Feed additives show varied results in reducing porcine delta coronavirus survival

There are currently three known coronaviruses that can affect the gastrointestinal health of pigs – transmissible gastroenteritis virus, porcine epidemic diarrhea virus and porcine delta coronavirus. Although clinical signs produced by PDCoV are less severe than those caused by PEDV, both viruses are often found together in co-infections, are excreted in feces and can be transmitted via feed contaminated with swine feces.

Inclusion of certain additives in swine feed has been shown to provide growth performance benefits, may be used as alternatives to antibiotics and may control pathogens such as Salmonella. The goal of this study was to determine if feed additives, at recommended or twice the recommended concentrations, are effective in reducing the survival time of PDCoV in feed.

A commercially available phase 2 nursery diet was obtained from Vita Plus of Madison, Wis. The complete feed was in meal form, did not contain any animal byproducts, and was negative for coronaviruses when analyzed by real time RT-PCR. Aliquots of feed (5 grams) were placed in plastic vials followed by the addition of 1X or 2X concentrations of eight different feed additives. Table 1 lists the brand names, manufacturer, active ingredients and concentrations added to 5 grams of complete feed for the eight additives evaluated. One milliliter of PDCoV (initial virus titer = 3.5 x 105) was added to each vial and vials stored at room temperature (25 degrees C). Vials were removed at various time points (zero to 10 days) and surviving virus was eluted and titrated in swine testicular cells.

A predictive model (Weibull model) was used to determine the kinetic values (delta) which predicted the survival of the virus in feed after the addition of the additives by indicating the time required to reduce the initial virus concentration by one log10 (or 90%). At 1X concentration, none of the eight additives was effective in reducing PDCoV survival because the delta values for the control and eight additives were similar (e.g. 0.86 days for control and 0.62-1.72 days for additives). However, adding 2X concentrations of these additives resulted in greater overall effectiveness in decreasing virus survival (Table 2).As shown in Table 2, UltraAcid P and KEMGEST required only 35 seconds to reduce PDCoV titer by one log10 (0.0004 days). However, after this initial decrease, the relative effectiveness of both additives was similar to the feed additives formic acid (4.95 days), sugar (4.94 days), and Activate DA (0.12 days), achieving a two log10 virus reduction in 10 days.

Adding Luprosil resulted in an initial one log10 reduction in 1.44 hours (0.06 days) and a 2.3 log10 reduction in 10 days. The addition of salt reduced the virus by one log10 within 2.16 hours (0.09 days) and achieved a total of three log10 virus reduction in 10 days, which was the greatest log reduction achieved for any additive evaluated in this study. Activate DA required 2.88 hours (0.12 days) to inactivate the initial one log10 of virus, and Acid Booster required the longest time to inactivate one log10 of the virus (0.28 days, 6.72 hours). However, Acid Booster was effective in reducing virus titer by 2.7 log10 within 10 days of storage.

Conclusions

  • None of the eight additives evaluated in this study were effective in reducing PDCoV survival when added at the recommended concentrations to a phase 2 commercial nursery diet.
  • The use of Luprosil, Acid Booster and salt at double the recommended concentrations are the most effective for inactivating PDCoV during a 10-day storage period of swine feed at room temperature.
  • Complete virus inactivation was not achieved using any of the additives at 2X concentration when evaluated after 10 days of storage.
  • Further research is needed to determine if there are interactive effects of active ingredients in various feed additives, as well as the relative effectiveness of individual active ingredients on the inactivation kinetics of PDCoV.
  • This study was conducted using a single dose of virus, but the relative effectiveness of these feed additives is unknown in complete feed that has been contaminated with different amounts of the virus.
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