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Acta veterinaria Scandinavica2026; 68(1); 7; doi: 10.1186/s13028-025-00839-0

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

Abstract: subsp. (), 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 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 finding were analysed using logistic regression analysis. In addition, we describe antimicrobial susceptibility of 535 equine . 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. Out of the 63 weanling foalssampled, was isolated in 26 (41%). There was a positive correlation between signs of respiratory infection and positive 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 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 S 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 ( = 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 ( = 403) were susceptible to tetracycline. Younger weanlings are more likely to have a positive finding and their clinical signs, including high temperature, are associated with this common opportunistic pathogen. A high level of genetic variation of is evident in weanling horses. Based on our findings, there is no evidence to suggest that exhibits reduced susceptibility to penicillin. Additionally, the susceptibility to trimethoprim-sulfamethoxazole in the horses under study remains high.
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Publication Date: 2026-01-27 PubMed ID: 41593672PubMed Central: PMC12837959DOI: 10.1186/s13028-025-00839-0Google Scholar: Lookup
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Summary

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Overview

  • The study investigates the occurrence, genetic diversity, and antibiotic susceptibility of Streptococcus zooepidemicus in Finnish weanling horses with and without respiratory infections.
  • Results show that younger foals and those with respiratory symptoms are more likely to carry S. zooepidemicus, which remains largely susceptible to key antibiotics like penicillin.

Background and Purpose

  • Pathogen Studied: Streptococcus zooepidemicus (S. zooepidemicus) is an opportunistic bacterial pathogen commonly present in the environment of horse stables and in the upper respiratory tract of young horses.
  • Health Impact: It can cause severe respiratory diseases, especially pneumonia, in Equidae (horses and related species).
  • Study Goals:
    • To determine how frequently S. zooepidemicus is found in weanling horses with or without respiratory infections.
    • To analyze the genetic variation among bacterial isolates from these horses.
    • To assess antimicrobial susceptibility patterns of S. zooepidemicus from both clinical and study samples.
    • To identify potential factors linked to infection such as stable type, age, breed, and clinical symptoms.

Methodology

  • Sample Collection: Samples were collected from 63 weanling horses housed either in loose or conventional stables.
  • Laboratory Testing:
    • Pulsed-field gel electrophoresis (PFGE) was used for bacterial typing to distinguish genetic clusters.
    • Multi-locus sequence typing (MLST) was used to identify sequence types and genetic variations.
    • Antimicrobial susceptibility testing was conducted on 535 equine S. zooepidemicus isolates obtained from clinical samples between 2011 and 2024.
  • Statistical Analysis: Logistic regression was used to analyze associations between various factors (stable type, age, breed, and clinical signs) and presence of S. zooepidemicus.

Key Findings

  • Occurrence:
    • S. zooepidemicus was isolated from 26 out of 63 (41%) weanling foals.
    • There was a significant positive correlation between respiratory infection signs and bacterial presence (Odds Ratio, OR = 5.15).
    • Younger foals had higher odds of being positive for S. zooepidemicus (OR = 0.972, meaning odds decrease slightly as age increases).
  • Genetic Diversity:
    • The isolates clustered into 11 distinct PFGE patterns, with the largest clusters containing six and five isolates.
    • MLST analysis identified a new allele type within one cluster and two new sequence types not previously documented.
    • The sequence types found differed from those reported in earlier Scandinavian studies, indicating geographic or population-specific variation.
  • Antibiotic Susceptibility:
    • All weanling isolates were susceptible to penicillin and trimethoprim-sulfamethoxazole.
    • No penicillin resistance was found in the larger clinical sample set (n=535).
    • Approximately 5.4% showed resistance or intermediate susceptibility to trimethoprim-sulfamethoxazole among 522 tested isolates.
    • Tetracycline susceptibility was much lower, with only 24% of 403 isolates being susceptible.

Interpretation and Implications

  • Clinical Relevance: Younger weanling horses are at greater risk of S. zooepidemicus colonization, particularly during respiratory infections, and the bacterium is a significant pathogen associated with fever and other clinical symptoms.
  • Genetic Variation: High genetic diversity of S. zooepidemicus exists among Finnish weanlings, highlighting the need for ongoing surveillance to detect emerging strains.
  • Antimicrobial Treatment: Penicillin remains effective and is the drug of choice for treatment, while there is some concern regarding tetracycline susceptibility.
  • Resistance Monitoring: Continued monitoring of antimicrobial susceptibility is essential to detect any emerging resistance, especially for commonly used drugs like trimethoprim-sulfamethoxazole.

Conclusions

  • S. zooepidemicus is a common opportunistic pathogen in Finnish weanling horses and is more likely to be present in younger foals and those with respiratory symptoms.
  • The bacterial strains present show significant genetic variability but maintain susceptibility to key antimicrobial agents such as penicillin.
  • The study provides important epidemiological data supporting targeted therapeutic strategies and ongoing surveillance of equine respiratory pathogens.

Cite This Article

APA
Junkkari R, Mykkänen A, Sulku P, Rantala M, Pohjanvirta T, Eklund M, Pelkonen S, Grönthal T. (2026). Occurrence, antibiotic susceptibility and genetic variation of Streptococcus zooepidemicus in Finnish weanling horses with and without respiratory infection. Acta Vet Scand, 68(1), 7. https://doi.org/10.1186/s13028-025-00839-0

Publication

Acta veterinaria Scandinavica
ISSN: 1751-0147
NlmUniqueID: 0370400
Country: England
Language: English
Volume: 68
Issue: 1
Pages: 7
PII: 7

Researcher Affiliations

Junkkari, Reija
  • Department of Equine and Small Animal Medicine, University of Helsinki, Helsinki, FI-00014, Finland. reija.junkkari@helsinki.fi.
Mykkänen, Anna
  • Department of Equine and Small Animal Medicine, University of Helsinki, Helsinki, FI-00014, Finland.
Sulku, Piia
  • Horse Clinic Anivet, Raviraitti 45, Turku, FI-20380, Finland.
Rantala, Merja
  • Department of Equine and Small Animal Medicine, University of Helsinki, Helsinki, FI-00014, Finland.
Pohjanvirta, Tarja
  • Research and Laboratory Department, Animal Health Diagnostic Unit, Finnish Food Authority, Neulaniementie 4, Kuopio, FI-70201, Finland.
Eklund, Marjut
  • Department of Equine and Small Animal Medicine, University of Helsinki, Helsinki, FI-00014, Finland.
Pelkonen, Sinikka
  • Research and Laboratory Department, Animal Health Diagnostic Unit, Finnish Food Authority, Neulaniementie 4, Kuopio, FI-70201, Finland.
Grönthal, Thomas
  • Department of Equine and Small Animal Medicine, University of Helsinki, Helsinki, FI-00014, Finland.

Conflict of Interest Statement

Declarations. Ethics approval and consent for publication: The study design was approved by the Finnish National Animal Experiment Board (ESAVI/6013/04.10.07/2013). The owners of horses were asked for their written approvals before the first farm visit. Consent for publication: Not applicable. Competing interests: The authors declare that they have no competing interests. Prior publication: Data have not been published previously.

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