Veterinary session

Modern equine husbandry presents many new challenges to veterinary practices and therefore the identification of pathogens that could potentially infect horses is of paramount importance. In addition to known pathogens, new bacterial and viral species can be detected thanks to the availability of new generation genomic methods. A previously unknown bacterium, Streptococcus hillyeri, was first isolated from a horse tracheal sample in 2010 and was reported in a paper published in 2019. Since then, it remains the only available publication on the species. Nothing is known about the prevalence or pathogenicity of the bacterium. This is why it attracted our attention when S. hillyeri was isolated in a near monoculture from the pleural effusion of a horse that died of septic pleuritis at the Department of Equine Medicine of the University of Veterinary Medicine Budapest, indicating the pathogenicity of the species. To learn more about the prevalence of the bacterium in horses, we aimed to develop a test method for its detection and to investigate samples from horses.

The test method chosen was a highly sensitive nucleic acid detection method, PCR. The system was tested to determine its analytical sensitivity (limit of detection, LOD) and specificity. The results of samples positive by real-time PCR were confirmed by conventional PCR. To estimate the carriage rate, we started from routine diagnostic samples of horses, most of which were nasal and uterine swabs. The 89 samples were from 2 different clinics.

The minimum amount of Colony Forming Unit (CFU) that could be reliably detected in a PCR reaction was 19.3 CFU/reaction and the test did not cross-react with other species of bacteria tested. From 89 clinical samples, a total of 7 strong and 4 weak positive (Ct > 35) results were obtained using real-time PCR. All positive results, except two weak positives, were confirmed by conventional PCR.

Based on our study, it can be concluded that the developed PCR system is suitable for the analysis of biological samples. Streptococcus hillyeri was present in 11.4% of the horses tested but was more frequent in respiratory samples (14.6%) than in those from the genital tract (2.5%). Our results suggest that S. hillyeri is present in the equine microbiome and may act as a facultative pathogen. Risk factors for its pathogenicity and a more accurate estimation of its carriage rate require further studies.