APPLICATION OF ELISA TO THE SEROLOGICAL DIAGNOSIS OF VIRULENT OVINE FOOTROT

The sensitivity and specificity of an ELISA that detects antibody in t he serum of sheep against Dichelobacter nodosus was investigated. The sensitivity of the assay was determined in two trials in which sheep f ree of virulent footrot were exposed to a virulent strain of D. nodosu s. In the first trial up to 96.6% of sheep that developed severe foot lesions were seropositive 2 weeks later. The proportion of seropositiv e sheep depended on the interval between exposure to D. nodosus and co llection of blood samples and reached a maximum 6 weeks after exposure in the first trial and plateaued from 8 weeks after exposure in the s econd trial. Of sheep with severe lesions present for only 1 week, 40. 0% became seropositive, while 98.3% of sheep with severe lesions prese nt for 3 or more weeks became seropositive in Trial 1. Of the unexpose d control sheep from the same flock, 93.3% remained seronegative. At l east 75% of sheep that were vaccinated with commercial footrot vaccine s remained seropositive for at least 24 months after last vaccination and 58.5% remained seropositive for at least 36 months. The specificit y of the assay in flocks of sheep from different geographical regions was determined also. Overall, 95.2% of 2978 sheep free of virulent foo trot were seronegative. However the rate of false positive serological reactions ranged from 1.6% to 11.3% between flocks and was higher in older sheep (7.0%) compared with sheep less than 1.5 years old (2.7%). Passively acquired colostral antibodies against D. nodosus were measu red in 22 lambs and did not persist beyond 55 days mean age. Non-speci fic reactions with cross reactive antigens were proposed to explain ag e related changes in serological results. Applications of this ELISA i n footrot control programmes are discussed.