Health authorities yesterday confirmed the city's first case of Japanese encephalitis this year.
A 26-year-old woman living on Tin Heng Estate in Tin Shui Wai is in stable condition in hospital, where she has been since June 10. She had developed a fever, headaches and vomiting the previous day.
A blood test on June 10 came back negative for Japanese encephalitis but the woman was confirmed with the virus yesterday after a second test on Sunday.
The woman had not travelled out of Hong Kong recently, said Dr Leung Ting-hung, controller of the Department of Health's Centre for Health Protection.
The mosquito-borne virus is often found in pigs and wild water birds, Leung said.
He added there were many grassy areas, ponds and marshes around Tin Shui Wai which attracted mosquitoes, and four pig farms were located within two kilometres of the woman's home.
The virus is not transmittable between humans, he said.
"We immediately began assessing the area - visiting residents in their homes and conducting questionnaires in the neighbourhood," Leung said.
There were six reported cases of Japanese encephalitis last year - two local cases, three involving people who had picked up the virus overseas, and one in which the source was unknown.
Three cases were reported in 2012 - one local and two from overseas.
Both of the local cases last year were people living in Tin Shui Wai.
Fok Ming-yan, pest control officer with the Food and Environmental Hygiene Department, said its staff would carry out more frequent patrols of piggeries in Yuen Long, especially during the rainy season, and would provide more information to farm managers.
She said officers would clear breeding grounds for mosquitoes in the area, such as ponds, and use pesticide and mosquitofish - a type of fish that eats mosquitoes - as necessary.