ASFv can be kept out of mills, but once in feed materials, it spreads…and stays

For the US, African Swine Fever (ASF) is a foreign animal disease; it is not present in the territory. Given the size of the country’s pork industry, everyone would very much like to keep things that way.

Learning from past foreign animal diseases can be instructive. Porcine Epidemic Diarrhea virus (PEDv) arrived in the US ten years ago, and is believed to have spread through feed. This served as a wakeup call to the dangers of feed-based viral transmission and catalyzed research in this area.

Work recently conducted by Dr. Jordan Gebhardt of Kansas State University is part of that effort. As he explains, research done at the university in years past had established that when feed mills handle ingredients contaminated with PEDv, it spreads extensively. “Anywhere we can find dust within a feed mill, which is just about everywhere, we can find virus. So our hypothesis is that virus attaches to dust particles [and] dust particles become widely distributed within that space,” he says. Now, with African Swine Fever potentially knocking at the door, he and his team wanted to establish whether the same would be true of ASFv, a larger and more complex virus which has been known to survive in feed ingredients.

For obvious reasons, they could not simply introduce one of the most dangerous animal pathogens known to science into a feed mill and record what happened. Work done with ASFv must be conducted in a BSL level-3 space equipped with appropriate safeguards to ensure the virus remains contained. But Kansas State University is one of a very few US institutions which has such facilities.

And so they basically built a tiny mill inside the Kansas State BSL-3 space. “We had a 50 kg capacity mixer, about a 2-3 meter tall bucket elevator, so somewhat smaller scale than a commercial mill but all the same equipment, all the same mechanisms,” Dr. Gebhardt explains. “So we set that equipment up in that facility, and then we did the same experimental design [as had been done with PEDv previously]. We ran some feed through it to prime the system, we ran an inoculated batch with ASF through, and then we ran some subsequent batches.”

Key Findings

Like with PEDv, they found that the ASF virus got everywhere. “If we go around that animal room, which is probably 5 meters by 6 or 7 meters in size, if we went to just about any surface within that room and took an environmental sample of that surface, we could find ASF virus. So within that experimental setting, that ASF virus became widely distributed.”

Moreover, they continued to find it in the subsequent batches of feed they ran after the inoculated batch, something which compares negatively with the PEDv experience. “That was even a little bit more clear within the ASF data, that carryover from batch to batch was even a little bit more significant than what we observed with PED,” he says.

Dr. Gebhardt is careful not to jump to conclusions from that data point alone; after all, they were using a different virus, different means of measuring the inoculation dose, etc. However, it is perhaps noteworthy: “Within that PED project, eventually, after running enough batches of feed through, we could no longer find the virus. Under the conditions of this experiment with ASF — the dose we started with, the number of batches we used — even after that last batch of feed, we could still find the virus…under the conditions of this study, we were able to detect it a little bit longer than we were with PEDv.”

Finally, there was also the persistence of the virus. “When we look at day zero, there’s about four logs of virus. And even when we store those surfaces for 180 days, we still have between 2 and 3 logs of virus,” Dr. Gebhardt recounts. “So statistically, there was no decrease in ASF DNA, even after six months.”

Again, he warns, one needs to be cautious about extrapolating from that; the presence of a piece of genetic material does not indicate whether the virus can still cause infection in an animal. Still, in his words, “this data was absolutely eye-opening, and so to answer the question of the stability of ASFv in the environment within a feed mill: it is extremely stable.”

Thus, this research could provide useful insight for diagnostic purposes — understanding after an outbreak if ASF was present in the feed mill or not. “If ASF was in that feed, we [now] know that virus becomes widely distributed within the mill, and if we go in and collect dust samples or environmental surface samples, we know that if ASF was there, we would find the DNA, even months later.”

Beyond that, it also demonstrates that you cannot simply hope for the virus to flush itself out of a contaminated mill. “We do need to worry about some of those later batches of feed. Therefore, if we really are concerned about [ASF], we probably need to layer additional biosecurity or mitigation techniques on top of just our normal batching procedures.”

BIOSECURITY, DONE RIGHT, WORKS WELL

This confirmation that the ASF virus spreads widely and lingers for a long time in a mill is admittedly not great news. However, Dr. Gebhardt also has more hopeful insight about ASFv in a feed mill setting, although this draws from earlier research.

In 2019-2020, a biosecurity-minded, vertically-integrated swine producer in Vietnam asked his team to see how the concepts they’d identified in PEDv spread would apply in the field, in a context where ASF was present in the area. “They wanted to understand what practices they should be considering, or what were the potential gaps in their existing biosecurity,” he explains.

The initial sampling identified trucks as the most problematic source of contamination. More specifically, he says, the big gap in the biosecurity was the truck cab. “They were disinfecting and washing the outside of feed trucks every time they would come back to the mill, but the cabs of those trucks, the insides, were not being decontaminated very well.”

Having identified the issue, they were able to identify a technique using a fan to spray disinfectant inside the cabs as well. “Once they made that change, we didn’t find any more positive samples,” he states.

There are a few lessons that can be drawn from this, including about the importance of people and moving objects in spreading viruses, and about the ability of properly-implemented biosecurity measures to control that spread. But it is also interesting to contrast these findings with the more recent experiment involving inoculated feed in a research setting. As Dr. Gebhardt notes, in a field setting known to have ASF circulating, they actually found vanishingly little trace of the virus in a biosecurity-focused mill, even when casting a wide net.

“We also looked at the feed mill itself, and going back to the PED work, instead of doing pinpoint ingredient samples or feed samples where we really have to find that inoculated spot in order to find the contamination, we focused on environmental samples — dust samples, primarily — because we thought that would be a more representative sample type over a long period of time… and what was interesting [was that in] just over 1,100 samples, we only found nine samples that were PCR positive.”

Of course, this should not minimize the threat of the ASF virus, nor the intense efforts that the company in question was taking to keep it out. “The biosecurity measures they implemented were terrific. So these results probably couldn’t apply to all scenarios,” he notes. “But in a situation where they do biosecurity well, we can keep ASF out.”

The inoculated feed study was described in the papers “Evaluating the distribution of African swine fever virus within a feed mill environment following manufacture of inoculated feed” in PLOS ONE and “Effect of mixing and feed batch sequencing on the prevalence and distribution of African swine fever virus in swine feed” in Transboundary and Emerging Diseases, as well as in an extension report available here and submitted for peer-review publication. The Vietnamese work was described in “Sampling and detection of African swine fever virus within a feed manufacturing and swine production system” in Transboundary and Emerging Diseases

By Shannon Behary, senior editor