Seroprevalence of Infectious Respiratory Agents in Thoroughbred Race Horses at the Seoul Race Park, Republic of Korea

Infectious respiratory disease is one of the most frequent causes of lost days in training and reduced performance of Thoroughbred racehorses. Equine influenza virus (EIV), Equine herpesvirus-1 (EHV-1) and -4 (EHV-4), equine rhinitis virus A (ERAV) and B (ERBV), and Streptococcus equi subspecies equi ( S. equi ) are important infectious agents of the respiratory tract of horses. The purpose of this study was to determine the seroprevalence of EHV-1, EHV-4, ERAV, ERBV, and S. equi and to measure EIV antibody levels of Thoroughbred racehorses at Seoul Race Park (SRP), Republic of Korea. All horses had previously been vaccinated against EIV and S. equi , but not against any of the other pathogens that were tested. A total of 94 serum samples, which were collected from race participants at the SRP were tested using the single radial haemolysis (SRH) test for EIV (H3N8), the complement-fixation (CF) test for EHV-1, EHV-4, ERAV, ERBV and an indirect enzyme-linked immunosorbent assay (iELISA) for S. equi . Serum samples from seventy eight out of 94 horses (83%) generated zones of over 85 mm 2 in the SRH test, which classified them as clinically protected against EIV (H3N8). The most sero-prevalent agent detected was EHV-4 (30.9%, 29/94), followed by EHV-1 (9.6%, 9/94), S. equi (2.1%, 2/94), ERAV (1.1%, 1/94) and ERBV (1.1%, 1/94). All horses showed no visual clinical signs. The present study showed that the seroprevalence of infectious respiratory agents was relatively low and provides evidence of low risk of respiratory infectious agents in Thoroughbred race horses at SRP. the SRP of management horse EIV, EHV-1, ERAV, ERBV, S. equi be by direct horse-to-horse contact aerosolized of secretions (17-19). of EIV, EHV-1, can severe and economic damage to the (20). important for the possible and of at respiratory disease, including nasal discharge, coughing and pyrexia. An examination for lameness was also conducted for all horses and only clinically healthy horses were allowed to participate in races. Only physically sound horses without signs of infectious respiratory disease were included in this study. Serum was separated by centrifugation, heat treated at 56°C for 30 minutes and stored at -20°C until use. The race results on race days were investigated. The race results were divided dual antigen A & C iELISA test was performed using the commercial Strangles ELISA kit (AHT-SEE-1/3/5, AHT, UK) as described by the manufacturer and Robinson et al . in 2013 (11). The normalized mean OD 450nm value ≥ 0.5 was classed as positive. Antibodies against H3N8 strain A/eq/Richmond/1/2007 cultured was measured using the SRH test according to the OIE terrestrial manual 2019 (4). Briefly, virus was coupled to sheep red blood cells (SRBC) with chromium chloride. Agarose plates were made with the sensitized SRBCs and guinea pig complement. Ten microlitres of 56°C heat-inactivated serum was aliquoted into 3 mm wells on the plate and incubated at 34°C for 20-24 hours. Control antiserum against EIV subtype 2 American-like strain A/eq/Richmond/1/2007 from the European Directorate for the Quality of Medicines and Healthcare (Strasbourg, France) was included on each plate as a control. The diameters of the resultant zones of haemolysis were measured with a calibrated viewer. SRH antibody levels were expressed as the area of haemolysis in millimetres squared. The results were interpreted referring to Gildea et al . (22); samples with haemolytic areas of 150 mm 2 or greater were categorized as virologically protected, samples with antibody levels less than 150 mm 2 , but greater than 85 mm 2 were categorized as clinically protected, samples with antibody levels less than 85 mm 2 , but greater than 50 mm 2 were categorized as partially protected and samples with antibody levels less than 50 mm 2 were categorized as susceptible. pathogen, with 30.9% seropositive horses, and both ERAV and ERBV were the least prevalent at 1.1% in this study. A trend towards reduced performance was found in EHV-1 seropositive racehorses in this study. Further studies are required to confirm the relationship between the EHV-1 infection and sub-optimal athletic performance in racehorses.