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 less than or equal to0.5 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 log(10) 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 log(10) 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, 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 log(10) 1.05 (1:11). Colostrum from first lactation animals showed mean SIA results of log(10) 4.06 (1:11,480) and early milk titres only levelled off approximately 11 days post partum (dpp). (C) 2001 Elsevier Science B.V. All rights reserved.