Smuggled pigs and illegal slaughtering are believed to be the source of the latest food poisoning case.
Four members of two Tseung Kwan O families fell ill last week after having pig lung soup, with organs bought from a nearby market stall.
It was the first time pig offal had been found contaminated with the banned anti-asthma drug clenbuterol since a new quarantine system was put in place eight months ago.
Director of Agriculture and Fisheries Lessie Wei Chui Kit-yee yesterday said the quarantine system was effective and the latest case was isolated.
The Government would not repeat last year's pig offal ban.