The nets have been found to be the most effective measure to fight the disease, especially in Tak, where more than 20,000 nets have been distributed over the past three years.

Dr Pajjuban Hamehongsa, chief of the Provincial Public Health Office in Tak, said the chemical soaked or sprayed on the nets was harmless to humans, but could instantly kill or "de-sexualise" mosquitoes.

"Families that use the nets no longer suffer from malaria," he said, adding that his office was trying to encourage villagers to treat their nets with repellent themselves.

Local nets were effective for only three years, unlike those imported from Japan, which worked efficiently for five years.

Tak province is heavily infested with mosquitoes and has a high rate of malaria. Some 12,784 people caught the viral disease in the first four months of this year, compared to 80,122 in the whole of 1999, a year when the incidence was very high.

Meanwhile, three people are reported to have died of dengue fever, while the number of those stricken by the potentially deadly disease is on the rise across the country.

A 37-year-old woman died late on Tuesday night in Phang Nga, three days after being admitted to Takua Pa Hospital. Doctors said the victim had earlier taken cold tablets, enabling the virus to "hide" before it reached a critical stage.

A male dengue victim died recently at a Khon Kaen hospital. The man, whose name and age were not disclosed, was admitted late last month after becoming unconscious with a high fever.

In Phang Nga, 78 people are undergoing hospital treatment for dengue fever. "Most are in a condition that requires special care," provincial public health chief, Dr Jessada Chaikhunrat, said.

In Khon Kaen, 200 people had sought hospital treatment and 40 of them were in a serious condition, said Dr Sirijit Wassanawat, a senior doctor at Khon Kaen Hospital.

In Prachin Buri, 119 people have been admitted to hospital for treatment for dengue fever, and one of them has died.