February 26, 2020
When we think about the African swine fever virus, Cassandra Jones says it's more appropriate to consider the good, the bad and the ugly.
"The good thing about the African swine fever virus, and one that I want to make sure is very clear, is that humans cannot contract this disease by consuming pork or affected pork from those pigs. It's also not transmissible to non-swine species and so your dogs and cats and cattle and poultry are not affected species from this devastating virus," Jones says. "That said, the virus to pigs is incredibly detrimental. It can be transmitted from wild boar or ticks to the domestic pig population, which makes understanding the epidemiology difficult, but it also makes eradication very, very challenging in naïve environments. The other challenge with ASFV is the way that the case presents itself to veterinarians. It certainly is well known to have a high case fatality rate, but some strains may be less pathogenic and initial symptoms easy to confuse with other endemic diseases, reducing the likelihood for quick diagnosis and subsequent eradication."
The ugly part of the virus is there is no available vaccine or treatment, it can survive in fomites for extensive periods and the virus is spreading faster than the speed of research. On Tuesday, the associate professor and member of the Kansas State University Feed Safety Team, told attendees at the Farm Foundation Forum in Washington, D.C., that while the most likely route ASFV would enter the United States would be through direct transmission, the industry has sharpened its focus on efforts to control and prevent indirect methods of transmission.
"This is where human movement, smuggled pork products and feed or ingredients could come into play. If any of those were contaminated, made their way into the United States and in front of a susceptible host, that's how we could have African swine fever virus enter our borders," Jones says.
Jones says it's important to understand how ASFV has historically entered naïve populations. For example, Russia investigated 284 outbreaks to determine the epidemiological root causes. Thirty-five percent attributed the root cause or the primary source of entry of new ASF infections was due to contaminated feed. Contaminated vehicle movement was responsible for 38%, and 23% of the time they could never actually identify a root cause.
"I want to point out that only 4% of the time was direct nose-to-nose contact with affected pigs, whether that's from neighboring farms or from wild boar populations," Jones says. "Even though that's the most direct and most likely method of disease transmission, the entry into a naïve population is very different and, in this case, it was mostly centered around contaminated feed or the vehicles themselves."
In China, after examining the first 68 outbreaks reported by their Ministry of Agriculture and Rural Affairs, it was found about half of the initial infections were linked to vehicle and people movement. Another 32% were associated with contaminated swill feed and backyard feeding populations. Nineteen percent were associated with transportation.
The USDA Animal and Plant Health Inspection Service has conducted a qualitative assessment of the likelihood of the ASFV entry into the United States and most of the methods of entry for indirect transmission are negligible to moderate. Even though feed of animal origin or plant origin and feed supplements are listed as low to moderate in terms of a potential method of entry, Jones says there's a high degree of uncertainty around this area and one reason the industry continues to research to better understand it.
"Whenever we think about the potential for feed-based transmission of a foreign animal disease and in this case, ASFV, again, I think it's important for us to return to the basics and think about how could this transmission occur," Jones says. "It does not magically jump from reservoir to host, and so how can feed be contaminated in the first place? What ingredients are we most concerned about? What's the likelihood for their contamination and if they're contaminated, can that virus actually survive within the ingredient itself upon import into the United States?"
Even if there are negligible or moderate levels of virus in an imported feed or an ingredient, a consistent or minimum dose is all that is needed to cause infectivity within the animal. Viruses do degrade over time, with varying temperatures, but in order for infectivity and transmission to truly occur, that virus needs to maintain its infectivity over time.
"Unfortunately, all three of these are possible and so because of that, a lot of our research has really focused on prevention mechanisms as well as mitigation," Jones says.
While the researcher is often asked what ingredients are at risk for getting infected, there are other concerns surrounding that question. For example, where are the countries/regions with active disease outbreaks and where are those pigs with disease located relative to the ingredient production? What agricultural practices are being implemented in that country where feed is manufactured? Are they drying grains on roadways that are typical highways for pig transport or in close proximity to feral pigs? How are the feed products packaged? Is the manufacturer using single-use bags and totes or are they working with re-used totes and bulk trailers?
"If those vitamins are sealed in a bag and shipped to the United States, it's much less likely that they could be cross-contaminated along the way," Jones says. "However, if we put these same products into trucks or vessels with little knowledge of what was previously hauled in those containers, then we really have an open question of how safe is this product and what are things that we need to do to make sure that we're not transmitting ASFV."
The USDA APHIS recently released a literature review on non-animal origin feed ingredients and the transmission of viral pathogens of swine, however Jones says it still does not provide conclusive evidence for the sources of contamination and the epidemiology of virus transmission to swine under field conditions. That type of research can be difficult to conduct, even under real-world application.
For example, Jones described research in Vietnam, led by Jordan Gebhardt and Steve Dritz from Kansas State University. Since last fall, the team collected samples at multiple sites within a single integrated swine production system that had cases of ASFV and was located in one of the most pig-dense provinces in Vietnam. The production system was using a mitigant in all diets.
The researchers collected 40 feed/ingredients samples within Vietnam to assess the likelihood or the probability of contamination of the ingredients coming into their facilities. None contained detectable levels of ASFV. They also collected 724 different environmental surface samples from feed manufacturing or feed delivery vehicles and about 1% of those were ASFV positive, but almost all of them were from feed delivery trucks that were going to contaminated farms, picking up that contamination and potentially returning back to the feed mill and then taking that contamination to potentially naïve farms. Only one of the 175 environmental samples in the feed mill was ASFV-positive, but it wasn't feed or an ingredient. It was the floor where delivery drivers frequently walk.
"In this case, the facility was utilizing a feed mitigant to reduce their risk because they knew that they had ASF circulating throughout the system subsequently to the submission," Jones says. "We've recently gotten a new report where there was another positive found in the feed mill, but this time from a feed line. That line was for feed not having the mitigant added."
Some viruses are worse than others and have the ability to survive in ingredients longer than others. The ingredients themselves, that have been tested thus far, also have unique characteristics. For example, soybean meal and choline chloride tend to retain virus better than other tested ingredients. Even if virus is still present, it must be in an infectious form with enough infectivity to cause transmission to an animal.
"Unfortunately, this is where the real challenge with feed-based contamination rises. We initially would see a dilution of African swine fever virus or the contaminant, but the same dynamics would lead to multiple exposures to a single animal over time and multiple animals being exposed simultaneously," Jones says.
For example, Jones says every day in the United States, ingredients are mixed in three- or four-ton batches of feed. The mixers optimize uniformity of nutrients and ingredients, but if a contaminant was present, it would be evenly distributed throughout an entire batch of feed.
"That batch of feed gets delivered to a farm, and most of the farms have about 1,200 pigs per barn. What that means is when we consider how much pigs eat, how frequently they are going to eat it, one batch of feed can actually lead to about 20 meals being consumed over that day or over a day and a half. Or if we think about this in disease transmission language, about 24,000 potential meals, or exposures, by the time that entire batch of feed has been consumed in about 36 hours," Jones says. "In summary, one contamination event within the feed supply may lead to 1,200 co-housing animals having more than 20 simultaneous exposures."
Jones reiterates feed is just one of many potential vehicles for ASFV transmission, but if it enters the feed supply, infectivity is almost certain because of the way the industry manufactures and delivers feed and raises animals. That is why her research team has tried to focus their efforts on preventing pathogen transfer through feed, such as excluding high-risk ingredients, extending biosecurity practices from farms to mills and implementing active mitigation practices through feed additives, thermal processing, storage times, whatever can be done to further eradicate the virus.
Many times, Jones says she is asked, as an academic, what do we need to do to understand the complexity of this problem? What are the things that would help resolve this issue and continue to work on prevention measures?
"What we need is continued additional support and I appreciate that our pork industry, our feed industry, the U.S. government has leveraged significant resources to this issue," Jones says. "We also need collaboration and dialogue. It's important for us as scientists to not work in isolated towers and make sure that we're collaborating not just within ourselves and among land grant institutions, but also with our peers across international borders. We need more access to those controlled facilities so that we can do research faster, but we also need research and access to naturally contaminated environments. It's really when the epidemiology and those controlled research projects come together that we can find our best information."
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