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).