Research explores survival of ASF virus in digestive system of pigs using an ASFV-like algal virus in corn- and soybean-based feed ingredients and complete feed
By Jerry Shurson, Christian Ramirez-Camba, Pedro Urriola, and Declan Schroeder
We know that African swine fever virus (ASFV) is very stable and can survive for more than a year in pig carcasses, blood, tissues, and body fluids, and that it can be transmitted orally (Shurson et al., 2022). We also know that a surrogate African swine fever-like algal virus, Emiliania huxleyi virus (EhV), remains bound to soybean meal after extraction for PCR or bioassays, which makes soybean meal an ingredient that is of potential concern for transmission of ASFV when consumed by pigs. However, we know nothing about the fate of ASFV in feed after it is consumed and subjected to the pig’s digestive processes.
There is no direct evidence in published scientific studies showing that feeding naturally contaminated feed causes disease under field conditions. Furthermore, conflicting findings have been reported in two studies that have been conducted to directly determine the infectivity of ASFV in feed when fed to pigs under experimental conditions. One study was designed to determine the minimum infectious dose of ASFV in a plant-based diet and showed that a minimum single dose of 104 TCID50 was needed to cause infection, but the median infectious dose was greater (106.8 TCID50), while the probability of infection increased at lower doses with repeated exposures or meals (Niederwerder et al., 2019). In contrast, feeding diets inoculated with 105 TCID50 of ASFV in liquid plasma to weaned pigs for 14 consecutive days did not cause disease, which indicates that even with multiple exposures from natural feeding behavior, the minimum infectious dose of ASFV is greater than 105 TCID50 in diets containing animal plasma (Blázquez et al., 2020). Reasons for the different responses between studies are unclear but may be related to the extent that ASFV is degraded during the digestion process after it is consumed by pigs.
Studying the extent of degradation or inactivation of ASFV in feed during the digestion process is very difficult to measure because of the extremely high biosecurity measures that must be maintained to conduct research on ASFV, and the tremendous difficulty in studying these effects in live pigs. Therefore, we conducted a series of experiments using the safest, most reliable and accurate experimental techniques available which included using a safe surrogate ASFV-like algal virus (Emiliania huxleyi virus; EhV) and well-established in vitro digestibility assays to determine the extent of EhV stability during three stages of the pig digestion process (Shurson et al., 2024). The objective was to determine the survival of EhV in corn- and soybean-based ingredients and complete feed during simulated stomach and small intestine hydrolysis and cecum and large intestine fermentation stages of the digestive process in pigs.
As shown in Figure 1, all feed matrices provided some level of protection against degradation of EhV. However, complete feed, extruded soybean meal, soybean hulls, and solvent extracted soybean meal had significantly greater protective effects on virus viability than corn and corn co-products. The average reduction in viable EhV concentrations in the in vitro stomach and small intestine digestion processes was 2.8 log units across all feed matrices. Although no differences in EhV viability were observed among feed matrices during the in vitro large intestine fermentation process, there was an average of 3.8 log unit reduction across all feed matrices. (Figure 2). Therefore, we estimate an average total reduction of 6.7 log units in viable EhV during the entire total tract digestion and fermentation process, with the least reduction of 5.3 log units for solvent-extracted soybean meal and the greatest reduction of 7.5 log units for corn. These differences in EhV degradation among feed matrices may explain the differences in infectious doses reported by Niederwerder et al. (2019) and Blázquez et al. (2020).
In conclusion, we are the first to report that using EhV as a surrogate for ASFV, there is between 5 to more than 7 log reductions in EhV in corn- and soybean-based feed ingredients and complete feed during the entire in vitro digestion and fermentation process. The differences in the timing of virus release from feed matrices during the stomach and small intestine stages of the digestion process may partially explain the inconsistencies of infection observed among studies when pigs consume infectious doses of ASFV contaminated feed.
References
Balestreri, C., Schroeder, D.C., Sampedro, F., Marqués, G., Palowski, A., Urriola, P.E., van de Ligt, J.L.G., Yancy, H.F., Shurson, G.C. 2024. Unexpected thermal stability of two enveloped megaviruses, Emiliania huxleyi virus and African swine fever virus, as measured by viability PCR. Virology Journal 21:1. doi: 10.1186/s12985-023-02272-z
Blazquez, E., Pujols, J., Segalés, J., Rodríguez, F., Crenshaw, J., Rodríguez, C., Ródenas, J., Polo, J. 2020. Commercial feed containing porcine plasma spiked with African swine fever virus is not infective in pigs when administered for 14 consecutive days. PLoS One 15:e0235895. doi: 10.1371/journal.pone.0235895
Niederwerder, M.C., Stoian, A.M.M., Rowland, R.R.R., Dritz, S.S., Petrovan, V., Constance, L.A., Gebhardt, J.T., Olcha, M., Jones, C.K., Woodworth, J.C., Fang, Y., Liang, J., Hefley, T.J. 2019. Infectious dose of African swine fever virus when consumed naturally in liquid or feed. Emerging Infectious Diseases 25:891–897. doi: 10.3201/eid2505.181495
Palowski, A., Balestreri, C,, Urriola, P.E., van de Ligt, J.L.G., Sampedro, F., Dee, S., Shah, A., Yancy, H.F., Shurson, G.C., Schroeder, D.C. 2022. Survival of a surrogate African swine fever virus-like algal virus in feed matrices using a 23-day commercial United States truck transport model. Frontiers in Microbiology 13:1059118. doi: 10.3389/fmicb.2022.1059118
Shurson, G.C., Palowski, A., van de Ligt, J.L.G., Schroeder, D.C., Balestreri, C., Urriola, P.E., Sampedro, F. 2022. New perspectives for evaluating relative risks of African swine fever virus contamination in global feed ingredient supply chains. Transboundary and Emerging Diseases 69:31-56. doi: 10.1111/tbed.14174
Shurson, G.C., Ramirez-Camba, C.D., Urriola, P.E., Schroeder, D.C. 2024. Stability of a surrogate African swine fever-like algal virus in corn- and soybean-based feed ingredients during extended storage and in vitro digestion processes. Frontiers in Veterinary Science 11:1498977. doi:10.3389/fvets.2024.1498977
Jerry Shurson, Christian Ramirez-Camba, Pedro Urriola, and Declan Schroeder are with the University of Minnesota.
