Reviewing the risk of feed as a vehicle for swine pathogen transmission

Abstract Background While porcine biological hazards have had the potential to be transmitted through feed and feed mills for decades, the emerging threat of foreign animal disease has elevated the concern that these may enter or be transmitted throughout the domestic swine herd via a feed vehicle. Objective The goal of this review was to describe the current classification for emerging porcine biological pathogen transmission through the feed supply chain so resources can be best directed towards those of highest risk. Methods By assessing the pathogen severity to pigs and the probability of pathogen transmission through feed, an overall risk can be established using a hazard analysis matrix. Results There is negligible risk for feed‐based transmission of a transmissible spongiform encephalopathy, Trichinella spiralis, Toxoplasma gondii, Salmonella Choleraesuis, Salmonella spp. except Choleraesuis and I 4,[5],12:i:‐, porcine deltacoronavirus, Senecavirus A, mammalian orthoreovirus 3, foot and mouth disease virus, classical swine fever virus or Chinese pseudorabies virus. However, the combined severity and probability of Salmonella enterica serotype I 4,[5],12:i:‐, porcine epidemic diarrhoea virus and African swine fever virus warrant a moderate risk characterization for transmission through the US feed supply chain. Conclusions This risk can be maintained below critical status by minimizing the likelihood that a pathogen can enter the feed supply chain, such as by excluding high‐risk ingredients from facilities, extending biosecurity to mills, and considering proactive mitigation strategies. In reality, all these actions may be necessary to prevent the detrimental transmission of porcine biological hazards into the US swine herd through the feed supply chain.


| INTRODUC TI ON
The US swine industry has made substantial gains in herd health by implementing farm biosecurity practices. Many of the primary routes of pathogen entry into the farm (i.e. other pigs, farm employees, visitors, air, etc.) have been minimized in high health systems, whereas less research has characterized the risk of feed as a vector of disease transmission. Much of the research thus far has focused on pathogens that are domestic threats in the United States, such as porcine epidemic diarrhoea virus (PEDV) or Salmonella. As foreign animal diseases, such as African swine fever virus (ASFV), classical swine fever virus (CSFV) and foot and mouth disease, circulate among a number of global trade partners, the concern for ingredients as a vector of transboundary disease entry has increased. While not all pathogens are strong candidates for feed-based transmission, it is important to characterize the risk for the feed supply chain to serve as a vehicle for pathogen entry into farms.
Research in this area is ongoing, so a review of current knowledge is important to form a foundation from which to strategically address research gaps. As such, our objective is to characterize pathogenic biological hazards that may be a risk for entering through the feed supply chain, and to describe their potential prevention or mitigation.

| Assessing the risk of swine pathogens in feed
In response to the Food Safety Modernization Act, the US Food and Drug Administration (FDA) published 21 CFR Part 507: Current Good Manufacturing Practice, Hazard Analysis and Risk-Based Preventive Controls for Food for Animals. This rule requires facilities that manufacture animal food for consumption in the United States to conduct a hazard analysis of agents that may cause illness or injury to humans or animals via the animal food. The rule requires that the assessment of severity and probability of an agent be used to determine overall risk, which allows a system to focus its resources on the most critical areas.
This review relies on the same required methodology. In the case of pathogens in feed, the severity of a hazard is typically consistent across production systems, feed mills and even countries. However, the assessment of hazard probability is highly variable, depending upon a wide variety of circumstances that are unique to each system. For example some pathogens, such as Salmonella, have the ability to impact both human and animal health, whereas others, such as PEDV, have no impact on human illness. Furthermore, the type of raw materials included in a diet, the environment in which feed is manufactured, the transportation of ingredients and feed to and from the facility, the prerequisite programs enforced and the training of the individuals involved in each process can all change the probability of hazard occurrence. Therefore, the classification of severity, and especially probability, should be conducted by each individual system. It is the authors' hope that this paper will facilitate that assessment, not replace it. For the purpose of this paper, assumptions include: (a) pathogen presence in the United States is as of September 1, 2019; (b) all entities manufacturing, processing, packing, holding, or transporting animal food are in compliance with all federal food safety regulations; (c) pigs are raised in indoor, conventional US commercial production.

| Severity
The assessment of severity includes evaluating its ability to cause illness or injury to humans or animals, regardless of its probability.
Classifications were determined as described in Table 1. Severity is associated with the pathogen itself, and is unrelated to its likelihood of contamination, survivability or infectivity. Its assessment is also indecent of systems placed for prevention or mitigation.

| Probability
The assessment of probability includes evaluating its likelihood of containing the pathogen, regardless of its severity. Classifications Classification Impact on human illness or injury Impact on animal illness or injury Potential minor illness from which full recovery is likely of few animals TA B L E 1 Classification of severity assessment based on impact on human or animal health were determined as described in Table 2. Probability is associated with the likelihood of contamination, survivability or infectivity of a pathogen. Its assessment is also impacted by systems placed for prevention or mitigation. However, probability is independent of the possible severity the pathogen may have on human or animal health.

| Assessment of risk
To assess overall risk to the system, the severity and probability must then be considered collectively, as demonstrated in Table 3. It is the combination of severity and probability that impact overall risk of feed as a vehicle for swine pathogen transmission.

| RE SULTS AND D ISCUSS I ON
3.1 | Pathogens of concern for feed-based transmission

| Prions and parasites
Prior to 2010, the primary biological hazard of concern for US feed mills were transmissible spongiform encephalopathies (TSE) caused by prions, such as bovine spongiform encephalopathy.
There is limited evidence of animal-to-human transmission and its impact on a large number of animals, leading to a very high severity. While TSE transmission to pigs has been established in experimental conditions, it is not known to occur naturally in swine (Hedman et al., 2016 , 2018). This leads to an overall risk ranking of negligible (Table 4).
Both Trichinella spiralis and Toxoplasma gondii have historically been linked to feed-based transmission and have potential impact on human health, leading to its high severity (Guo et al., 2015;Slifko, Smith, & Rose, 2000). However, biosecurity improvements and indoor all-in/ all-out management have largely eradicated the hazards from the domesticated swine heard (Davies, Morrow, Deen, Gamble, & Patton, 1998). In a study of North Carolina swine farms in 1998, just 1 of 2,175 (0.057%) pigs housed in confinement was seropositive for T. gondii and 1 of 2,183 (0.046%) pigs was positive for antibodies against T. spiralis (Davies et al., 1998). Even then, the resulting carcass processing and meat preparation must be inadequate for illness to result, leading to an unlikely probability. These data suggest that feed-based transmission of prions or parasites to domestic pigs is a negligible risk (Table 4).

| Bacteria
While the poultry feed and pet food industries have evolved to con-

TA B L E 3 Classification of risk based on the combination of severity and probability
Choleraesuis transmission through feed would be a high severity, but unlikely probability, leading to negligible overall risk (Table 4).
Outside of the regulatory environment, there is still concern for other serotypes of Salmonella spp. In particular, there is rising concern with an emerging serotype, Salmonella enterica serotype 4,[5],12:i:-in swine. This is a potential monophasic variant of Salmonella Typhimurium, and highly resistant to multiple drugs, including ampicillin, streptomycin, sulfonamides and tetracyclines (Elnekave et al., 2018). The serotype is most commonly linked to pork products, and was responsible for a substantial recall of whole roaster hogs from the state of Washington, but was not been linked to feed (Elnekave et al., 2018). (1) (Lomonaco et al., 2018). Notably, Salmonella Choleraesuis was not identified. Based on limited data linking other Salmonella serotypes to animal health or resulting human illness, the severity of Salmonella spp. beyond Salmonella Choleraesuis and I 4,[5],12:i:-is characterized as very low, whereas the probability is almost certain, leading to an overall risk of negligible (Table 4). As the swine industry has continued pressure to eliminate sources of antibiotic-resistant bacteria from pork, the feed-to-fork food safety will continue to be an emerging concern. It will become important to characterize the serotypes of Salmonella more fully and understand their source of entry and potential transmission to impact animal or human health.

| Viruses
The major swine-based viruses with concern for feed-based trans- . This leads its probability to be characterized as possible, and an overall risk of negligible (Table 4)

| Viral survivability
If an ingredient is contaminated with a virus, the pathogen must survive transport to cause infectivity in the US swine herd. For foreign animal diseases, this would involve either trans-Atlantic or trans-Pacific shipment in varying temperatures and humidity. The potential for transboundary entry through ingredients has been determined directly for ASFV, and indirectly via surrogates for FMDV, CSFV and PSV (Dee et al., 2018). When subjected to conditions mimicking

| Infectivity of the virus
To cause illness within an animal, there must be sufficient quantities of virus within a feed or ingredient to cause infectivity. One of the reasons that PEDV is so easily spread through the feed supply chain is its low infectious dose, just 5.6 × 10 1 TCID 50 /g in feed has been demonstrated to be infectious via bioassay (Schumacher et al., 2016). This equates to one gram of faeces from an acutely infected pig having the potential to contaminate 500 tonnes of feed with PEDV (Jones, Stark, Dritz, Rigdon, & Woodworth, 2015).

| Overall risk of feed as a vehicle for foreign swine pathogen transmission
Based on current knowledge of the likelihood of contamination, survivability or infectivity of a pathogen, prevention or mitigation, the probability for FMDV, CSFV and PSV transmission through feed is unlikely, leading to a negligible overall risk (Table 4). If ASFV enters the feed supply chain, viral levels may range widely based on the method of contamination. For example environmental crosscontamination may be similar to inoculation levels of 10 1 or 10 2 .
However, direct faecal contamination may lead to up to 10 7 . Due to its unpredictability in dose, and that the probability of African swine fever disease virus (ASFV) transmission by feed is possible, leading to a moderate overall risk (Table 4).
As with all models, this risk assessment is liable to change. For example if ASFV enters the United States, its probability of transmission by feed may increase to likely or almost certain, which may increase its overall risk to critical. The US swine and feed industries must continue to increase their knowledge and prevention of biological pathogen transmission in feed so that risk of disease remains negligible to moderate. Furthermore, resources must be directed towards understanding the potential for contamination, survivability and infectivity of S. enterica serotype I 4,[5],12:i:-, PEDV, and ASFV in feed and ingredients. It is necessary to prevent their entry into the feed supply chain and implement science-based mitigation measures to limit their likelihood of occurrence.

| Prevention of entry
If a foreign animal disease contaminates an ingredient in a sufficiently high dose to survive transboundary shipment and cause infectivity, we must prevent its entry into the domestic swine feed supply chain. This is best accomplished by eliminating high-risk ingredients from mills. Ingredients that are likely to be contaminated based on their combination of geographical location, agricultural practice and transportation methods should be completely eliminated from the entire facility, not just high-risk diets. This is because once a virus enters a feed manufacturing facility, it tends to spread to surfaces and stay present until sanitation, which is extremely difficult to complete in a feed mill. In a 2017 study, a PEDV-contaminated ingredient was introduced into a mixer . That process caused nearly all feed contact surfaces (inside of mixer, interior of conveyors, etc.) to have measurable PEDV after the initial diet was manufactured, and the virus stayed present even after four flushes of PEDV-negative feed batches. Even more concerning, nearly all nonfeed contact surfaces (walls, floors, equipment exteriors, etc.) had measureable virus as soon as PEDV was introduced and remained on these surfaces after all 4 PEDV-negative feed batches were manufactured. Dust collected from PEDV-positive, non-feed-contact surfaces subsequently caused infectivity in a pig bioassay (Gebhardt et al., 2018). In order to decontaminate the facility, all equipment had to be disassembled and pressure washed with a 10% bleach solution.
It has therefore been established that if an ingredient carries a foreign animal disease into a feed mill, there is potential for the mill itself to become contaminated and become a source of disease transmission.
Thus, it is warranted to exclude high-risk ingredients from the facility altogether. A decision tree has been developed by the Swine Health Information Center to help facilities identify high-risk ingredients (Swine Health Information Center, 2018).

| Mitigation of pathogens in feed
Even with the best efforts to prevent foreign animal disease entry into a feed mill, there is still the potential for its presence and subsequent transmission through feed. As a final hurdle to prevent transmission to pigs, facilities may consider proactive mitigation through quarantining ingredients, thermal processing or the use of feed additives.
Quarantining ingredients to allow for natural viral degradation may be an effective method of mitigation; however, there is limited information to carry it out successfully. The concept of viral decay can be used to calculate half-life estimates for quarantine time recommendations (Dee et al., 2014). Unfortunately, currently available data are based only on two data points (d 0 and 30) with one time × temperature combination. More robust thermal decay curves are needed to more accurately estimate quarantine times across a broader range of environmental conditions before the mitigation can be used confidently.
Another mitigation method, thermal processing, has had demonstrated success to reduce the infectivity of PEDV in feed . Again, the method's success relies on time × temperature combinations that have not yet been fully established. In the case of ASFV, pelleting is not a plausible mitigation measure because thermal decay curves involving temperatures achieved through a traditional steam conditioner have never been established. Furthermore, quarantine time and thermal processing are both considered pointin-time mitigation measures. Both may, under ideal conditions, lead to viral inactivation. However, neither protect the ingredient nor feed from subsequent downstream cross-contamination that may occur during conveyance, load-out or transportation.
Due to the potential for cross-contamination, feed additives may be more successful mitigants. Formaldehyde-based ingredients or those containing medium chain fatty acids have had demonstrated success as mitigants of porcine pathogens (Gebhardt et al., 2018).
Their potential is still being evaluated in mitigating foreign animal diseases in feed and ingredients. Still, these ingredients must be used safely and in compliance with regulatory requirements. For example formaldehyde is an approved food additive for the prevention of Salmonella in feed, but its use for PEDV or ASFV control would be outside of the current regulatory approval. As research continues to identify products and additives to successfully mitigate pathogens, it will be a key to maintain dialogue with regulatory agencies so the products can be used legally and safely.
In conclusion, there are two domestic porcine pathogens (S. enterica serotype I 4,[5],12:i:-and PEDV) and one foreign animal disease (ASFV) that pose the greatest risk for entry and transmission through a feed vehicle in the United States. Additional research is urgently needed to fully assess the probability of their occurrence, as well as methods to reduce their likelihood of entry or potential mitigation.

E TH I C S S TATEM ENT
The authors confirm that the ethical policies of the journal, as noted on the journal's author guidelines page, have been adhered to. No ethical approval was required as this is a review article with no original research data.

CO N FLI C T O F I NTE R E S T
The authors have no conflicts of interest to declare.