Streptococcus equi subspecies equi M protein (SeM) titers are useful for confirming complications of strangles infection (titer ≥1:12,800) or for evaluating immunologic risk of complications prior to vaccination (titer ≥1:1600). There are generally-accepted interpretations of SeM titers; however, specific references for data interpretation are lacking. The purpose of this study was to compare SeM titers with clinical disease displayed by horses post-outbreak to determine if the generally-accepted titer interpretation was consistent with clinical signs in these horses. Following an outbreak of S. equi, serum was collected to measure SeM titers on all horses at 8 weeks post-outbreak and on select horses at 12 weeks and 28 weeks post-outbreak. Horses were categorized as having no disease, uncomplicated strangles, persistent guttural pouch infection (>40 days post-infection), and/or complicated cases (metastatic abscess formation or purpura hemorrhagica, for example) based on clinical findings. Twenty-eight out of forty-eight (58.3%) developed clinical signs of S. equi infection. Of the 28 affected horses, 11 (39.3%) had uncomplicated strangles, 12 (42.8%) had persistent guttural pouch infection, and 8 (28.6%) were complicated cases, 3 of which were dually-categorized as persistent guttural pouch infection and complicated cases. Sixteen out of 28 (57.1%) horses had titers ≥1:12,800 at 8 weeks post-outbreak. Only 6/16 horses with SeM titers ≥1:12,800 had evidence of strangles complications at that or later time points. SeM titers declined in all horses tested at 28 weeks (n=36) to levels ≤1:6,400. This outbreak demonstrates that SeM titers can be significantly elevated post-outbreak (≥1:12,800) and the horse may not have strangles infection complications. It is still prudent to monitor a horse with a very high titer to ensure that complications are absent.
Clinical Assistant Professor
Kansas State University
Friday, June 15
3:00 PM – 3:15 PM
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