Epidemiological studies of inflammatory airway disease in horses

Newton, Jonathan Richard (2002). Epidemiological studies of inflammatory airway disease in horses. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000e7fe


Studies of respiratory disease in different populations of horses were undertaken, including a case control study, using clinically apparent respiratory disease in Thoroughbred racehorses as the case definition. Controls were matched on date and training yard and data were analysed using conditional logistic regression. Disease was statistically associated with several infectious and non-infectious risk factors. Younger horses, those entering training within the last 3 months and those with Actinobacillus/Pasteurella spp. or Mycoplasma felis isolated from the trachea were at increased risk of clinical disease. When all controls with sub-clinical disease were excluded, tracheal Streptococcus zooepidemicus infection was significantly associated with disease.

An experimental bacterial vaccine against S. zooepidemicus and Actinobacillus spp. was evaluated for its effect on natural respiratory disease in Welsh Mountain pony foals using a blinded, randomised, controlled trial. Weekly examinations were conducted in 29 ponies, of which 12 received vaccine, 12 received placebo and 5 were untreated. Data were analysed using a multilevel modeling approach, with autoregressive variables to adjust for significant effects of disease in previous time periods. Tracheal infection with S. zooepidemicus was a significant risk factor for aggregated clinical scores and clinical signs of nasal discharge, cough and dyspnoea. There was evidence for a significant dose response between S. zooepidemicus infection and dyspnoea and inflammatory airway disease. Both pony-level and observation-level analyses demonstrated significant variation in clinical scores between ponies with transferrin haplotypes D or F2.

Tracheal and nasopharyngeal isolates of S. zooepidemicus from the ponies in the vaccination study were typed by polymerase chain reaction of the hypervariable region of the M-protein (5 possible types) and the 16S-23S RNA gene intergenic spacer (8 possible types). More S. zooepidemicus types were isolated from the trachea than the nasopharynx. There was evidence for clonal succession of types over time and novel S. zooepidemicus types were identified. There were apparent differences in the strength of association of different S. zooepidemicus types with respiratory disease in these ponies during the study period.

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