Introduction
Strangles was first described in 1256 (Ruffo, 1256), although the disease and its causative organism Streptococcus equi subspecies equi (S. equi ), first identified by Schütz (1888), have likely been around for much longer. In the 17th century, strangles was considered an inevitability; indeed, it was suggested that the disease was transmitted in utero due to the high numbers of horses that contracted the infection across varied backgrounds, genetic profiles, and management systems (Solleysel, 1664, Paillot et al., 2017). By the 20th century, risk factors had been determined (Todd, 1910), including age and body condition; however, these are still in dispute (Ling et al., 2011). Management-related factors are commonly the only factors consistently associated with outbreaks (Libardoni et al., 2016, Laing et al., 2021).
As was long suspected (Todd, 1910) and later confirmed (George et al., 1983, Newton et al., 1997a, Timoney et al., 1998), S. equipersists in the guttural pouch, without causing clinical disease in a proportion of animals. S. equi survives in this low nutrient state, intermittently shedding into the environment, allowing the organism to spread to naïve individuals; indeed, its success as a pathogen can be attributed to the ability to cause both acute and persistent disease. Chronically infected equids rarely show clinical signs, presenting a major obstacle to the prevention and control of outbreaks (Verheyen et al., 2000). The challenges associated with detecting carriers are a key reason for the perpetual spread of S. equi (Pringle et al., 2020b).