title: Descriptive epidemiologic study of disease associated with influenza virus infections during three epidemics in horses.
abstract: OBJECTIVE
  To describe 3 epidemics of respiratory tract disease caused by influenza virus infections in a large population of horses.
  
  
  METHODS
  Cross-sectional and prospective longitudinal observational studies.
  
  
  METHODS
  All horses stabled at a Thoroughbred racetrack.
  
  
  METHODS
  During a 3-year period, descriptive information was collected as horses arrived at the racetrack and throughout race meetings. Routine observations and physical examinations were used to classify horses' disease status. Cause of epidemics was established by use of serologic testing and identification of influenza virus in nasal secretions.
  
  
  RESULTS
  An epidemic of respiratory tract disease caused by influenza virus infections was identified during each year of the study. Attack rates of infectious upper respiratory tract disease (IURD) ranged from 16 to 28%. Incidence of disease caused by influenza virus infections during racing seasons in the second and third years was 27 and 37 cases/1,000 horses/mo, respectively. Physical distributions of stall locations revealed that affected horses were stabled throughout the population; horses affected later in epidemics were often clustered around horses affected earlier. Mucopurulent nasal discharge and coughing were observed in 83 and 62% of horses with IURD, respectively. Median duration of clinical disease was 11 days. Serologic testing was the most sensitive method used to detect influenza virus infections; 76% of affected horses seroconverted to influenza virus.
  
  
  CONCLUSIONS
  Epidemics of IURD were observed annually in association with influenza virus infections. Few precautions were taken to limit spread of infection. Preventing or decreasing the likelihood of exposure and improving immunity in the population could substantially decrease risk of disease in similar populations.
authors.affiliation: Department of Veterinary Internal Medicine, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Canada.
authors.forename: P S
authors.initials: PS
authors.lastname: Morley
authors.forename: H G
authors.initials: HG
authors.lastname: Townsend
authors.forename: J R
authors.initials: JR
authors.lastname: Bogdan
authors.forename: D M
authors.initials: DM
authors.lastname: Haines
mesh_terms: D000818:Animals; D000914:Antibodies, Viral / Q000097:blood; D003430:Cross-Sectional Studies; D004196:Disease Outbreaks / Q000662:veterinary*; D005260:Female; D006734:Horse Diseases / Q000453:epidemiology* / Q000635:transmission; D006736:Horses; D015994:Incidence; D008137:Longitudinal Studies; D008297:Male; D009976:Orthomyxoviridae Infections / Q000453:epidemiology / Q000635:transmission / Q000662:veterinary*; D010805:Physical Conditioning, Animal; D011446:Prospective Studies; D012525:Saskatchewan / Q000453:epidemiology; D013177:Sports
chemical_list: D000914:Antibodies, Viral