Predicting herd protection against foot-and-mouth disease by testing individual and bulk tank milk samples

Four groups of cattle were tested for antibodies against foot-and-mouth disease (FMD) virus type O(1) over three 70 day vaccination cycles using the liquid-phase-blocking-ELISA (LPBE). First lactation cows showed the lowest titres and group protection levels (GPLs) against FMD virus strains with 'r' values < or =0.5 while second lactation animals gave the highest results. When mean serum titres for each group and sampling date were plotted against GPL a strong correlation was found. Revaccination was indicated at a mean titre of approximately log10 2.88 (1:760; R=0.93; n=86) if the herd was threatened by field strains with an 'r' value of 0.25, or log10 2.62 (1:420; R=0.83; n=48) if this ratio was 0.5. Significant overall correlation (R=0.53; n=624) was obtained between serum titres and milk IgG(1) results derived from the modified specific isotype assay (SIA). Milk titres equivalent to 1:100, 1:200, 1:400 and 1:800 were 1:3.8, 1:6.3, 1:10.4 and 1:17.1, respectively, in first lactation cows. Bulk tank milk samples demonstrated a repeating pattern of results corresponding to the vaccination cycle with no titre lower than log10 1.05 (1:11). Colostrum from first lactation animals showed mean SIA results of log10 4.06 (1:11,480) and early milk titres only levelled off approximately 11 days post partum (dpp).