Epidemiology of infectious upper respiratory tract disease in horses

The goals of this research were to characterize the occurrence of infectious upper respiratory tract disease (IURD) as it is seen among horses stabled at a racetrack, to identify strategies that might be used to prevent the occurrence of IURD, and to evaluate the importance of IURD as a problem affecting racehorses. An observational investigation was conducted at a Thoroughbred racetrack during an influenza epidemic identified during the 1990 race meeting as were prospective studies during the 1991 and 1992 race meetings. A double-blind field trial was also conducted to evaluate a commercially available vaccine containing inactivated influenza viruses and inactivated equine herpesvirus type 4 (EHV-4).;A total of 109 IURD cases were caused by influenza virus during the 1990 epidemic. Among cases identified during 1991 and 1992 (n = 227), 57% were caused by influenza virus during two epidemics, 18% were caused by EHV-4 during two epidemics, 11% were caused by Streptococcus equi subsp. equi during one epidemic, and 14% had an unknown etiology. The incidence of undifferentiated IURD averaged throughout the 1991 and 1992 race meetings was 49.0 cases per 1,000 horse-months.;Significant risk factors for disease occurrence were similar among all etiologies; young horses, horses with low antibody concentrations to influenza virus or herpesvirus, and horses with frequent horse-to-horse contact had increased risk of IURD. Relatively small differences in antibody concentrations were associated with substantial risk reduction and serologic testing could be used to identify horses at greatest risk of disease. As horses with frequent contact had greater risk of disease, decreasing the rate of contact between healthy and diseased horses may provide an inexpensive control measure when epidemics occur.;The strength of the association between low serum antibody concentrations and disease occurrence, and the relatively low antibody concentration needed for protection suggested that effective immunization might have prevented a large proportion of cases that occurred. However, administering a vaccine considered representative of commercial vaccines did not significantly reduce the risk of IURD during an influenza epidemic. This may have been caused by especially poor response to vaccination among naive horses.