Risk factors for disease associated with influenza virus infections during three epidemics in horses.

The research aims to identify risk factors that increase the likelihood of horses developing respiratory diseases during influenza epidemics. The study focuses on horses kept at a Thoroughbred racetrack and identifies age, serum antibody concentrations, and frequent contact with other horses as significant risk factors. However, contrary to expectations, vaccination history did not seem to mitigate the risk.

Study Design and Method

• The study was a blend of cross-sectional and prospective, longitudinal observational studies conducted over three years at a Thoroughbred racetrack. This combined approach allowed for a comprehensive view of risk factors over time and across different horses.
• The research involved 1,163 horses stabled at the racetrack—horses infected with the influenza virus during three separate epidemics served as the subject of interest.
• Classification of the horses’ disease statuses was achieved through routine observations and physical examinations, providing authentic and first-hand data for the study.
• The extracted data were then analyzed to identify any patterns or factors associated with the development of the disease.

Results and Findings

• The results between the epidemics were relatively consistent, enhancing the reliability of the findings.
• Key risk factors identified were age and concentrations of serum antibodies against the influenza virus—younger horses and those with lower antibody concentrations were found to have the highest risk of disease.
• It was inferred that respiratory disease could have been prevented in approximately 25% of the affected horses if all the horses had high serum antibody concentrations before potential exposure to the virus.
• Contrary to expectation, a recent history of vaccination did not correlate with a reduction in the risk of disease—suggesting that such vaccinations may not be as effective in controlling the risk of disease as assumed.
• The study also found that exercise ponies, likely due to their more frequent contact with other horses, were at higher risk than racehorses.

Conclusions and Implications

• According to the study’s findings, when designing and implementing control programs for respiratory tract disease caused by influenza virus infections, extra attention should be given to younger horses, horses with low serum antibody concentrations, and horses frequently interacting with other horses.
• The study also recommended that control programs should not count solely on the efficacy of commercial vaccines to substantially reduce disease risk.