Bacterial isolates and antimicrobial susceptibility patterns in tracheal wash samples from horses with equine asthma in Poland.

Overview

• This study investigated the types of bacteria found in tracheal wash samples from horses with equine asthma (EA) in Poland and analyzed how these bacteria respond to antibiotics.
• The research also examined whether the presence and type of bacteria were related to the season of sampling or the level of airway inflammation indicated by neutrophil counts.

Background

• Equine asthma (EA) is a chronic inflammatory disease affecting the lower airways of horses, leading to difficulty in breathing.
• Bacteria can often be detected in tracheal wash (TW) samples from affected horses, but it is unclear whether their presence indicates infection or simply colonisation (non-harmful presence).
• Understanding which bacteria are present and their susceptibility to antibiotics is important for appropriate treatment decisions.

Study Design and Methods

• Subjects: The study included 165 adult horses diagnosed with EA based on criteria set by the American College of Veterinary Internal Medicine (ACVIM).
• Sample Collection:
  • Endoscopy and tracheal wash sampling were performed using a double-guarded catheter to minimize contamination.
  • Bronchoalveolar lavage fluid (BALF) cytology was conducted to assess airway inflammation, specifically the percentage of neutrophils.
• Bacterial Culture and Identification:
  • TW samples were cultured using conventional microbiological techniques.
  • MALDI-TOF MS (Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry) was used to accurately identify bacterial species.
• Antimicrobial Susceptibility:
  • Isolated bacteria were tested against various antibiotics to determine resistance or sensitivity.
• Data Analysis:
  • Associations between bacterial culture results and variables such as month or season of sampling and neutrophil percentage were evaluated using appropriate statistical tests (chi-squared/Fisher’s exact with Monte Carlo simulations).
  • Effect size was measured using Cramer’s V to understand the strength of any relationships.

Key Findings

• Bacterial Cultures:
  • Out of 165 horses, 117 (70.9%) had positive bacterial culture results from their TW samples.
  • In total, bacteria were isolated from 116 horses (70.3%).
  • The most commonly found bacteria included subtypes of common equine respiratory bacteria (specific species names were not provided in the abstract).
• Seasonal Variation:
  • Whether the culture was positive or not did not significantly depend on the month or season of sampling.
  • However, the types (genus) of bacteria present showed a modest but statistically significant association with the month of sampling, suggesting some variation in bacterial populations throughout the year.
• Neutrophil Percentage:
  • There was no clear relationship between bacterial presence or type and the percentage of neutrophils in the airways, indicating that bacterial colonisation does not necessarily correlate with airway inflammation severity.
• Antimicrobial Susceptibility:
  • Resistance patterns varied depending on the bacterial species identified.
  • This underscores the importance of performing antibiotic susceptibility testing to guide therapy rather than relying on empirical treatment.

Conclusions and Clinical Implications

• Many horses with EA have cultivable bacteria in their tracheal wash samples, but this often reflects colonisation rather than active infection requiring treatment.
• Positive bacterial culture results alone should not be used as a definitive sign of bacterial infection in horses with EA.
• When antimicrobial treatment is clinically justified based on symptoms and cytological evidence (such as increased neutrophils or other signs of infection), it should be guided by culture and sensitivity results to ensure effective therapy and reduce antibiotic resistance risk.
• Overall, this research supports careful interpretation of bacterial cultures in equine asthma and highlights the need for targeted antibiotic use informed by diagnostic testing.