Japanese Encephalitis is a mosquito-borne viral infection that primarily causes reproductive issues. When infected while pregnant, sows will exhibit pregnancy loss or abortion, prolonged gestation length, an elevated number of stillbirths and mummified foetuses and weak or deformed piglets at farrowing. Prior to 2021, Japanese Encephalitis Virus (JEV) was not endemic in Australia, resulting in a pig population immunologically naïve to the virus. The widespread use of naturally ventilated housing systems elevated the risk of infection, due to increased mosquito exposure. The objective of this study was to investigate the existence of tolerance of sows to JEV under natural challenge conditions during an outbreak for sows mated between September 2021 and January 2022. Indicator phenotypes for JEV affected sows were developed based on exposure to the virus during gestation. The JEV exposure period was identified from change-point analysis applied to reproductive data, independently verified by seroconversion testing. However, the exact timing when individual sows were infected by mosquitos was unknown. Binary indicator traits (0 = no adverse outcome; 1 = poor outcome) were as follows: farrowing failure (FF), no farrowing occurred after mating (including abortions); number of mummified piglets within litter > 1 (MUM); total born (born alive + stillbirth + mummified piglets).