-exposure, the team found that the minimum infectious dose in liquid media was extremely low; the lowest dose that can reliably be administer to pigs infecting 37.5 percent of those exposed. With complete feed, on the other hand, she found that the lowest infectious dose was 104 TCID50.
Tools for mitigating risk of ASFV transmission through feed
The third part of Dr. Niederwerder’s approach has been to assess strategies to mitigate the risk of transmission of ASF in feed and feed ingredients. The team determined some critical questions:
- Is the ingredient necessary for swine nutrition, or can it be excluded from the diet?
- Can the ingredient be sourced locally or from a country that does not have circulating African Swine Fever Virus?
- What are the risks of the ingredient becoming contaminated based on manufacturing protocols?
- What are the risks of the ingredient supporting the development of the virus during shipment?
For imported ingredients that can’t be replaced and represent risk, the team noted mitigation techniques in three categories (hold time, heat and antiviral chemical controls).
Through the transoceanic shipment model, Dr. Niederwerder examined the half-lives of the virus in the nine ingredients capable of supporting infectious virus for 30 or more days. She found an average half-life of 12.2 days among these nine ingredients. Interestingly, she also found that ASF had a longer half-life in each of the nine ingredients than it had in the media control, indicating that these ingredients enhance the stability of the virus.
This half-life data was used to calculate holding time recommendations which were published by the Swine Health Information Center and other major pork-producing organizations.
Dr. Niederwerder and her team have also conducted research on the efficacy of medium chain fatty acid and formaldehyde-based feed additives on inactivation of ASFV. She found that both types of feed additives were capable of reducing the viral quantity below the level of detection on cell culture with doses of 0.7 percent and 0.35 percent, respectively.
She and her team went on to explore the efficacy of these feed additives when mixed with feed ingredients that were subjected to a transboundary feed model. Though she found ASF DNA in each of the samples tested, she also noted that use of a formaldehyde-based feed additive led to substantial reduction in the ASF DNA quantity in certain feed ingredients.
Thank you to Dr. Niederwerder for an excellent presentation. Thanks to her work we’ve learned that: 1) ASFV survives in feed ingredients exposed to transoceanic shipment conditions. 2) ASFV is transmissible through the natural consumption of plant-based feed. 3) ASFV stability is increased in feed, with half-lives ranging between 9.6 and 14.2 days and 4) MCFA and formaldehyde-based feed additives reduce the infectivity of ASFV in feed, but do not eliminate DNA.