Seventeen horses from the stables of three Hong Kong trainers have tested positive for the banned drug zilpaterol, a substance used to produce muscle growth in livestock, the Hong Kong Jockey Club said on Wednesday.
Thirteen of the horses have been withdrawn from Wednesday's public holiday races following testing that began earlier this week, the city's racing authority said.
"Seventeen horses have been found to have the substance," Hong Kong Jockey Club spokeswoman Carmen Lok told AFP.
The discovery follows a series of doping scandals in Britain involving identities including Godolphin trainer Mahmood al-Zarooni who was handed an eight-year ban in April for using prohibited steroids on 11 horses.
The Jockey Club said that 12 of the horses were trained by Ricky Yiu, and the South China Morning Post daily said the remainder were trained by Paul O'Sullivan and Me Tsui Yu-sak.
The Jockey Club announced the initial positive tests on the Yiu-trained horses in a statement on Tuesday and said tests are under way to identify the source of the substance and determine if feed products had been contaminated.
"Certain registered feed products used by Mr Yiu are also used by several trainers in Hong Kong," it said.
Zilpaterol is used to increase mass and muscle in livestock but it is also used as a doping agent in racing, where side effects on horses include profuse sweating, muscle tremors and a dramatic increase in heart rate.
Merck, the substance's manufacturer, said it is only to be used in feed for cattle and advises against administering it to horses.
In the latest doping case to hit British racing, Gerard Butler, a trainer from the sport's Newmarket headquarters was last week charged with giving anabolic steroids to horses. He could face up to a 10-year ban if found guilty.