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

OBJECTIVE: To identify risk factors associated with respiratory tract disease in horses during 3 epidemics caused by influenza virus infections. DESIGN: Cross-sectional and prospective longitudinal observational studies. ANIMALS: 1,163 horses stabled at a Thoroughbred racetrack. PROCEDURES: Investigations were conducted during a 3-year period. An epidemic of respiratory tract disease caused by influenza virus infections was identified in each year. Routine observations and physical examinations were used to classify horses' disease status. Data were analyzed to identify factors associated with development of disease. RESULTS: Results were quite similar among the epidemics. Concentrations of serum antibodies against influenza virus and age were strongly associated with risk of disease; young horses and those with low antibody concentrations had the highest risk of disease. Calculation of population attributable fractions suggested that respiratory tract disease would have been prevented in 25% of affected horses of all horses had high serum antibody concentrations prior to exposure. However, recent history of vaccination was not associated with reduction in disease risk. Exercise ponies had greater risk of disease than racehorses, which was likely attributable to frequent horse-to-horse contact. CONCLUSIONS AND CLINICAL RELEVANCE: Particular attention should be paid to young horses, those with low serum antibody concentrations, and horses that have frequent contact with other horses when designing and implementing control programs for respiratory tract disease caused by influenza virus infections. It appears that control programs should not rely on the efficacy of commercial vaccines to substantially reduce the risk of disease caused by influenza virus infections.