Producer-initiated field research leads to a new diagnostic test for footrot

Abstract

The Cicerone Project was formed in 1998 to address problems faced by wool producers. In the New England area, the issue of suspected false positive diagnoses of virulent footrot, which can be a significant cause of economic loss to individual producers, was investigated. In New South Wales, footrot diagnosis is primarily a field diagnosis supported by the gelatin gel laboratory test. The principal causative agent of footrot is Dichelobacter nodosus. If the gelatin gel test finds strains of D. nodosus to be thermostable (gel stable), a finding of virulent footrot is likely and quarantine of the affected property follows. However, livestock producers and inspectors reported that there were a considerable number of cases where laboratory tests found strains to be stable but these strains did not cause virulent footrot in the field. Preliminary results using DNA markers associated with virulent footrot showed that one of these markers, intA, was absent in gel stable, field benign strains but present in all strains tested which caused field virulent footrot. A trial conducted at Uralla, New South Wales, demonstrated conclusively that there were strains of D. nodosus which were stable in the gelatin gel test but did not cause virulent footrot in the field. All of these strains were negative in the intA DNA test. These results were confirmed in a second field trial at Molong, New South Wales. These trials were instrumental in establishing that the gelatin gel test at times gave results inconsistent with the clinical expression of footrot, potentially leading to a false positive diagnosis of virulent footrot. Subsequent research led to confirmation of the intA test, which is now available as an additional tool for footrot diagnosis.