"We cannot say for sure the virus has mutated, but we suspect it is the cause of the considerably more-severe symptoms seen in some patients," said Dr Tawee Chotpitayasunondh, director of Queen Sirikit National Institute of Child Health. The ministry is studying the enterovirus 71, which causes hand, foot and mouth disease, to determine whether it is a mutated form or not, said Dr Kumnuan Ungchusak, the head of the Bureau of Epidemiology.

The ministry will have to compare the genetic profile of the strain found in recently infected Thai victims with those from cases in countries like Taiwan, Singapore and Malaysia, said the doctor.

"It is not just in Thailand. Enterovirus 71 seems to be becoming more virulent in those countries," Kumnuan said.

For example, during a 1998 outbreak of hand, foot and mouth disease in Taiwan, up to 300,000 people were infected, and 78 died, Kumnuan said.

He added that medical officials suspected then a mutation was to blame for the severity, but they were unable to confirm it.

"It was still the same subtype B and C, but the clade [a group of biological species that comes from a common ancestor] of these subtypes may have changed. Further study is needed to be sure," said Kumnuan.

Four people died in Thailand this year of common hand, foot and mouth disease.

Nine people died of the severe type of the disease. In these cases the patients' symptoms were unclear. They suffered and died of complications, said Kumnuan.

Most of these cases were children younger than five years old, said the doctor. If the ministry's study confirms that the enterovirus 71 has mutated, the government will have to revise its disease-control measures, its method of diagnosis and the treatment, said Tawee.

Public Health Minister Pinij Charusombat said that 1,360 cases of hand, foot and mouth disease had been reported this year.

Four people have died of the disease. There were close to 300 cases of the disease reported in Bangkok in the past year.

In the past week, at least nine schools in Bangkok have been temporarily shut down because of the outbreak, said Pinij. He said a rapid response unit is on call around the clock to control the disease.

If someone is infected, he or she must not go to school. If more than five cases of the disease are reported at the same school, that school would be shut down until the disease was contained, said Pinij.

Arthit Khwankhom

The Nation