A study of immunologic relationships among serologically heterologous strains of bovine viral diarrhea virus by cross immunity tests.

: Three serologically heterologous strains of bovine viral diarrhea (BVD) virus were compared in reciprocal cross immunity tests in calves. Each strain was inoculated into five calves. Thirty-eight days later one calf in each group was reinoculated with homologous virus while the immunity of the remaining four was challenged with the serologically heterologous strains of virus, using two calves for each strain. Of the 12 calves in the cross immunity tests, nine were refractory to immunity challenge with the heterologous viral strains while one in each group responded with mild, atypical signs of BVD. On the basis of immunity testing it appears that significant serologic differences among strains of BVD virus are unrelated to the immunologic properties of the strains. It was demonstrated, moreover, that each of the three strains used in the present study evoked the production of antibody to the heterologous as well as to the homologous strain of virus.