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TDK conference 2016

Association between cytological findings and bacterial isolates from tracheal wash samples in horses of different ages in Great Britain
Lean Melanie - year 5
University of Veterinary Medicine, Department of Microbiology & Infectious Diseases; Rossdales Equine Hospital, Newmarket, UK; Beaufort Cottage Laboratories, Newmarket, UK
Supervisors: Dr. Alastair Foote, Dr. Claire Wylie, Dr. Laszlo Fodor


Respiratory disease accounts for a major source of wastage within the UK racing industry. The terminology ‘Inflammatory airway disease’ (IAD) describes airway inflammation and accumulation of mucus, the aetiology of which is poorly understood. Uncertainty remains in the role of infectious agents in the disease process and whether infectious and non-infectious inflammatory airway disease can be reliably differentiated.

Aims of the study were to evaluate any association between cytological findings and bacterial isolates from tracheal washes of horses. The hypothesis being that degenerative changes in neutrophils in horses with inflammatory airway disease was predictive of isolating significant bacterial isolates, whereas non-degenerate neutrophils feature in non-septic lower airway inflammation. Using two statistical tests we were able to establish whether there was any association between bacterial growth (of the isolates of interest) and neutrophils (indicating respiratory inflammation), whether there was any association between non-degenerate or degenerate neutrophils and the presence of significant bacterial isolates and whether an age-related association between young and old horses both exhibiting respiratory inflammation, was observed. A P-value of P<0.05 was considered significant.

A total of 1,977 tracheal wash aspirate samples were collected by Rossdales Equine Hospital veterinary surgeons and referring vets across Great Britain and referred into Beaufort Cottage Laboratories from the period of January 2013 to July 2015.

Overall results investigating inflammation versus no inflammation indicated that there was an association between inflammation and having growth of any of the significant bacteria compared to no growth (P<0.001). The following individual bacterial isolates were significantly associated with inflammation: Streptococcus equi subsp. zooepidemicus and β-haemolytic Streptococcus spp. (grouped together) (P<0.001), Streptococcus pneumoniae (P<0.01), Pasturella spp. (P<0.001) and Bordatella spp. (P<0.01) compared to α-haemolytic Streptococcus spp. and Actinobacillus spp., which were not (respective P-values of P<0.76 and P<0.16).

Comparing association between the presence of degenerate neutrophils and growth of the significant bacterial generated a P-value of P<0.001, suggesting an association between inflammation and the presence of degenerate neutrophils. A P-value of P<0.001 was generated following overall analysis of the horses grouped by age (≤4 years old versus ≥8 years old) suggesting an association between the presence of any of the significant bacterial isolates and inflammation.

Results of this study suggest that there is an association between degenerate neutrophils and significant bacterial isolates. These results help to further examine aetiological factors associated with inflammatory airway disease and its pathogenesis in the young horse population, and in particular the role of bacteria.

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