Oral infectivity and 50% porcine infectious dose of classical swine fever virus JPN/1/2018 strain isolated from the first case in Japan.

We determined the 50% porcine infectious dose (PID) of the classical swine fever virus (CSFV) JPN/1/2018 strain currently circulating in Japan. Twelve piglets were orally inoculated with 10, 10, or 10 50% tissue culture infectious dose (TCID) of the virus. All piglets receiving 10 TCID developed typical clinical signs with viral RNA detection in their blood from 2 days post-inoculation (dpi). One piglet receiving 10 TCID was confirmed to be infected at 4-5 dpi, while the remaining piglets in this group became infected later, with evidence of infection detected at 11-14 dpi. These results suggest that only the first infected piglet was directly infected by the viral inoculum, while the remaining 3 piglets were indirectly infected through horizontal transmission from the first infected piglet. No piglets inoculated with 10 TCID became infected. Based on these infection patterns, the PID was calculated as 10 TCID, providing essential data for infection risk assessment and establishing target reduction titers for disinfection against the JPN/1/2018 strain.