sometimes epithelial ulcers in the
mouth, esophagus and forestomachs. This syndrome is called "mucosal disease". The pathogenesis is very
complicated. Briefly, noncytopathic strains of BVDV are endemic in cattle populations. If a fetus is
infected in-utero, it becomes tolerant to the noncytopathic strain and persistently infected. If this animal is
later super-infected with a cytopathic strain or the noncytopathic strain mutates to a cytopathic strain, the
animal is incapable of mounting an appropriate immune response because of tolerance and the virus
causes severe disease