Humoral immune responses in ovine footrot

The analytical sensitivity and precision of an enzyme linked immunosorbent assay (ELISA) for the detection of antibody against Dichelobacter nodosus were optimised. Humoral immune responses were evaluated in sheep affected with virulent footrot, free of virulent footrot, immunised with footrot vaccines (including malnourished sheep) and with passively acquired (colostral) antibody in order to determine assay sensitivity, assay specificity and other factors necessary for interpretation of results. Most sheep seroconverted after infection with D. nodosus provided that mild underrunning of the horn in one foot was present for at least 3 weeks. The magnitude of the humoral response initially was strongly correlated with the severity of lesions and later also with their duration. But six weeks after introduction of D. nodosus to a flock, humoral responses were associated with the duration of underrun lesions and not with the number or severity of such lesions. These factors also influenced the anamnestic response elicited following subcutaneous injection of D. nodosus antigen (anamnestic test). Spontaneous anamnestic responses were detected in sheep following recurrent clinical episodes of virulent footrot. Up to 11.3% of sheep free of footrot were classified as seropositive. The rate of false positive reactions increased with age but was higher in some flocks than others regardless of age. Exposure of sheep to cross reactive antigens was proposed to explain these findings. Many species of bacteria were shown to have antigens in common with the antigen used in ELISA and the anamnestic test. This antigen was extracted from D. nodosus with KSCN and was visualised in the cell envelope of D. nodosus by immuno—gold electron microscopy. Some components of the antigen were shown to be specific for infection with D. nodosus by irnmunoblotting with the serum of infected sheep, but the majority of components were not specific and were recognised by sera from sheep apparently free of footrot