Relationship between the likelihood of footrot elimination from a flock and the virulence of the strain of Dichelobacter nodosus present

Objective: To assess ability to eliminate different strains of footrot in sheep using inspection and culling of affected sheep.
Methods: A flock of 1417 Polwarth sheep that had deliberately been infected with seven different strains of Dichelobacter nodosus and undergone different control measures prior to eradication, including zinc sulfate footbathing and vaccination, were subjected to visual foot inspection on a number of occasions. Sheep identified as infected or having any foot abnormalities were removed from the flock at each inspection. The experiment had three replicates and a small number of untreated control animals. Sheep were examined following the inspections to assess the effectiveness of elimination and additional measures were implemented in two of the three replicates in an attempt to eliminate all strains of D. nodosus.
Results: Three strains of D. nodosus were apparently successfully eliminated from all replicates (strains A, E and H). Strains B and C were detected in one replicate each, despite additional inspections. The three stains that were eliminated were the more virulent strains and the two strains that remained were the least virulent. No assessment could be made on a further two strains.
Conclusions: The application of a rigorous inspection and culling program resulted in the elimination of the more virulent D. nodosus strains, but did not result in the elimination of all D. nodosus strains on all occasions.