Laboratory tests are most efficient (high positive predictive value) when the samples to be tested have a high chance of having being taken from animals with the disease, that is, following veterinary assessment and suspicion that the disease may be virulent (Whittington et al., 2016). Laboratory tests conducted in the absence of history and assessment of clinical signs will be subject to a higher error rate.
Laboratory tests include:
- examination of a stained smear
- culture of D. nodosus
- tests for protease activity
Confirming the presence of D. nodosus is required in order to make a diagnosis of footrot. This can be achieved by examination of a stained smear, by culture or by polymerase chain reaction (PCR).
The virulence of D. nodosus can be estimated by protease tests (elastase, gelatin gel and other tests), to help support a diagnosis of virulent footrot. The pathogenicity of specific isolates of D. nodosus arguably can be assessed only by experimental inoculation of a pure culture onto the feet of susceptible sheep, or placing sheep with pre-existing, mild lesions onto suitable pasture or wet mats to allow lesions to reach their maximum potential severity. There are a number of reports from pathogenicity trials of this nature but in general they are of low quality because of missing information: clinical and epidemiological data from the farms of origin not given, number of sheep in trial not given, results given for only some isolates, the frequency of lesions induced not given, unconventional protease test methods used (Whittington et al., 2016). Furthermore, the approach of experimental inoculation could be fallible if a unique microbial ecology is required on the foot for clinical expression of footrot lesions i.e. if important causal organisms are absent, or not favoured on the wet mat environment. The published data suggest an association between positive results in protease tests and virulence defined by the capacity of an isolate to cause severe lesions. However, the frequency of anomalous results has not really been determined (Whittington et al., 2016).
Serogrouping of D. nodosus is required if outbreak specific vaccination is to be used as a control measure.
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