In 2018, Dr. Maureen Sutter worked with a handful of authors to publish a journal article titled “Strangles, convalescent Streptococcus equi subspecies equi M antibody titers, and presence of complications”.
Streptococcus equi subspecies equi is the causative agent of strangles, a highly contagious upper respiratory tract infection of horses. Typical clinical signs of disease include fever, inappetance, lethargy, submandibular or retropharyngeal lymphadenopathy or purulent drainage, or purulent nasal discharge. Complications of S. equi infection can occur and include airway obstruction from lymphadenopathy, disseminated abscesses from hematogenous spread, or purpura hemorrhagica and various diseases caused by immune‐mediated processes.1, 2, 3, 4, 5 Streptococcus equi M protein (SeM) antibody titers are typically measured to determine if a horse has developed a complication of strangles, such as purpura hemorrhagica or metastatic abscess formation, or to determine if a horse is at risk of purpura hemorrhagica if they were to be vaccinated. Both the 2005 and 2018 American College of Veterinary Internal Medicine consensus statements on strangles state that a very high titer (≥1:12 800) is associated with metastatic abscess formation or purpura hemorrhagica and that high titers (1:3200‐1:6400) are detected 4‐12 weeks after infection.1, 2 Anecdotally, horses can have high titers (≥1:12 800) 4‐8 weeks after infection and no signs of complications (authors’ personal observations, KMD, LAB, ACT). The objective of this study was to measure SeM antibody titers on horses after outbreak to determine if titers detect the presence of complications.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.