Analysis of the presence of Beta-hemolytic streptococci and cytological profiles in horse tracheal lavage samples with diverse clinical respiratory symptoms.

Background: Riding clubs face significant financial losses due to upper respiratory infections caused by Beta-hemolytic (β-hemolytic) streptococci, leading to strangles and related diseases that negatively impact the health and performance of horses. Our objective was to determine the relationship between these inflammatory cells, the presence of β-hemolytic streptococcal species, and the clinical symptoms exhibited by the horses. This study investigated the presence of β-hemolytic streptococcus in 133 tracheal lavage samples from horses exhibiting various clinical respiratory symptoms and quantified the populations of neutrophils, eosinophils, macrophages, lymphocytes, and mast cells. The horses included in this study exhibited a variety of clinical respiratory symptoms categorized into groups: (I) Horses with no apperent symptoms (Control Group), (II) Horses with only coughing, (III) Horses with only lymph node swelling, (IV) Horses with coughing and lymph node swelling, (V) Horses with all symptoms (coughing, lymph node swelling and nasal discharge). After obtaining the bacterial cultures from tracheal wash samples, additional diagnostic tests were performed to identify the suspected colonies. The cytological analysis determined the proportions of inflammatory cells, while the statistical analysis assessed the significance of differences in inflammatory cell populations and pathogen presence across the different symptom groups.

Results: Among 133 horses categorized into five clinical groups, S. zooepidemicus was isolated in 18% overall, with the highest isolation rate in horses exhibiting both coughing and lymph node swelling (36%). Statistical analysis showed a significant difference in S. zooepidemicus prevalence between groups (p = 0.019), with post hoc tests confirming significance between asymptomatic horses and those with combined symptoms. Neutrophil proportions differed significantly among groups (p = 0.010), with the highest counts in symptomatic horses with both coughing and lymph node swelling. Furthermore, bacteriologically positive samples exhibited significantly higher neutrophil counts (median 83.5, IQR 76) than negatives (median 22, IQR 46, p < 0.001), suggesting an acute inflammatory response linked to infection. In contrast, macrophage, lymphocyte, and eosinophil counts showed no significant variation among groups or infection status.

Conclusions: Horses with coughing and lymph node swelling showed the highest prevalence of β-hemolytic streptococci. S. zooepidemicus infection was associated with increased neutrophils and decreased macrophages and lymphocytes, highlighting the role of neutrophils in inflammation. Asymptomatic horses may also carry the bacteria, indicating their role as carriers. Distinct inflammatory profiles correspond to clinical signs, underscoring the complexity of equine respiratory disease and the value of cytology and tracheal wash in diagnosis.