Melioidosis has claimed six lives this year among 29 cases reported, including 20 from Sham Shui Po.

The Centre for Health Protection said Yuen's team from Hong Kong University collected 31 household samples of patients living in the area.

Two samples collected from the same unit - a vase and the other bathroom tap water - had tested positive of the bacterium Burkholderia pseudomallei, the pathogen of melioidosis.

"The center has learned that the HKU nucleic acid tests are extremely sensitive and the two samples were found with an extremely low viral count, which could be residual bacteria, or environmental contamination by the patient," it said.

The center said it collected 107 samples from 16 buildings where patients lived. The rest came back negative.

Although there is no evidence pointing to the source of tap water contamination, the center has asked the water supplies department to check records over the past three months.

"After the review, the department had not found any reports of contamination in the water supply system in the last three months, while tests in water samples from relevant reservoirs have been normal and complied with standards," it said.

Since the announcement, the center has not received reports of new infections, while private hospitals are reminded to stay alert.

The Department of Health held a seminar on prevention of melioidosis.

Senior medical officer Gladys Yeung Chin-chin said people need not worry about transmission of melioidosis, because they are very rare and "almost impossible."

She added that the three most common transmission paths are through contaminated oil and water; inhalation of contaminated dust or water droplets; or consumption of contaminated water.

Yeung called on people to avoid contact with mud, especially after the latest typhoon Nesat, as wind could bring the bacterium to the surface.

She said common symptoms of melioidosis are fever, headache, chest pain, coughs and swelling of regional lymph nodes. The symptoms are similar to that of flu.