However, in these very young piglets, no PEDV antigens or nucleic acids were detected in epithelial cells lining the crypts, and positive-staining cells were lacking in the colon by IHC staining (Kim et al., 1999) and in situ hybridization (Kim and Chae, 2003) . Similarly, different levels of virulence among TGEV strains were also reported (Kim and Chae, 2002) . Therefore, the pathogenicity of TGEV/PEDV infection in pigs depended on the viral strains, the age of pig, how piglets were derived (CD) or if naturally farrowed, the colostrum/milk status (CD) and environment variations (Table 4 ).