Ever since the introduction of Porcine Epidemic Diarrhoea virus into the United States in 2013, the US has been extra aware of the risk of animal diseases entering through feedstuffs. No wonder that the role of feed gets a major emphasis in US research on African Swine Fever. What role exactly can feed play in transmission and mitigation of the virus?
Swine enteric coronaviruses, including Porcine Epidemic Diarrhoea virus (PEDv) and porcine deltacoronavirus (PDCoV), are considered the last major transboundary swine diseases introduced into the US pig herd in 2013 and 2014, respectively. Several epidemiological analyses into the introduction and rapid spread across new farms revealed the potential source of the virus as contaminated feed and feed ingredients. Although there are other risk factors, such as illegally smuggled pork products, for introduction of ASFv into the US, plant-based feeds and feed ingredients are of particular concern due to several unique characteristics. What are the relevant questions to ask?
Identifying which feed ingredients provide an environmental matrix that supports ASFv stability is an important step in determining risk. To evaluate this risk using a transboundary shipment model, in 2018 a team of researchers led by Dr Scott Dee selected 12 feeds, ingredients or products of animal origin based on import volume and use in swine feed for evaluating ASFv stability. Ingredients included conventional soybean meal, organic soybean meal, soy oilcake, distillers dried grains with solubles (DDGS), lysine, choline, vitamin D, moist cat food, moist dog food, dry dog food, pork sausage casings and complete feed.
After 30 days of transatlantic shipment conditions, ASFv Georgia 2007 was broadly stable across diverse ingredients, with infectious virus being detected in 75% of the tested ingredients, including conventional soybean meal, organic soybean meal, soy oilcake, choline, moist cat food, moist dog food, dry dog food, pork sausage casings and complete feed.
ASFv transmissibility through the oral route has been appreciated for a century and was recognised early on as having more variability than parenteral routes of inoculation. However, transmissibility of the modern ASFv Georgia 2007 isolate through natural drinking of contaminated liquid and natural eating of contaminated plant-based feed was only recently characterised. For this study, infectivity rates in nursery pigs were determined at various ASFv doses consumed naturally in small volumes of liquid media (100ml) or complete feed in meal form (100g). Confirmed infection occurred through both drinking and eating routes, with lower doses required for transmission in liquid compared to feed. Specifically, the minimum infectious dose of ASFv Georgia 2007 in liquid was 100 50% tissue culture infectious dose (TCID50), whereas 104 TCID50 was the dose necessary for infection in feed.
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Statistical modelling of repeated exposures to small volumes over time (i.e. consuming a contaminated batch of feed or drinking contaminated water) revealed an increased likelihood of infection as the number of exposures or total consumption volume rises. Taken together, ASFv is orally transmitted through natural consumption of contaminated plant-based feed, with the infection probability dependent on the quantity of virus present and the volume of feed consumed.
As a relatively new area of specialisation in the biosecurity realm, feed biosecurity has become an important and widely recognised biosecurity target critical for the prevention of porcine viral disease entry onto farms. When examining feed ingredients as a potential pathogen source, several factors influence this biosecurity risk.
In addition to biosecurity and sourcing considerations, physical and chemical treatments of feed or ingredients can be tools for risk mitigation of ASFv.
Implementing feed quarantine, or storage of ingredients after import from high-risk countries and regions, is one strategy intended to allow virus decay prior to incorporation of the ingredients into swine diets.
Heat treatments and storage of crops and plant-based ingredients have demonstrated experimental efficacy in reducing the infectivity of swine viruses such as ASFv and PEDv. For example, research in 2020 by Melina Fischer and others from the German Friedrich-Loeffler-Institut contaminated field crops, including wheat, barley, rye, triticale, corn and peas, with ASFv Armenia 2008 prior to subjecting the crops to a two-hour drying period at 20°C. After two hours of storage at room temperature, no infectious virus could be isolated from the unprocessed crops.
Feed additives with antimicrobial activity against ASFv and other swine viruses have gained substantial interest in the wake of feed risk awareness and the need for antibiotic alternatives. Primary additive classes investigated for antiviral activity include aqueous formaldehyde, medium-chain fatty acids (MCFA), short-chain fatty acids, organic acids and essential oils. Mechanistically, these antimicrobial products inactivate viruses in different ways, and regulations on use vary by country.
For example, MCFA are believed to reduce virus infectivity by disrupting the viral envelope, leading to deconstruction of the virion and an inability to bind to the host cell for entry.
A second example is aqueous formaldehyde, which is believed to reduce virus infectivity through alkylation and cross-linking of viral nucleic acids and proteins. The efficacy of both MCFA and aqueous formaldehyde has been experimentally confirmed for ASFv.
Taken together, both physical and chemical treatments provide opportunities to reduce virus risks in feed; however, it is important to note that most methods of mitigation do not eliminate ASFv DNA or other viral nucleic acid from feed, underscoring the importance of determining virus biological infectivity after mitigation is applied.
With economic losses of ASFv introduction into the US swine herd estimated at >$ 15 billion due to production losses and market disruption, the importance of preventing entry cannot be overstated. As thousands of metric tonnes of swine feed ingredients are imported each year into the US from countries with active ASF outbreaks, it is critically important that mitigation strategies be investigated and adopted to reduce the risk of ASFv entry through this route.
* This article is an abridged and approved version of a recent article that was published a peer-reviewed journal: Niederwerder M.C., Risk and Mitigation of African Swine Fever Virus in Feed. Animals. 2021; 11(3):792.
References available on request. Funding was provided by the US National Pork Board.
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Dr Megan Niederwerder, DVM, PhD, assistant professor, Kansas State University, USA