CORD-19:16dd2eb417bbac88ac2235efe705761a57807ae6 JSONTXT 8 Projects

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 Abstract 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. tions. 2 An additional objective was to follow SeM antibody titers out to 7 months after infection to determine immunoglobulin decay and to monitor for development of additional complications. We hypothesized that the magnitude of SeM antibody titer after infection (SeM titer ≥1:12 800) will be useful to monitor for the presence of complications or for the risk of development of complications. The horses that were euthanized were all complicated cases, including metastatic abscess formation, infarctive purpura hemorrhagica, secondary pleuropneumonia, and dysphagia. This reported outbreak is novel because there was a high proportion of horses with SeM antibody titers ≥1:12 800 (33%). There was also a high proportion of horses with complicated disease (29%) compared to reported complication rates (2%-20%). 1, 2, 4 In this outbreak, case fatality in complicated cases (50%) was high compared to reported case fatality in complicated cases (up to 40%). 2 Lastly, there was a high proportion of horses developing persistent GP infection (43%); this is higher than previous reports (up to 10%) 2 but consistent with a more recent report (up to 40%). 1, 4 Possible explanations for the higher complication rates and higher convalescent SeM antibody titers include a higher dose of exposure, a more virulent S. equi strain, 10, 11 Lastly, 13 horses were not tested for carrier status and were presumed not to have persistent GP infection; however, this could have led to misclassification. At initial testing for carrier status, some horses were tested only once with nasopharyngeal lavage largely based on economics; however, the current recommendations for detection of carrier status are endoscopy and GP lavage. 1, 4 This limitation could have led to underestimating the number of horses with persistent GP infection in this outbreak. It could have also led to overestimating the sensitivity and specificity of SeM antibody titers for detecting persistent GP infection as all of these horses had a SeM antibody titer ≤1:3200; however, this outbreak and other studies have indicated that SeM titers are not useful in this manner. 1, 8, 9 This outbreak illustrates the utility of SeM antibody titers after infection with S. equi. This study demonstrates that a horse may have complications of strangles without a SeM antibody titer ≥1:12 800 and that a horse may have a SeM antibody titer ≥1:12 800 without complications. A convalescent SeM antibody titer ≥1:12 800 warrants additional investigation for complications or persistent GP infection but does not necessarily confirm a horse has complicated disease. Results of this study were presented as an abstract at the 2018 American College of Veterinary Internal Medicine Forum, Seattle, WA. There was not off-label antimicrobial use. Authors declare human ethics approval was not needed for this study. https://orcid.org/0000-0003-0259-7210