Serological testing
Indirect ELISAs detect antibodies generated by the host: they are used for screening animals (Craig, 2021), identifying exposure following an outbreak (Robinson et al., 2013, Rendle et al., 2021), and diagnosing the complications of strangles (Boyle et al., 2009). Carrier status cannot be determined using commercially available ELISAs (Durham and Kemp-Symonds, 2021, Van Maanen et al., 2021). Commercially available ELISAs detect antibodies produced against the SeM surface protein, or both antigen A (SEQ2190, a non-SeM target) and antigen C (a fragment of SeM) of S. equi , the so-called dual-target ELISA (Robinson et al., 2013, Boyle et al., 2018).
SeM-based ELISAs can be used to aid in the diagnosis of purpura haemorrhagica or metastatic abscessation (≥12,800), as well as identify animals predisposed to developing purpura haemorrhagica (>1:3,200) (Boyle et al., 2018, Boyle et al., 2009). They can also be used to indicate recent infection (≥4-fold increase in paired samples ten days apart) (Boyle et al., 2009, Boyle et al., 2018); although, a single reading is not a measure of protection or active infection.
Cross-reactivity with a SeM homologue in S. zooepidemicus (Kelly et al., 2006) combined with the failure of the SeM-based ELISA to detectS. equi strains not containing SeM (Harris et al., 2015) led to the development of the dual-target ELISA (Duran and Goehring, 2021, Robinson et al., 2013). Following an outbreak, it is advised to use the dual-target ELISA to identify horses exposed to S. equi (Boyle et al., 2018, Duran and Goehring, 2021). The dual-target ELISA is reported to have similar sensitivity but greater specificity than the single target SeM-based ELISA (Robinson et al., 2013). It can be used to identify recent exposure, from as little as two weeks post-infection, and has been used to determine exposure in populations across the globe (Ling et al., 2011, Štritof et al., 2021, Ivens et al., 2011).