Objective: Ovine footrot is a contagious bacterial disease that reduces meat and wool production and can trigger on-farm quarantine in New South Wales. Field diagnosis is based on the prevalence and severity of foot lesions, environmental conditions and flock history. The study evaluated whether a PCR assay or gelatin gel test for virulence in Dichelobacter nodosus isolated from hoof material could aid in the clinical diagnosis of virulent footrot.
Methods: A quantitative polymerase chain reaction (qPCR) used for diagnosis of virulent footrot in some Australian states was evaluated on 218 hoof swabs taken from 44 sheep flocks from 36 NSW properties, quantifying both the aprV2 positive and aprB2 positive acidic protease genotypes of D. nodosus.
Design: The same flocks/swabs were used to evaluate test agreement between the aprV2/B2 qPCR and the gelatin gel test, and a multiple logistic regression was used to identify factors critical for field diagnosis of virulent footrot.
Results: Only fair to moderate agreement (kappa test) and significant disagreement (McNemar's) was shown between the gelatin gel test and the ratio of aprV2 positive to total D. nodosus. The proportion of aprV2 positive D. nodosus was not significantly different between foot lesions scores of increasing severity. Field diagnosis of virulent footrot was best explained by the prevalence of score 4 and 5 lesions, wet and warm environmental conditions, and recent footrot diagnosis.
Conclusion: Although the apr2 gene could differentiate between benign and virulent strains of D. nodosus, the apr2 qPCR was of minimal use for field diagnosis of virulent footrot, where disease expression relies on host genetics, immunity and environmental conditions.
Keywords: Dichelobacter nodosus; acidic protease; field diagnosis; ovine footrot.
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