Chief Scientific Officer Intervacc AB Stockholm, Stockholms Lan, Sweden
Abstract: Background – Strangles, caused by Streptococcus equi, remains endemic in UK horses. The transmission of S. equi from healthy carrier horses that have recovered from the disease, but remain persistently infected, has been proposed to be the primary cause of new outbreaks of disease. Hypothesis/Objectives – We hypothesised that genome sequencing could be utilised to track outbreaks of strangles in the UK, shedding new light on the evolution and transmission of this important pathogen.Animals – With ethical approval (URN20201973-2), S. equi isolates (n=510) were recovered from clinical samples taken from horses in the UK that were submitted to six UK diagnostic laboratories between 2016 and 2022.Methods – The genomic DNA of isolates was purified, sequenced and the population structure determined by Bayesian Analysis of Population Structure (fastBAPS).Results – Nine fastBAPS groups were identified, although 82% of strains belonged to only two groups (fastBAPS3, n=230, 45%; fastBAPS5, n=189, 37%). Over the study period there was a significant increase in the prevalence of fastBAPS3 (7% in 2017 rising to 93% in 2022) and decreasing proportions of fastBAPS5 (69% in 2017 falling to “<1% in 2022). Conclusions and clinical importance – The rapid change in population structure cannot be explained by transmission from carriers. Instead, most transmission appears to occur following the movement of horses incubating strangles or those that have recovered recently. Our data suggest that interventions such as screening, quarantine and vaccination of horses could have the greatest benefit towards reducing the prevalence of strangles.
