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Equine veterinary journal2025; doi: 10.1111/evj.14558

Unwelcome neighbours: Tracking the transmission of Streptococcus equi in the United Kingdom horse population.

Abstract: Strangles (Streptococcus equi infection) remains endemic in the UK, with ~300 laboratory diagnoses annually. Sub-clinically infected long-term carriers are considered a key driver of endemicity. Analysing genomes of circulating strains could provide valuable transmission insights of this pathogen. Objective: To determine the population structure and diversity of UK S. equi isolates and to model transmission using epidemiological and whole genome sequencing data. Methods: Retrospective cross-sectional epidemiological and genomic surveillance. Methods: A dated phylogenetic tree derived from 511 S. equi isolates collected from UK horses between 2015 and 2022 was reconstructed. Bayesian Analysis of Population Structure (BAPS) identified clusters of related genomes, while iGRAPH identified clusters of sequences appropriate for transmission analysis, performed using Transphylo. Results: BAPS identified nine groups, with 82% of strains clustering into two (McG-BAPS3, McG-BAPS5). A statistically significant association (p < 0.001) was found between the year of recovery and trends in the frequency of McG-BAPS groups, with McG-BAPS3 increasing and McG-BAPS5 decreasing in prevalence over the study period. Eight transmission clusters encompassing 64% of total sequences (n = 286/447) underwent analysis. Sixteen direct transmission pairs were identified; 10 were between horses from different UK regions. A transmission chain extending over a 6-month period was inferred from isolates from nine horses. Conclusions: Bacterial strains from sub-clinically infected carrier horses may be underrepresented due to data collection via positive laboratory diagnoses. Furthermore, a low sampling proportion relative to overall UK cases provided only a snapshot of broader, unsampled transmission events. Conclusions: The rapid change in S. equi population structure indicates acutely infected/recently convalesced short-term carrier horses play a more influential role in transmission than long-term carriers. Our work provides novel insights to our understanding of S. equi transmission dynamics. Transmission of genetically related strains across diverse regions suggests a real-time sequence-based surveillance system could inform interventions to minimise transmission.
© 2025 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2025-07-20 PubMed ID: 40684376DOI: 10.1111/evj.14558Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The study investigates the transmission of Streptococcus equi, the bacterium causing strangles in horses, within the UK horse population. By using sequencing and modeling, the researchers discovered trends in bacterial strains, determined their diversity, and highlighted how strain changes might inform disease control measures.

Research Methodology

  • The study was conducted retrospectively, comparing epidemiological and genomic data from 511 S. equi isolates collected from UK horses between 2015 and 2022.
  • The researchers used Bayesian Analysis of Population Structure (BAPS) to identify clusters of related genomes as well as iGRAPH to identify the right clusters for transmission analysis.
  • The transmission of the bacteria was analyzed using the Transphylo method. This method uses data on the time and location of infections to create a transmission tree, showing how an infection might have spread in a population.

Research Findings

  • The BAPS identified nine groups of related genomes, with 82% of strains clustering into two groups (McG-BAPS3 and McG-BAPS5).
  • A statistically significant association was found between the year of recovery and trends in the frequency of McG-BAPS groups. Specifically, the prevalence of the McG-BAPS3 strain increased while the McG-BAPS5 decreased over the study period.
  • 16 direct transmission pairs were identified, 10 of which were between horses from different UK regions. Proof of a transmission chain extending over a 6-month period was inferred from isolates from nine horses.

Conclusions and Implications of the Research

  • This study revealed that due to data collection methods, bacteria from sub-clinically infected carrier horses might be underrepresented. Also, the low sampling proportion relative to the total UK cases only provided a snapshot of broader, unsampled transmission events.
  • The study’s results suggest that short-term carrier horses, that are acutely infected or have recently recovered, play a bigger role in transmission than long-term carriers.
  • Transmission of genetically related bacteria strains across diverse regions indicates that a real-time sequence-based surveillance system could potentially help to minimize transmission by informing intervention measures.

The research provides novel insights to our understanding of S. equi transmission dynamics and can serve as groundwork for developing more effective disease control strategies for the UK horse population in the future.

Cite This Article

APA
McGlennon AA, Verheyen KL, Newton JR, van Tonder A, Wilson H, Parkhill J, de Brauwere N, Frosth S, Waller AS. (2025). Unwelcome neighbours: Tracking the transmission of Streptococcus equi in the United Kingdom horse population. Equine Vet J. https://doi.org/10.1111/evj.14558

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English

Researcher Affiliations

McGlennon, Abigail A
  • Royal Veterinary College, Hertfordshire, UK.
  • Department of Veterinary Medicine, University of Cambridge, UK.
Verheyen, Kristien L
  • Royal Veterinary College, Hertfordshire, UK.
Newton, J Richard
  • Department of Veterinary Medicine, University of Cambridge, UK.
van Tonder, Andries
  • Department of Veterinary Medicine, University of Cambridge, UK.
Wilson, Hayley
  • Department of Veterinary Medicine, University of Cambridge, UK.
  • Public Health Genomics Foundation, Cambridge, UK.
Parkhill, Julian
  • Department of Veterinary Medicine, University of Cambridge, UK.
de Brauwere, Nicolas
  • Redwings Horse Sanctuary, Norwich, UK.
Frosth, Sara
  • Department of Animal Biosciences, Swedish University of Agricultural Sciences, Uppsala, Sweden.
Waller, Andrew S
  • Intervacc, Hägersten, Sweden.

Grant Funding

  • G2019 / Horse Trust

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Citations

This article has been cited 1 times.
  1. Rask E, Righetti F, Ruiz A, Bjerketorp J, Frosth S, Frykberg L, Jacobsson K, Guss B, Flock JI, Henriques-Normark B, Hartman E, Gustafsson A, Paillot R, Waller AS. Closing the Stable Door on Strangles: Serological Responses of Vaccinated Horses on a Farm Following the Arrival of a New Horse. Animals (Basel) 2025 Dec 13;15(24).
    doi: 10.3390/ani15243584pubmed: 41463869google scholar: lookup
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