SEVERITY AND PERSISTENCE OF FOOTROT IN MERINO SHEEP EXPERIMENTALLY INFECTED WITH A PROTEASE THERMOSTABLE STRAIN OF DICHELOBACTER-NODOSUS AT5 SITES

Objective To test the hypothesis that ovine footrot associated with a thermostable protease strain of Dichelobacter nodosus undergoes self c ure or is sustained as an annually recurring disease, depending on the environment. Design and procedure Forty Merino sheep from a single bl ood line were infected with a protease thermostable strain of D nodosu s at each of five sites in Western Australia. Footrot lesions and micr oscopic evidence of D nodosus were recorded every fortnight for 2.5 ye ars, supplemented by laboratory culture. Rainfall, soil and air temper ature, pasture quantity and composition and soil types were also recor ded. Flocks that apparently self cured were relocated to a more favour able site for footrot in the final spring season. Results The maximum prevalence of feet with clinical footrot lesions was 80.6, 1.3, 14.4, 3.8 and 88.1% at the five sites. Severe footrot occurred for three con secutive spring seasons at one site that had clay loam soil and at lea st 3500 kg/ha total pasture dry matter annually. However, the infectio n was asymptomatic for up to 10 weeks between outbreaks. D nodosus was isolated from flocks for 2.5 years at only two sites, although there was microscopic evidence of the organism at other sites in the final y ear. A thermolabile variant (strain U6) of D nodosus was isolated from the two sites where footrot persisted, Conclusion Depending on time a nd location, ovine footrot induced by a protease thermostable strain o f D nodosus either self cured or persisted as annual outbreaks intersp ersed with periods of asymptomatic infection.