Contact tracing and DNA testing of strangles-causing bacteria reveal how outbreaks spread

Every year, a considerable number of Swedish horses are affected by strangles, but there's potential to shield them from this ailment through vaccination. This project traced outbreaks using DNA tests of bacterial strains and interviews with horse owners. Several subvariants of the strangles bacterium were identified among riding schools, trotting horses, and private horses. DNA analyses either corroborated or dismissed potential connections between distinct outbreaks.

Strangles, a globally pervasive equine disease, is highly contagious, excruciatingly painful, and economically burdensome. It is caused by the bacterium Streptococcus equi subspecies equi. The Swedish Board of Agriculture receives reports of around one to two new strangles outbreaks every week. Symptoms include fever, purulent nasal discharge, and abscesses in the head and neck. A previous study examining the relatedness of 670 bacterial isolates from strangles-affected horses across 19 countries on different continents revealed that the strangles bacteria could be categorized into six distinct genetic groups or clusters. The majority of European outbreaks were classified under the same cluster, BAPS-2. By utilizing similar technology in conjunction with interviews, our aim was to discern whether strangles in Sweden primarily disseminates through contact between Swedish herds or if new strains continually enter the country through horses traveling in from abroad. We also aimed to ascertain whether the available subunit vaccine for strangles aligns with the bacterial subvariants present in Sweden.

Results

The results from the DNA testing of 42 strangles isolates in Sweden between 2021-2022 using whole-genome sequencing indicated that all of them belonged to the BAPS-2 cluster. However, at least 13 distinct subvariants were detected. The available strangles vaccine matched the detected strangles variants with an impressive 99.9% accuracy. However, none of the affected horses on the farms had been vaccinated against strangles.

Most outbreaks were linked to the introduction of a new horse to the facility, often one recently imported. The most prevalent strangles subvariant was found in 31% of the tested samples. In a trotting stable with unvaccinated horses, where new cases emerged over an extended period while new horses continued to be introduced, we demonstrated that they were initially affected by one strangles variant and later by another variant a few months later.

When DNA testing revealed different bacterial subvariants at different facilities, the possibility of cross-contamination between them could be dismissed. When tests indicated the same subvariant, the infection might have been directly transmitted between the facilities, or there could have been a common source of infection. This common source could involve a widely spread subvariant, and the shared link might have existed much earlier and several steps away, potentially even in a different country.

Through interviews with farms, we often managed to map out the paths of infection. For example, we found instances where the same infectious subvariant had been transferred to different facilities via infected horses transported in shared horse carriers. Another time, the connection was through an inadequately cleaned horse transport where the infection persisted between different trips.

Engaging with strangles in this manner represents a pioneering area of research for the Swedish National Veterinary Institute (SVA) and is in alignment with contemporary infection control strategies. By meticulously tracing infections and transmission routes, we can motivate horse owners to implement targeted preventive measures, such as vaccination and quarantine, and monitor signals that might necessitate updates to the strangles vaccine.

The study was led by Gittan Gröndahl and Anna Aspán (SVA) in collaboration with Andrew Waller (Intervacc, SLU), Karin Lindgren (SVA), Ellinore Jansson (SVA), Anita Ekmann (District Veterinarians), and Sara Frosth (SLU). Funding for the study was provided by the SVA Research Fund and Intervacc.