Strangles, convalescent Streptococcus equi subspecies equi M antibody titers, and presence of complications

BACKGROUND: Streptococcus equi subspecies equi infection elicits M protein antibody titers in equids. Interpretation of titers is not generally accepted. HYPOTHESIS: The magnitude of S. equi M protein (SeM) antibody titer after infection (titer ≥1:12 800) will be useful to monitor for the presence of complications or the risk of development of complications. ANIMALS: Forty-eight horses on 1 farm involved in strangles outbreak. METHODS: Clinical and observational study. S. equi M protein antibody titers were measured on all horses 8 weeks after infection and select horses 12 and 28 weeks after infection. Horses were categorized: no disease, uncomplicated case, persistent guttural pouch (GP) infection, or complicated cases (metastatic abscesses, purpura hemorrhagica, secondary infections, and dysphagia). Category was compared to titer. RESULTS: Twenty-eight of 48 (58\%) developed clinical signs of S. equi infection. Of those, 11 (39\%) had uncomplicated strangles, 9 (21\%) had persistent GP infection, 5 (18\%) were complicated cases, and 3 (11\%) had both persistent GP infection and complications. Thirty-three percent of horses (16 of 48) had SeM antibody titers ≥1:12 800 eight weeks after infection. Of horses with titers ≥1:12 800, 6 of 16 had evidence of complications. Of complicated cases, 6 of 8 had titers ≥1:12 800. In this outbreak, the sensitivity (75\%; 95\% CI [confidence interval] 45-105) for a SeM antibody titer ≥1:12 800 detecting complications was higher than the specificity (43\%; 95\% CI 23-64). CONCLUSIONS AND CLINICAL IMPORTANCE: This outbreak demonstrates that SeM antibody titers can be increased after infection (≥1:12 800) in the absence of complications of strangles.