References
Unsaddling Streptococcus equi infection of horses
Abstract
Infection with Streptococcus equi, which forms abscesses in the lymph nodes of the head and neck in horses, is endemic in almost all countries around the world. The identification and isolation of horses with fever, an early sign of disease, is critical to minimising the number of horses affected and the severity of an outbreak, while the identification and treatment of persistently infected ‘carrier’ horses can reduce the risk of recurrent outbreaks and transmission between equine populations. Rapid diagnostic testing plays a key role in the identification of infected horses, which can then be isolated before the development of acute disease or treated to clear persistent infection. Vaccination can also be used to reduce the number of horses that become infected and the severity of their ensuing disease. This review describes the tools available to veterinarians and the journey towards the development and launch of a multi-component fusion protein vaccine that does not trigger positive diagnoses with any of the available diagnostic tests for strangles. The use of vaccination, alongside conventional methods of biosecurity and diagnostic testing, has the potential to unsaddle S. equi, reducing the number of strangles outbreaks and enhancing the health of horses.
Strangles, caused by Streptococcus subspecies equi (S. equi), was first described in 1251 by Jordanus Rufus (Ruffo, 1256). However, the disease remains endemic in horse populations throughout the world, with the notable exception of Iceland, where a ban on the import of horses has been in place for over 1000 years (Björnsdóttir et al, 2017; Mitchell et al, 2021). This article describes the mechanisms that underpin the transmission of S. equi and the tools available for preventing and resolving this important disease.
Horses become infected with S. equi via the nose or mouth, most likely through ingestion of contaminated food or water (Figure 1) (Boyle et al, 2018). The bacteria attaches to and invades the mucosal surface of the nasopharynx, using an array of ‘sticky’ proteins on its surface, including the collagen-binding protein, CNE, which is believed to stick out from the surface of S. equi on hair-like projections (Lannergård et al, 2003; Holden et al, 2009; Steward et al, 2017). However, S. equi does not colonise the nasopharynx and samples taken from recently infected horses and those incubating the disease often return a negative culture or polymerase chain reaction (PCR) result, confounding their diagnosis (Boyle et al, 2018; Rendle et al, 2021). Instead, S. equi translocates to the sub-mandibular and retropharyngeal lymph nodes within only a few hours of infection (Timoney and Kumar, 2008).
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