There is a specific diagnostic requirement where vaccination is the chosen control method. After confirmation of virulence it is necessary to confirm the serogroup(s) of D. nodosus that are present in the flock, so that a specific bivalent vaccine can be used.
There are major 10 serogroups of D. nodosus (A, B, C, D, E, F, G, H, I and M), defined in slide agglutination tests using rabbit antisera (Claxton, 1986b; Claxton et al., 1983). The classification can be extended to many more subtypes, but this is not important with respect to choice of serogroup for inclusion in a vaccine. More than one serogroup can be present in a flock of sheep, and even on the same foot of a sheep. Serogroup is completely independent of virulence, that is, all of the serogroups are associated with benign and virulent footrot. Serogroup is determined by the pilus (fimbrial) antigen which is a hair-like structure on the outside of the bacterial cell, encoded by the gene pilA.
A polymerase chain reaction (PCR) has been developed for classification of serogroup based on the DNA sequence of the pilus gene pilA (Dhungyel et al., 2002) (Figure 1). It has been validated for direct application to footrot lesion swabs at the University of Sydney in a project funded by Meat and Livestock Australia (McPherson et al., 2018).
Figure 1. Results of a multiplex PCR reaction to determine serogroup. The white bands against the black background of an agarose gel are interpreted against standards to determine which serogroup-genes are present in a sample.
Sample size is important in this application. Sampling 22 sheep and 2 colonies of D. nodosus per sheep was determined by simulation modeling to be an optimum sampling strategy for accurate identification of the relevant serogroups from among the 10 possible (Hill et al., 2010).
The University of Sydney has produced guidelines for laboratory diagnosis when outbreak specific vaccination is being considered as a control option. Please click on this link: Footrot Fact Sheet 1 Diagnosis for specific vaccination 270417
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