A case-control study of respiratory disease in Thoroughbred racehorses in Sydney, Australia.
Authors: Christley R M, Hodgson D R, Rose R J, Wood J L, Reids S W, Whitear K G, Hodgson J L
Journal: Equine veterinary journal
Summary
# Editorial Summary Lower respiratory tract disease in racing Thoroughbreds remains poorly understood, with infectious agents thought to play a significant role despite inconsistent evidence. Christley and colleagues conducted a matched case-control study in Sydney, comparing 100 horses presenting with cough to 148 controls from the same training establishments, analysing tracheal wash samples for viral serology, bacterial culture and mycoplasma isolation alongside endoscopic examination. Whilst respiratory viruses and mycoplasma showed no meaningful association with coughing, bacterial isolates exceeding 10³ colony-forming units/ml correlated strongly with disease, with Streptococcus zooepidemicus, S. pneumoniae, Pasteurella spp and Bordetella bronchiseptica identified as significant pathogens. Notably, 58% of affected horses yielded few or no bacterial isolates at the time of clinical presentation, indicating that bacterial superinfection may represent a secondary phenomenon in many cases, and that non-infectious inflammatory mechanisms warrant investigation. For practitioners, these findings suggest that isolating bacteria from tracheal wash samples should prompt treatment, but negative culture results do not exclude lower airway disease, and diagnostic strategies should account for the possibility of viral, environmental or mechanical aetiologies.
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Practical Takeaways
- •Bacterial infection is an important cause of lower respiratory disease in racehorses; tracheal wash culture with quantification (>10³ CFU/ml) can guide diagnosis and treatment decisions
- •Routine viral serology has limited diagnostic value for acute coughing in racehorses and should not be relied upon as sole basis for diagnosis
- •More than half of coughing cases have no identifiable infectious agent at presentation, suggesting need for investigation of environmental, inflammatory, and other non-infectious factors
Key Findings
- •Bacterial isolation of >10³ CFU/ml from tracheal wash was strongly associated with coughing in racehorses
- •Streptococcus zooepidemicus, S. pneumoniae, S. suis, S. sanguis, Pasteurella spp, and Bordetella bronchiseptica were individual bacterial species associated with respiratory disease
- •Serological evidence of respiratory virus infection and mycoplasma isolation showed no significant association with coughing
- •In 58% of coughing cases, few or no bacteria were isolated, indicating other infectious or non-infectious aetiologies remain unidentified