Occurrence of different strains of Dichelobacter nodosus in new clinical lesions in sheep exposed to footrot associated with multi-strain infections

Objective To investigate the occurrence of S1, U1 and T strains of Dichelob acter nodosus in new clinical lesions in sheep exposed to footrot associate d with multi-strain infections. Design Seventy-seven donor sheep were grazed with 84 recipients for 33 week s. The donor sheep were Merinos with a history of clinically virulent footr ot associated with protease type S1, U1 and T strains of D nodosus that hyb ridised with gene sequences pJIR314B, pJIR318 and/or pB645-335. The recipie nt sheep were Merinos with no history of footrot. Procedure Each fortnight, all feet were examined, their lesion scores were recorded and samples of lesion material were taken for laboratory tests. Results Eighty-nine percent (299 of 336) of feet of recipient sheep develop ed new clinical lesions. S1, U1 and T strains of D nodosus were recovered f rom 58%, 22% and 18%, respectively, of these lesions at a ratio that remain ed constant during two apparent peaks in footrot transmission. Gene sequenc es homologous to pJIR314B and pB645-335 were detected in 56% (93 of 166) an d 29% (48 of 166), respectively, of S1 strains of D nodosus at a ratio that was not constant during the experiment. Conclusions S1 was the dominant protease type of D nodosus in new clinical lesions. The occurrence of S1 strains did not increase relative to U1 and T strains of D nodosus during the experiment. S1, U1 and T strains of D nodo sus remained in equilibrium despite changes in environment, genetic types i n the population of S1 strains, and host resistance to footrot.