Occurrence, antibiotic susceptibility and genetic variation of Streptococcus zooepidemicus in Finnish weanling horses with and without respiratory infection

Abstract Background Streptococcus zooepidemicus subsp. zooepidemicus (S. zooepidemicus), an opportunistic pathogen often found in the stable environment and upper respiratory tract of young horses, can cause severe pneumonias in Equidae. In this study we investigated the occurrence, genetic variation and antimicrobial susceptibility of S. zooepidemicus isolates from 63 weanling horses kept in loose housing or conventional stables. The bacterial isolates were typed by using pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST). The possible associating factors (stable type, age, breed and clinical signs) for positive S. zooepidemicus finding were analysed using logistic regression analysis. In addition, we describe antimicrobial susceptibility of 535 equine S. zooepidemicus isolates in Finland derived from clinical samples sent to the Clinical Microbiology Laboratory of the Faculty of Veterinary Medicine, University of Helsinki from 2011 to 2024. Results Out of the 63 weanling foalssampled, S. zooepidemicus was isolated in 26 (41%). There was a positive correlation between signs of respiratory infection and positive S. zooepidemicus finding (OR = 5.15, 95% CI: 1.169–22.694). Age was also a significant factor (OR = 0.972, 95% CI: 0.952–0.992), as younger foals were more likely to be S. zooepidemicus positive. Bacterial isolates were distributed into eleven PFGE clusters. The largest cluster contained six isolates, followed by clusters with five and three isolates, respectively. The cluster with three isolates appeared to have a new proS allele in MLST analysis. In addition, two more new sequence types were observed. Sequence types observed in our study differed from those previously identified in two earlier equine studies conducted in Sweden and Iceland. All isolates from weanling foals in our study were susceptible to penicillin and trimethoprim-sulfamethoxazole. There was no resistance to penicillin in the patient samples (n = 535) from 2011 to 2024. Of the 522 isolates tested, 28 (5.4%) were classified as either resistant or intermediate to trimethoprim-sulfamethoxazole. Only 24% of the isolates tested (n = 403) were susceptible to tetracycline. Conclusions Younger weanlings are more likely to have a positive S. zooepidemicus finding and their clinical signs, including high temperature, are associated with this common opportunistic pathogen. A high level of genetic variation of S. zooepidemicus is evident in weanling horses. Based on our findings, there is no evidence to suggest that S. zooepidemicus exhibits reduced susceptibility to penicillin. Additionally, the susceptibility to trimethoprim-sulfamethoxazole in the horses under study remains high.