International horse movements play role in spread of strangles, researchers find

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Side view of a two-year-old Shetland pony with strangles.
A two-year-old Shetland pony with strangles. His first symptom was a runny nose. He then developed a fever, and swelling to the right side of his face. This is a side view before the abscesses burst. ©AkaEmma, Public domain, via Wikimedia Commons

The latest DNA sequencing techniques have been used to track the bacteria responsible for strangles in horses around the world, in the largest study of its kind into the pathogen.

Researchers said they identified many examples of closely related strains of  Streptococcu equi in geographically distant nations, highlighting that the lack of pre-export testing, used routinely for many animal diseases, facilitates what they describe as the unbridled international transmission of the bacterium.

Their results, published in the journal Microbial Genomics, provide evidence of the important role played by the movement of horses in spreading this disease, providing new opportunities for interventions to prevent future outbreaks.

Strangles is the most frequently diagnosed infectious disease of horses. The bacterium invades the lymph nodes of the head and neck of horses, causing them to swell and form abscesses that can, in around 2% of cases, literally strangle the horse to death.

Some horses who recover from strangles remain persistently infected. These apparently healthy animals shed bacteria and spread the disease to other horses that they come into contact with.

Using standard diagnostic testing, the Streptococcus equi strains look almost identical. But by carefully examining their DNA, the researchers were able to track different variants as they spread across the world.

The research used the new online Pathogenwatch resource, developed at the Wellcome Sanger Institute, to visualise and share genome data to track the course of infections.

“Piecing the puzzle together, we showed that cases in Argentina, the United Kingdom and the United Arab Emirates were closely linked,” said Professor Matthew Holden of the University of St Andrews, who was involved in the study.

“Along with other examples, we provide evidence that the global trade and movement of horses is helping to spread the disease.”

Professor Julian Parkhill in the University of Cambridge’s Department of Veterinary Medicine, who was also involved in the study, said the research again shows the power of genomic data to uncover the fine detail of pathogen transmission locally and globally.

“Using whole-genome sequences we can track the movement of pathogens with very high precision, showing how and where to intervene to prevent the disease spreading.”

Strangles was first described in Medieval times and, with the exception of Iceland, affects horses in all corners of the world. Iceland’s freedom from the disease is because of a ban on the import of horses, which has been in place for more than 1000 years.

“This has been an incredible team effort, which was only possible through the collaboration of leading researchers from twenty-nine different scientific institutes in eighteen countries,” said Dr Andrew Waller, of Intervacc AB.

Horses are transported all over the world as they move to new premises or attend competitions and events. New cases of strangles can be prevented by treating carriers before they pass on the bacteria.

The authors noted the significant impact of strangles on horses, with mortality rates of up to 10 % in some outbreaks.

“We urge the OIE (World Organisation for Animal Health) to include strangles on the list of infectious diseases of Equidae of international importance.”

Such a policy change would encourage intervention measures that will rein in the onward transmission of Streptococcus. equi, they said, yielding significant health, welfare and economic benefits to the equestrian industry throughout the world.

The collaborating institutions in the study were:

  • Argentina: Clinica Equina, Buenos Aires
  • Australia: University of Melbourne
  • Belgium: Ghent University, Merelbeke
  • France: LABÉO Frank Duncombe, Caen
  • Germany: Labor Dr. Böse GmbH, Harsum
  • Ireland: Irish Equine Centre, Naas; University College Dublin
  • Israel: Kimron Veterinary Institute, Bet Dagan
  • Italy: University of Camerino
  • Japan: Japan Racing Association, Tochigi
  • Poland: Institute of Veterinary Medicine, Warsaw University of Life Sciences – SGGW
  • New Zealand: Massey University, Palmerston North; University of Waikato, Hamilton
  • Saudi Arabia: Al Khalediah Equine Hospital, Riyadh
  • Spain: Exopol, Zaragoza; Universidad Complutense, Madrid
  • Sweden: Department of Biomedical Science and Veterinary Public Health, Swedish University of Agricultural Sciences, Uppsala; Intervacc AB, Stockholm
  • The Netherlands: Royal GD, Deventer
  • United Arab Emirates: Central Veterinary Research Laboratory, Dubai; Emirates Racing Authority, Dubai
  • United Kingdom: Animal Health Trust, Newmarket; Big Data Institute, Li Ka Shing Centre for Health Information and Discovery, Nuffield Department of Medicine, University of Oxford; Centre for Genomic Pathogen Surveillance, Wellcome Trust Sanger Institute, Cambridge; Redwings Horse Sanctuary; University of Cambridge; University of St Andrews
  • United States of America: Gluck Equine Research Center, Lexington; Weatherford Equine Medical Centre, Texas

Mitchell, C. et al. ‘Globetrotting strangles: the unbridled national and international transmission of Streptococcus equi between horses.’ Microbial Genomics, March 2021. https://doi.org/10.1099/mgen.0.000528

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