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
Animals—1,163 horses stabled at a Thoroughbred
Procedure—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 if 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.
Conclusion 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. (J Am Vet Med Assoc 2000;216:545–550)