Authorities in Thailand say they are still not certain what caused the death of six tourists in the northern city of Chiang Mai but suggest some may have died from being exposed to pesticide or other toxic chemicals.

The deaths include that of a Thai tourist guide, two tourists from Britain, one tourist from France, one tourist from New Zealand and one from the United States. The tourists were staying in three different hotels in the city during January and February 2011. Three other tourists also fell ill but recovered.

The latest news comes from a posting on the Thai government’s Department of Disease Control website on Tuesday, that said the authorities have have conducted an investigation into the mysterious deaths but the results were inconclusive, and the “specific agents that caused the deaths and illnesses in these events cannot be identified”. It also says the authorities cannot establish exactly how the victims came to be exposed to them.

The American tourist who died was a 33-year-old woman. She died on 11 January from injury to the heart muscle. The website report suggests here death was probably caused by a “chemical or biotoxin” pesticide and that lab tests ruled out drugs and viral causes. Her travelling companion, a 29-year-old woman from Canada, also got sick but recovered. Both were staying in the same hotel room at one of the three hotels.

A 25-year-old French woman died 19 January. In her case, the investigation concluded her death was unrelated to the others. They found she had died of severe myocarditis (inflammation of all the heart muscles), most likely due to a viral infection rather than exposure to a poison. She had checked into a second of the three hotels, and a female travelling companion who checked in at the same time did not get sick.

Most of the evidence so far concerns four women: a 47-year-old from Thailand, who was found dead on 3 February, a 23-year- old from New Zealand, who died on 6 February and her two friends, also 23 years old, who recovered.

The three New Zealand women fell ill on 3 February and were found to have developed severe metabolic acidosis (abnormal acid level in circulation) and two of them had injury to the heart muscle. The Thai woman probably died from sudden arrhythmia, said the report. All four women were staying in the same hotel, the third of three investigated.

The investigators suggest the cause of the sickness in the four women was probably the same, given the proximity of their rooms, and it is unlikely to be bacterial or viral and may be “some toxic chemical, pesticides or gas”. They ruled out pesticides in the “organophosphate, organochlorine and carbamate group, such as cholpyrifos, are also unlikely to be the cause because they conflict with clinical specimens and blood test”.

Tests also ruled out other chemicals, such as “sodium monofluoroacetate (compound 1080) and phosphine gas”, but the report also emphasizes that negative results do not always confirm absence, as chemicals can dissipate before the tests are done. The investigators also found aluminium traces in some of the rooms and elsewhere in the hotel, but no evidence it came from the pesticide aluminum phosphide.

The two British deaths were of a couple in their 70s. They were found dead in their hotel room on 19 February. They were staying in the same hotel as the Thai woman and the women from New Zealand.

The Thai authorities sent chemical and tissue samples to laboratories in Thailand, the United States, Japan and Germany during the course of their investigation. The investigation included interviews with other tourists travelling with the victims, and some of their family members.

The findings were reviewed by outside infectious disease experts, toxicologists, pathologists, forensics experts, epidemiologists and specialists in environmental science and pesticides.

The report concludes that:

“Despite the best efforts of the Thai authorities and their international partners in undertaking an exhaustive investigation, the specific agents that caused the deaths and illnesses in these events can not be identified and it can not be determined exactly how people were exposed to them.”

However, the Thai authorities said they are taking measures to reduce future risks to visitors to Chiang Mai, that will also apply to other provinces in Thailand. These include:

  • A panel will be set up to investigate and recommend stricter controls on use of chemicals and pesticides in hotels and markets.
  • A special channel for advising on illness of tourists and expatriates: these will be verified and investigated.
  • Already in effect, is the surveillance of tourists admitted to hospitals, and new protocols for investigating fatal cases.
  • Shops selling household and agricultural pesticides must declare if they sell items on a watchlist of products, and monitor who buys them. They will be checked periodically.
  • Hotels must only use licensed pest control operators and specify in contracts which chemicals to use, and send samples for testing twice a year.
  • Local authorities to give advice on safety and monitor use of ritual papers and other materials during festivals and other community, traditional and religious activities.
  • Developing food safety standards for eateries, street vendors, especially in areas in and around the Night Bazaar, which is popular with tourists.
  • Health advice cards for tourists on food safety and other measures.

Written by Catharine Paddock PhD