Background: Infection with Streptococcus equi subspecies equi ( S. equi) is characterised by acute disease with about 10% of infected animals entering a carrier state. Clinically, infection with S. equi cannot readily be distinguished from infection caused by other respiratory pathogens including Streptococcus equi subsp. zooepidemicus ( S. zooepidemicus), equine influenza virus and equine herpes virus. Screening protocols, with appropriate quarantining facilities, are important to detect carriers of S. equi and avoid strangles outbreaks. Objectives: Evaluate the effectiveness of the screening process implemented at a UK welfare centre to prevent introduction of strangles. Study design: Retrospective cross-sectional study. Methods: Clinical records of 626 equids admitted to a UK welfare centre between 2017 and 2021 and from horses that developed respiratory signs after admission were reviewed. Results: The screening protocol, which included a clinical examination, paired serology samples (iELISA) taken 6 weeks apart, and bilateral guttural pouch endoscopy to identify abnormalities such as chondroids with lavage for quantitative PCR and culture analysis for S. equi (and often S. zooepidemicus) detected 34 potential carriers of S. equi or S. zooepidemicus. Of these, 24 (3.8%) were PCR-positive for S. equi, 8 were PCR/culture positive for S. zooepidemicus and 2 were PCR/culture negative but had chondroids. Bilateral guttural pouch endoscopy, with PCR analysis of lavage material, was the most cost-effective method of detecting S. equi carriers. There were no cases of strangles within the general herd after screening and admission of new horses. Main limitations: Variation in the level of detail of clinical records. Conclusions: The screening process resulted in the identification of carriers and maintained a strangles-free herd. Further research is required to elucidate the significance of S. zooepidemicus infection in the guttural pouch.

Streptococcus equi subspecies equi (S. equi) is the causative organism of the upper respiratory disease of equids, strangles, characterised by pyrexia, lymphadenopathy, and mucopurulent nasal discharge. Strangles was first reported over 750 years ago and continues to be of significance in equine populations across the globe. This review discusses how S. equi has adapted, the clinical manifestation of strangles, and how clinicians and caregivers can tackle the disease in the future. S. equi evolved from the commensal, and occasionally opportunistic pathogen, Streptococcus equi subspecies zooepidemicus refining its capabilities as it became host restricted. The success of S. equi can be attributed to its ability to cause both acute and persistent infection, the latter occurring in about 10% of those infected. In this carrier state, S. equi persists in the guttural pouch without causing clinical signs, intermittently shedding into the environment, and encountering naïve animals. Insight into the S. equi genome and lifestyle has led to advances in diagnostic assays and the development of a safe and efficacious recombinant-fusion vaccine, giving clinicians and caregivers the tools to better combat this infection. Alongside rigorous biosecurity protocols and pragmatic control measures such as screening new arrivals for exposure and carrier status, these new technologies demonstrate that strangles can be an increasingly preventable infection.