The polymerase chain reaction (PCR) test for AprV2/B2 detects one of the genes in D. nodosus that codes for protease – it is an indirect test. While the AprV2/B2 PCR does not necessarily require prior culture, it can be undertaken on cultured bacteria.
This test was developed in Europe and is based on detection of a point mutation in one gene that is assumed to differentiate benign and virulent strains of D. nodosus (Frosth et al., 2015; Stäuble et al., 2014). The underlying gene sequence differences were determined by whole genome sequencing and bioinformatics, and rely on the prior classification of bacterial isolates as virulent or benign being reliable (Kennan et al., 2014); the authors of that research stated that such classification was presumptive.
While the test appears to be gaining acceptance in some jurisdictions in Australia, caution is required. Using samples collected from foot lesions from 960 sheep from 40 flocks in four different geographic regions across Australia, we evaluated the analytical characteristics of the qPCR tests for the protease gene alleles aprV2 and aprB2, and compared these with results from elastase and gelatin gel tests (McPherson et al., 2017).
There was a low level of agreement between clinical diagnosis and the qPCR test and poor agreement between the qPCR test and the protease tests. The diagnostic specificity of the qPCR test was low at both the flock and individual swab levels (31.3% and 18.8%, respectively). By contrast, agreement between the elastase test and clinical diagnosis was high at both the flock level (diagnostic sensitivity [DSe], 100%; diagnostic specificity [DSp], 78.6%) and the isolate level (DSe, 69.5%; DSp, 80.5%).
False positive diagnosis of virulent footrot is likely to occur more often with the aprV2/B2 qPCR and gelatin gel tests than it is with the elastase test.
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Footrot @ Sydney 