ISG15 and viral infectious disease: In the challenge conducted with the P VHSV isolate, the fold induction of the three genes in poly I:C-stimulated animals (poly I:C + P VHSV group) was significantly lower than those recorded in the poly I:C + L15 group, mainly at 3 h post-2nd injection, for Mx and Isg15, and at 3 and 6 h post-2nd injection, for Pkr. However, from 12 h post viral infection onward, relative fold change values were similar in both groups or even higher in infected animals (poly I:C + P VHSV group) than in the poly I:C + L15 group (Figure 4A–C).