This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
In 2014, it was first reported that pigs could become infected with porcine epidemic diarrhoea virus (PEDV) following consumption of contaminated feed via natural feeding behaviour (Dee et al., 2014).Since that time, similar observations have been reported for Seneca virus A (SVA), porcine reproductive and respiratory syndrome virus (PRRSV) and African swine fever virus (ASFV) (Dee et al., 2020a;Niederwerder et al., 2019,). These and other studies have also confirmed that certain feed ingredients, that is, soy-based products, are protective to viruses and enhance their survival for extended periods under simulated conditions of transoceanic shipping. (Dee
A longitudinal study was conducted to assess the impact of different antimicrobial exposures of nursery-phase pigs on patterns of phenotypic antimicrobial resistance in fecal indicator organisms throughout the growing phase. Based on practical approaches used to treat moderate to severe PRRSV-associated secondary bacterial infections, two antimicrobial protocols of differing intensity of exposure [44.1 and 181.5 animal-treatment days per 1000 animal days at risk (ATD)] were compared with a control group with minimal antimicrobial exposure (2.1 ATD). Litter-matched pigs (n = 108) with no prior antimicrobial exposure were assigned randomly to the treatment groups. Pen fecal samples were collected nine times during the wean-to-finish period and cultured for
Escherichia coli
and
Enterococcus spp.
Antimicrobial susceptibility testing was conducted using NARMS gram-negative and gram-positive antibiotic panels. Despite up to 65-fold difference in ATD, few and modest differences were observed between groups and over time. Resistant patterns at marketing overall remained similar to those observed at weaning, prior to any antimicrobial exposures. Those differences observed could not readily be reconciled with the patterns of antimicrobial exposure. Resistance of
E. coli
to streptomycin was higher in the group exposed to 44.1 ATD, but no aminoglycosides were used. In all instances where resistance differed between time points, the higher resistance occurred early in the trial prior to any antimicrobial exposures. These minimal impacts on AMR despite substantially different antimicrobial exposures point to the lack of understanding of the drivers of AMR at the population level and the likely importance of factors other than antimicrobial exposure.
IMPORTANCE
Despite a recognized need for more longitudinal studies to assess the effects of antimicrobial use on resistance in food animals, they remain sparse in the literature, and most longitudinal studies of pigs have been observational. The current experimental study had the advantages of greater control of potential confounding, precise measurement of antimicrobial exposures which varied markedly between groups and tracking of pigs until market age. Overall, resistance patterns were remarkably stable between the treatment groups over time, and the differences observed could not be readily reconciled with the antimicrobial exposures, indicating the likely importance of other determinants of AMR at the population level.
The hypothesis that feed ingredients could serve as vehicles for the
transport and transmission of viral pathogens was first validated under
laboratory conditions. To bridge the gap from the laboratory to the
field, this current project tested whether three significant viruses of
swine could survive in feed ingredients during long-distance commercial
transport across the continental US. One-metric ton totes of soybean
meal (organic and conventional) and complete feed were spiked with a 10
mL mixture of PRRSV 174, PEDV, and SVA and transported for 23 days in a
commercial semi-trailer truck, crossing 29 states, and 10,183 km.
Samples were tested for the presence of viral RNA by PCR, and for viable
virus in soy-based samples by swine bioassay and in complete feed
samples by natural feeding. Viable PRRSV, PEDV, and SVA were detected in
both soy products and viable PEDV and SVA in complete feed. These
results provide the first evidence that viral pathogens of pigs can
survive in representative volumes of feed and feed ingredients during
long-distance commercial transport across the continental US.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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