Immuno-serological studies on a foot and mouth disease virus type A strain involved in breakdown of vaccine immunity

A foot and mouth disease virus type A strain, isolated from an out­ break in a vaccinated herd, was compared serologically with the vaccine virus by the two-dimensional microneutralisation test and immunologically through a cross-immunity test in cattle. The antigenic diversity of the field virus from the vaccine virus as revealed by serological analysis was substantiated by lack of cross-protection in primo-vaccinated animals. The anamnestic response con­ sequent to infection was compared with that obtained through booster vaccination in terms of antibody levels and heterologous immunity. It was observed that anamnestic response had little effect on heterologous neutralisa­ tion, owing to the wide antigenic divergence.