Discussion
To the knowledge of the authors, the pre-processing (grinding) of infectious pig carcasses before composting has not been evaluated for potential environmental contamination. Although this study does not reduce any risk to zero, it does appear aerosolization of viral pathogens, and swine DNA is a low risk during the grinding process. The low risk is supported by Table 5, where no aerosolized swine DNA was detected at 137m, and by the only PCR viral positive being a relatively weak result attained only after the vortex of the filter itself. The air collectors were placed downwind from the grinder, giving the best situation for potential collection of virus and porcine nucleic acid. A limiting factor in this study was that we were only able to collect samples out to 137m from the grinding process.
The carbon material that was placed underneath the carcass and carbon source ground material likely prevented significant leaching of viruses into the groundwater during this study. With ASF, previous studies have shown soil to be a poor fomite for viral survival, which may protect from groundwater leaching (Mazur-Panasuik & Wozniakowski, 2020). In a study completed on an above-ground burial with Seneca Valley Virus positive pigs, carcasses were placed more intimately with the surrounding soil and leaching of the virus down to a depth of 46cm but not 91cm was found (B. Thaler, personal communication, April 22, 2020). In areas or times of more considerable rain or snowfall, the potential for leaching of pathogens into the soil and groundwater may be higher (Chatterjee et al., 2013; Grisey et al., 2010). Since this study was performed in the winter, the frozen soil under the windrows may have also prevented potential pathogen leaching. Further research is needed to assess the risk of pathogen aerosolizing from grinding and the potential leaching of swine viruses to the groundwater from contaminated carcasses.
This study was designed to assess the capability of using the grinding (pre-processing) of carcasses for composting in cold weather conditions to dispose of infected mortality and prevent pathogen transmission in the event of FAD break. While studies have shown that composting can eliminate swine diseases like FMD, PEDV, and PRRSV many of these studies were performed under very controlled and enclosed environmental conditions and not in exposed outdoor weather situations (Costa & Akdeniz, 2019; Guan et al., 2010; Vitosh-Sillman et al., 2017). In the pile sections treated with microbial stimulate, the most considerable numerical differences were seen in the mixed biomass windrow, but overall treated and untreated windrows sections were numerically similar. According to the US Environmental Protection Agency, pathogen reduction by time-temperature is classified as Class A or Class B, which is used to assess virus elimination capability (Costa & Akdeniz, 2019). As seen in Table 2, all three biomass types in this study meet the qualification of Class A, which requires the composting temperature of 55°C for three consecutive days. Specifically pertaining to ASF, maintaining 60°C for 15-20 minutes is documented as adequate to eliminate the virus, which all compost types would have completed (Mazur-Panasuik et al., 2019; United States Department of Agriculture, 2018). Table 2 shows pre-processing compost was able to reach temperatures reported to eliminate pathogens even under the adverse cold weather conditions.
Although widely used around the world as a standard method for mortality disposal, there are limited peer-reviewed reports on the survival of pathogens in composting swine mortality (Wilkinson, 2007). On the day of windrow formation, each biomass type was confirmed positive by PCR for the presence of PEDV and PRRSV nucleic acid. Outer and inner regions of the compost pile have the potential for differences in pathogen elimination, and previous literature supports the locations that were chosen for compost sampling in this study (Christensen et al., 2002; Wilkinson, 2007). In this study, direct compost sampling was targeted at a shallow outer location of the carcass/biomass blended material and a deeper depth (minimum 0.91m into the carcass/biomass blended material). A previous study looking at PEDV detection in compost only collected samples after the 1st and 2ndtemperature cycles were completed and did not perform a regular sample collection as conducted in the current study (Vitosh-Sillman et al., 2017). The present study and the literature demonstrate similar results where PEDV nucleic acid was not detected at the end of the first compost heat cycle (Vitosh-Sillman et al., 2017). ASF survival in different matrices was found to be dependent on the moisture content, with the most persistence in drier materials like straw and hay (Mazur-Panasuik & Wozniakowski, 2020). A weakness of the current study is that the moisture content of the biomass material was not evaluated. However, woodchips used for compost can have a moisture content of 14.4% or lower, whereas cornstalks can have moisture content up to 76% (Ima & Mann, 2007; Tannous, 2015; Xu et al., 2020). The lower moisture content of the woodchips may have contributed to the more extended detection of viral nucleic acid by PCR compared to windrows containing cornstalks. This theory is further supported by the increased infectious ability of PEDV seen in the woodchips on bioassay (Table 6). The results of this study suggest that cornstalks may be a better compost material than woodchips for the elimination of swine viruses.
The pathogens utilized in the current study are both RNA viruses. The higher stability of double-stranded DNA (as found in mammalian cells) is well known compared to RNA (Grosjean, 2009). The difference in RNA and DNA stability may explain the greater length of detection of swine DNA in the compost material compared to viral RNA. Also, the differences in the quantity of nucleic acid were likely higher in the swine carcasses compared to the magnitude of PRRSV and PEDV nucleic acid present from infection (Tables 3 and 4). To the knowledge of the authors, this is the first study looking at the detection of swine DNA in compost material over time. Detectable swine DNA decreased over time, as observed by the increasing negative results and the rising ct values attained in all windrow types. However, the expected length of time porcine DNA could be detected from composted carcasses remains unknown.
The use of bioassay to determine the infectious capability of a detected virus is a highly sensitive and conclusive method (Zimmerman et al., 2012). However, the use of bioassay to determine the infectious ability of compost material in swine has not been well documented. The results of the current study showed that PRRSV infectivity in compost material is short-lived regardless of the biomass used. However, PEDV nucleic acid detected in the compost material remained infectious in the woodchips out to four days after windrow formation but appeared non-infectious at time points afterward. The data in the current study suggests PEDV is not readily transmissible from windrows containing cornstalks showing a difference between composting materials. Although a single pig was positive by PEDV PCR in pens of week two woodchip and week two combination compost samples, no clinical signs of disease were observed. The positive pigs were also negative on repeat testing three days later, suggesting the PEDV detected at that time was not genuinely infectious.
This current study documents the potential ability for compost to eliminate domestic and foreign swine diseases successfully. However, it also pointed out the differences compost material can provide in disease elimination over time. Woodchips are commonly used as biomass for compost. However, this study shows that woodchips may not be as effective or as rapid at destroying viral nucleic acid compared to other material. In a mass depopulation event where multiple swine sites must dispose of large volumes of carcasses, the limitations of available carbon sources for compost is a concern. Having various options for compost disposal is needed for a rapid response to disease elimination efforts. The advantage of the method of grinding carcasses (pre-processing) is that less land space is required when compared to traditional full carcass composting. Grinding also allows for high temperatures to be reached very readily, and the current study shows that it may remove the need to turn the windrow and still be able to render pathogens non-viable. This study supports that the risk of environmental contamination from the grinding process is minimal, but requires further evaluation. Current research on the effect of compost, the compost biomass types used, and moisture content on pathogen survival in swine is limited and more research in this area is warranted.