Effects of the severity and duration of lesions on the primary and anamnestic humoral responses of sheep to Dichelobacter nodosus and observations of natural resistance to footrot.

In a flock of 137 sheep naturally infected with Dichelobacter nodosus the severity of the lesions was the principal factor associated with the humoral response early in the period of spread of D. nodosus, underrun lesions having the greatest effect. However, after five to six weeks, the duration of underrun lesions rather than their severity or number primarily influenced the response. Sheep first affected late in the period of spread had fewer affected feet, milder lesions and a lower humoral response than those affected earlier. An anamnestic humoral response was stimulated by injecting membrane-protein antigen of D nodosus subcutaneously 18 weeks after the sheep had been treated parenterally with antibiotics and antiseptic footbathing. The anamnestic response was related to the antibody level reached during the infection phase, and hence with the duration and severity of the lesions, and with the residual antibody level at the time of the anamnestic challenge, suggesting that the population of memory B cells specific for D nodosus was proportional to the size of the originally activated B cell population. Even after allowing for differences between the duration and severity of the lesions differential responses were detectable among the sheep. Primary and anamnestic responses of a non-specific nature occurred in control sheep.