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

AprV2andaprB2are variants of the apr gene ofDichelobacter nodosus, 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 ofaprV2andaprB2in flocks with and without footrot. Our hypotheses were that both strains are present in endemically affected flocks, withaprB2andaprV2associated with mild and virulent phenotypes respectively but thatD. nodosusis not present in flocks without footrot. Alternatively,aprB2persists in flocks without footrot. Despite extensive searching over 3 years only three flocks of sheep without footrot were identified.D. nodosuswas not detected in these three flocks. In one further flock, only mild interdigital dermatitis was observed, and onlyaprB2was detected. Twenty-four flocks with endemic footrot of all severities were sampled on three occasions and all were positive forD. nodosusand theaprV2variant;aprB2was detected in only 11 of these flocks.AprB2was detected as a co-infection withaprV2in the 22% of samples positive foraprB2and was more likely in mild footrot phenotypes than severe.Dichelobacter nodosusserogroups were not associated with footrot phenotype. We conclude thatD. nodosus, evenaprB2strains, do not persist in flocks in the absence of footrot. Our results support the hypothesis thataprB2is 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.