CD40LG and infection: What is more important, our study indicate significant down-regulation of IgM levels noticed in koi on day 11 post-infection to CEV genogroups I or IIa compared to the control group which is an accordance with the findings of Adamek et al. (2021), who reported the down-regulation expression of IgM in the gills of CEV infected koi with two-fold at day 3 and five-fold at day 9 p.i..