Genomic study reveals drivers of strangles spread
Researchers combined whole genome sequencing with epidemiological data.
A genomic study from the Royal Veterinary College (RVC) has explored the main causes of strangles transmission among horses in the UK.
The findings revealed a ‘significant change’ in the population structure of Streptococcus equi, the bacteria which causes the respiratory tract infection.
Strangles is a highly contagious and potentially life-threatening infection, affecting horses, ponies and donkeys of all breeds and ages. Approximately 300 laboratory-confirmed cases are reported in the UK each year, though there is expected to be many more undiagnosed cases.
The RVC sought to improve understanding about the transmission of strangles, particularly the contributions of short-term and long-term carriers of the disease.
Researchers from the RVC, the University of Cambridge, the Swedish University of Agricultural Sciences and Redwings Horse Sanctuary examined 500 samples of S. equi, collected from across the UK.
The samples were collected from clinical cases, which had been sent to diagnostic laboratories by veterinary surgeons seeking confirmation of infection. Surplus samples from the Surveillance of Equine Strangles network were cultured at the RVC, before the DNA was extracted and sequenced to reveal complete genetic codes.
Using a novel approach, combining whole genome sequencing and epidemiological data, researchers traced how the strains were related and how they might have evolved. Computer programmes also tracked how genetically related strains spread across horses and regions.
This allowed the team to pinpoint transmission events and track chains of infection.
The findings revealed a ‘significant and rapid’ change in the S. equi population structure in the UK between 2016 and 2022. Contrary to previous assumption, acutely infected or recently convalesced short-term carrier horses could be playing a more influential role in transmission than long-term carriers.
The disease was found to be frequently transmitted across the country, with two-thirds of transmission pairs identified as horses from different regions.
This included a significant transmission chain spanning the UK across a six month period. The RVC says this demonstrates a national and international importance in controlling horse movements.
Most of the detected cases were transmitted from horses displaying overt clinical signs or had been convalescing. Researchers say that there is a critical need for awareness and adoption of post-outbreak screening protocols.
Abigail McGlennon, former PhD student in the Department of Pathobiology and Population Sciences at the RVC, and lead author of the paper, said: “This study shows how important enhanced surveillance utilising epidemiological and genomic data is to investigate and understand strangles transmission across the UK.
“Our data highlights the importance of greater awareness and adoption of post-outbreak screening protocols to confirm freedom from infection, rather than owners assuming recovery based on the resolution of clinical signs.”
The full study can be found in the Equine Vet Journal.
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The BSAVA has opened submissions for the BSAVA Clinical Research Abstracts 2026.
