Data were collected from January 2001 to December 2005 by the Korea Racing Authority Equine Registration and Health Management System (Table 1). With the Analyse-it program (Analyse-it Software, UK), the χ² and one-tailed p values from the Mantel-Haenszel χ² test and the two-tailed probability from the Fisher Exact test were used to analyze significant differences in IURD incidence with different risk factors (season, age, and stabling period). The Park had yearly IURD epizootics with a 29.6% average incidence rate from 2001 to 2005. This was significantly higher than the 1.5% IURD incidence rate per quarter in non-racetrack horse populations in the USA reported at the annual convention of the American Association of Equine Practitioners. The difference between the incidence of IURD in our study and the USA might be caused by the dense population and high turnover rate at the Park (Table 1).

A number of risk factors for IURD occurrence were identified in this study including the frequent stabling seasons, the stabling period, and age of the horses. There was frequent movement of horses in and out of the Park in the spring, summer, and fall and the least amount of movement in winter (Fig. 1A, p < 0.05). The IURD incidence rate (Fig. 1B) was significantly higher in the summer and fall (p < 0.05). The main populations of newly stabled horses and IURD patients were 2 to 3 years old. The incidence rate of IURD in the 2 and 3 years old group (Table 2) was significantly higher than in other age groups (p < 0.05). The IURD incidence rate in horses stabled for less than 3 months (Table 2) was significantly higher than that for horses stabled for other periods of time (p < 0.05).

Bacterial identification and confirmation were performed using nasal swap specimeans with the API 20 Strep (bioMérieux, France). In vitro antimicrobial susceptibility testing was conducted using the disk-diffusion testing method with Sensi-disc test discs (Becton, Dickinson and Company, USA) in accordance with the guidelines of the Clinical and Laboratory Standards Institute (CLSI) [3,4]. S. equi subsp. equi ATCC 33398, S. equi subsp. zooepidemicus ATCC 43079, and Staphylococcus aureus 25923 were used as quality control organisms. The zone diameters were determined using criteria defined by the CLSI and Food and Drug Administration, and other protocols from France and the USA [5,7]. The most commonly isolated organisms were Pseudomonas spp. (16.3%), Escherichia coli (14.3%), S. equi (10.2%) [S. equi subsp. equi (5.1%), and S. equi subsp. zooepidemicus (5.1%)], Enterobacter cloacae (9.2%), and Staphylococcus spp. (8.2%) among the aerobic bacterial isolates (n = 98). The bacterial isolates most commonly identified in this study are consistent with previous reports, but notable differences existed in their isolation frequency [1,2,6]. Results from a study of acute respiratory disease in horses in Ontario, Canada suggested that the most frequently isolated bacteria were α-hemolytic streptococci, coagulase-negative staphylococci, Actinobacillus equuli, and S. equi subsp. zooepidemicus [2]. Another study of foals with upper respiratory tract infections in Poland reported that coagulase-negative staphylococci and Acinetobacter sp. were the dominant identified species [1]. A German study of respiratory symptoms in horses found that S. equi subsp. zooepidemicus and S. equi subsp. equi were the dominant species [6]. A possible explanation of these differences in the frequency of major bacterial species isolated from cases of IURD may be due to differences in geography and stabling environments. Antimicrobial susceptibility of bacterial isolates is summarized in Table 3, suggesting that the current initial (trimethoprim/sulfamethoxazole, oxytetracyclin, penicillin, or amoxicillin/clavulanate), secondary (amikacin and gentamicin nebulization), and tertiary antibiotic (ceftiofur) treatments for IURD were effective.

In conclusion, this study revealed that host factors have a critical role in the development of IURD. Further analysis is required to determine the viral contribution to IURD and the detailed relationships between risk factors and IURD incidence rates at the Seoul Race Park in Korea. Regulatory veterinarians should target and segregate horses with the greatest IURD risks and design more effective preventative measure, such as implementing specific respiratory pathogen-free certification systems prior to the Park arrival, and removing infected horses.