The cause of mild fevers in 132 horses at Hong Kong’s Sha Tin Racecourse is still under investigation, according to Brian Stewart, BVSc (hons), MBA, head of Veterinary Regulation and international liaison for the Hong Kong Jockey Club. The Associated Press reported earlier this week that the fevers were caused by equine herpesvirus (EHV).

Stewart said Club veterinarians have seen an average of two or three horses with very mild fevers accompanied by occasional raised monocyte (monocytes are a particular type of white blood cells found in the circulation that convert into active macrophages, specialized white blood cells that fight infection, when they enter the tissue) counts occurring daily over the last few months (since Feb. 10). No other clinical signs or neurological complications have been reported. The horses have been recovering quickly and generally are able to return to full training within a week.

Temperatures are recorded twice daily for all horses at Sha Tin, and all horses are required to be vaccinated for EHV, Japanese B encephalitis, and equine influenza twice yearly.

Veterinarians have been taking test samples from all febrile horses. These tests include nasopharyngeal swabs collected to test for influenza and EHV, and for culture. Stewart said some of the horses have tested positive for EHV on polymerase chain reaction (PCR) tests and an increase in EHV antibodies has been noted in paired serology. Further tests (full serology and genetic typing of the EHV strain) are being conducted at a laboratory in the United Kingdom