Equestrian events were switched from host city Beijing because of difficulties in establishing a disease-free zone in mainland China, and Hong Kong's leader tried to quash any concerns the outbreak may have triggered.

"Horses (are) like human beings. They get sick, they get cured. I do not think we should be fussed ... it will not be an issue by the time we hold the Olympics in Hong Kong," chief executive Donald Tsang told reporters after a tour of Olympic facilities in Beijing.

A total of 132 horses fell ill with the disease between February 10 and April 9 but containment measures at the club in Shatin in the New Territories appeared to be working and there was no obvious threat of the outbreak worsening.

"The affected horses have shown a mild fever and occasionally, mild blood abnormalities and mild swelling of the legs," the Hong Kong Jockey Club said in a statement.

"Most horses have returned to training within a week without complications that may affect the horse's health or future racing performances," it added, blaming the outbreak on the climate.

"The recent changeable weather conditions have probably contributed to the recent cases."

The viral outbreak comes as Hong Kong police investigate a device that was found embedded in the turf of the Happy Valley racecourse in late March.

The mechanism, which was uncovered during a routine check, was designed to shoot poison darts into horses at the start of a race. The Beijing Olympics equestrian chief in Hong Kong quickly pledged that security would be tight for the 2008 Games.