The World Health Organisation is still stumped over the cause of the string of "puzzling" deaths in Chiang Mai.

Dr Maureen Birmingham, the WHO representative in Thailand, said a number of laboratory tests - both inside and outside the kingdom - still needed to be done on the patients' bodies, food and environment to find out what killed them.

"As frustrating as it is, a single common link or cause has yet to be clearly identified, although many specimens are still undergoing analysis. All possibilities are being considered, including something infectious or toxic."

Dr Birmingham said she was confident an "exhaustive" investigation was being carried out and there was no clear evidence the cause of the deaths was within the Downtown Inn.

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"We have no basis to challenge the decision of the local authorities to keep the hotel open."

Echovirus was detected on a swab in one case and coxsackie virus in an antibody test in another, but further tests were needed as antibodies in the blood could be related to an infection many years prior, she said.

If echovirus caused the deaths, infection probably occurred outside Chiang Mai because most people became sick within 24 hours of arriving in the city, and the virus normally took longer to incubate.

Wellington-based virologist Dr Sue Huang, head of the WHO polio virus reference laboratory, said echovirus and coxsackie virus belonged to the enterovirus group - its most famous member was polio.

Enteroviruses are spread through food, touching an infected person, using infected facilities such as swimming pools, or even touching objects such as door handles.

While they are most common in Third World countries, in 1999 six people in Wairarapa became ill with symptoms of myocarditis - the same heart inflammation that killed Sarah Carter.

Tests eventually identified the coxsackie B virus as the cause.

Dr Huang said most people who contracted an enterovirus showed no symptoms, with only about 10 per cent becoming ill with symptoms ranging from mild to severe.

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"It's a really big spectrum, only a very, very small percentage will have a severe outcome like myocarditis."

Dr Robert Doughty, soon to take up the inaugural Heart Foundation chair in heart health at Auckland University, said that while myocarditis was normally caused by a viral infection, it could be very difficult to determine the cause.

That was sometimes done putting a cardiac biopsy under a microscope.

"Even then, identification of the virus in the heart muscle itself can be difficult and take a long time," Dr Doughty said.

While the irritation could be directly caused by the virus or infection, in most cases the cells the body produced to fight the infection actually damaged the heart they had been released to protect, he said.

The inflammation could cause problems with the heart pump function, which lead to breathlessness and heart failure. It could also cause sudden death when the heart suffered rhythm problems.