Melioidosis can manifest as conditions such as a localised infection, pneumonia, meningoencephalitis and sepsis, which is an extreme response to infections where the immune system starts to attack healthy tissue. Depending on the location of infection, symptoms include fever, headache, cough, chest pain, ulceration and localised pain or swelling.

The incubation period commonly ranges from two to four weeks. The fatality rate is between 40 and 75 per cent. It can be treated with antibiotics, but there is no vaccine targeting the bacteria.

People with underlying diseases including diabetes, lung disease, liver disease, renal disease, cancer, or immunosuppression are more likely to be infected. Workers in agricultural, laboratory and healthcare are also susceptible to exposure.

The disease is considered endemic in Southeast Asia and northern Australia. It can also affect animals such as sheep, goats and pigs.

2. How is the disease affecting Hong Kong?

An abnormal number of melioidosis cases, 34 so far, have been recorded in Hong Kong this year, with 23 found in the Sham Shui Po district. Seven deaths were reported.

The city logged 17 cases last year. An average of 10 cases have been detected each year over the past five years, according to official statistics.

According to the Centre for Health Protection (CHP), between August and October, 20 people, of which 16 were aged 60 or above, living in 19 blocks across Sham Shui Po were infected. They have no contact history with each other. The buildings are all located within a 1km radius of the Lee Cheng Uk and Pak Tin estates.

Most of the patients from the Kowloon cluster over the past three months were immunocompromised because of underlying diseases, according to the centre.

Among the 26 workers from a construction site in Pak Tin estates who had chronic illnesses and were selected to undergo a test by the authorities, eight males and two females aged 39 to 66, all asymptomatic, were found to have antibodies against melioidosis bacteria on Saturday.

It was believed that they might have had an asymptomatic infection in the past after being exposed to soil or water contaminated with the bacterium.

3. Have the authorities found the cause of the outbreak?

On October 21, the CHP revealed that DNA traces of the bacterium were found in the four soil samples from the construction site at Pak Tin Estate, but it could not be reproduced in a laboratory because of its low bacterial load. None of the infected people known at the time had worked, entered or walked near the area.

After finding positive swab samples obtained from a water pipe and the surface of a flowerpot at a patient’s home in Sham Shui Po, health authorities last Tuesday inspected the Shek Kip Mei and Butterfly Valley reservoirs in the district with University of Hong Kong microbiologist Yuen Kwok-yung.

Although no positive readings were reported during the inspection, Dr Albert Au Ka-wing, CHP’s principal medical and health officer, said one direction of the investigation was whether the patients had drunk or come into contact with contaminated water.

Considering another possibility of the spread, Yuen’s team had also brought some cheap face masks from the district to see if they were contaminated.

Infectious diseases expert Dr Wilson Lam previously said wind and rain might have brought the bacteria to the air, which was then breathed in by the patients.

4. What did the government do to contain the spread?

Health authorities announced a plan to make melioidosis a statutory notifiable disease, meaning doctors would be required to report cases to officials.

The Water Supplies Department said last week it had increased chlorine levels at the Sha Tin Water Treatment Plant facility, the largest in the city, which services Sha Tin, Kowloon Central and part of Hong Kong Island, to kill the bacteria that might have caused the outbreak. It was also considering Yuen’s suggestion of installing UV light to eliminate more bacteria.

Calcium hydroxide, which can be used as a disinfectant of the bacterium, was also added to the soil at the construction site at Pak Tin Estate to protect workers and those living nearby.

5. Should we be worried?

Respiratory disease specialist Dr Leung Chi-chiu said the chance of having a major outbreak was small as the disease was rarely transmitted from human to human.

As 38 per cent of the workers tested were found to be infected, he believed that the contamination at the construction site was severe, adding that the 10 workers were unlikely to develop symptoms unless they were severely immunocompromised.

He said it could not be ruled out that residents at the public housing estate were also heavily exposed, adding that the government should test high-risk residents to look at the infection rate.

Citing previous studies, infectious diseases expert Lam noted that only a small number of people who come in contact with the bacterium causing melioidosis will suffer an infection.

He said most patients were usually older, and there was no need to panic for healthy people with a normal level of immunity as they would not catch the disease after contact with low levels of the bacteria.

He said the best way for people to protect themselves was to boost their immune systems.

6. How should we protect ourselves?

Infectious diseases expert Dr Wilson Lam warned all residents, regardless of their medical condition, against using or consuming unboiled water found near soil that could be contaminated.

He said the public should also avoid using such water for other purposes such as washing, rinsing mouths or brushing teeth.

He added that patients with chronic illnesses, especially diabetes, should be “extra careful”.

Yuen also suggested the elderly as well as those with chronic illnesses run the tap for a minute first before using the water to brush their teeth, shower or drink.

According to the CHP, the public can also take precautions such as wearing gloves and boots during activities that could involve contact with soil or water, washing or showering after exposure to possibly contaminated water or soil, cleaning wounds as soon as possible, covering cuts or grazes with waterproof dressings and washing hands with liquid soap after handling soils.

Additional reporting by Harvey Kong