Impact of Strain Variation of on Disease Severity and Presence in Sheep Flocks in England.

and are variants of the apr gene of , the cause of footrot in sheep. They are putative markers for severe and mild disease expression. The aim of our study was to investigate the distribution of and in flocks with and without footrot. Our hypotheses were that both strains are present in endemically affected flocks, with and associated with mild and virulent phenotypes respectively but that is not present in flocks without footrot. Alternatively, persists in flocks without footrot. Despite extensive searching over 3 years only three flocks of sheep without footrot were identified. was not detected in these three flocks. In one further flock, only mild interdigital dermatitis was observed, and only was detected. Twenty-four flocks with endemic footrot of all severities were sampled on three occasions and all were positive for and the variant; was detected in only 11 of these flocks. was detected as a co-infection with in the 22% of samples positive for and was more likely in mild footrot phenotypes than severe. serogroups were not associated with footrot phenotype. We conclude that , even strains, do not persist in flocks in the absence of footrot. Our results support the hypothesis that is associated with mild footrot phenotypes. Finally, we conclude that given the small number of flocks without footrot that were identified, footrot is highly endemic in English sheep flocks.