African swine fever (ASF) is an important transboundary animal disease with a high mortality rate. The high African swine fever virus (ASFV) titers in the excretion of infected wild boar possibly contaminate the feed ingredient. Once contaminated, it could support persistent residual titer of the ASFV. The chemical inactivation of imported feed ingredients is a precautionary risk management measure to restrict the import risk of ASFV through international trade. The log ASFV titers were linearly reduced as a function of the inactivation time after exposure to 0.03%, 0.05%, 0.1%, and 0.2% formaldehyde-based product (FBP). A four-log reduction of ASFV titer was achieved after exposure to 0.2% FBP and 0.03%-0.1% FBP for 30-min and 60-min inactivation times, respectively. The decimal reduction time or D value is defined as the time required to inactivate the virus titer by 1 log. The ASFV inactivation rate from the independent experiment of FBP concentration was converted to a D value. The observed mean D0.2%, D0.1%, D0.05%, and D0.03% of FBP were 13.4, 44.9, 45.0, and 45.3 min per log reduction of ASFV, respectively. The interpretation of D0.2% of FBP is that the ASFV titer is inactivated by 1 log after being exposed to 0.2% FBP for every 13.4 min. A more effective chemical has a lower D value because of a shorter inactivation time required to achieve the same 1-log reduction. In addition, the hypothetical inactivation time by any chemical additive is scenario-specific and is calculated by the product of D value (at a certain concentration) and log titers of residual ASFV. This study introduces the concept and application of the D value to compare the virucidal activity of chemicals and to determine the hypothetical inactivation time of chemicals depending on the chemical concentration including the virus titer in the feed.
Keywords: D value; African swine fever; formaldehyde-based product; inactivation rate.
This study demonstrated the virucidal activity of 0.03%–0.2% formaldehyde-based product (FBP) against the African swine fever virus with cell culture. This product may have the potential to inactivate African swine fever (ASF) in feed or feed ingredients. The D value concept was lately introduced to compare the virucidal activity across various types of chemicals and different concentrations of FBP. Even though the concentrations of FBP tested were in a limited range, the decimal reduction time curve (equation) allows users to determine the exposure time of the desired virucidal activity of some other concentrations of FBP. Future studies are needed to verify viricidal activity in feed or feed ingredients along with pig bioassay to show it may or may not prevent ASF infection by feed.
© The Author(s) 2022. Published by Oxford University Press on behalf of the American Society of Animal Science. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.