A leading microbiologist on Tuesday suggested that the authorities increase the amount of residual chlorine in drinking water and use ultraviolet light for disinfection to help kill the bacteria that causes the potentially deadly disease melioidosis.

Yuen Kwok-yung from the University of Hong Kong made the recommendations after inspecting two service reservoirs – the Shek Kip Mei Service Reservoir and the Butterfly Valley Fresh Water Primary Service Reservoir – near Sham Shui Po district, where most recent cases of the bacterial infections have been found.

Hong Kong has recorded 31 melioidosis cases this year. Seven patients have died, all of whom had pre-existing conditions or were immunocompromised.

Yuen said medical experts had yet to ascertain the cause of the outbreak, adding that it is important to put in place measures to protect drinking water to reduce the risk of more cases.

He recommended that the Water Supplies Department raise the level of free residual chlorine in drinking water from one part per million (ppm) to 1.2 ppm. He said this would ensure that the chemical can kill the bacteria by the time water gets to people's homes from treatment facilities.

"Especially after one night, the amount of free residual chlorine remaining in the water will decrease," Yuen said.

"So I again urge the elderly and those who are chronically ill to, in the morning, avoid using the water that comes out within the first minute of turning on the tap for drinking, washing your face, brushing your teeth, or showering, because you might inhale droplets of water that contain the bacteria."

In the long run, Yuen added, ultraviolet light should be used for disinfection at service reservoirs. He said he also expected melioidosis infections to wane as the weather turns dry and cool.

Melioidosis is transmitted through muddy water and soil, though human-to-human and person-to-person transmission is rare. The Centre for Health Protection says the mortality rate can be between 40 and 75 percent, although it is treatable with antibiotics.