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.