THE revelation of a sixth death and the results of an investigation by Thailand's Department of Disease Control has shed new light on the series of mysterious fatalities in Chiang Mai.

The sixth death involved a Frenchwoman - one of two who fell sick, a media release from the department says. It highlighted the exceptional nature of a complaint that appears to have struck down six young women aged 23-33 between January 9 and February 4, killing three of them.

These three deaths - the unnamed Frenchwoman, New Zealander Sarah Carter, 23, and American Mariam Soraya Vorster (Pandola), 33 - have been linked by the media to three other deaths, British pensioners George and Eileen Everitt, and Thai guide Waraporn Pungmahisiranon, 47.

The joint investigation by the department with the Chiang Mai Provincial Health Office ''found four clinically-confirmed cases of myocarditis (inflammation of the heart muscle) and another two patients with mild symptoms.

''These six patients were among three separate groups of tourists visiting Chiang Mai, Thailand, between 9 January and 4 February. All were young women aged 23-33 years and were from the United States (1), Canada (1), France (2) and New Zealand (3).''

Five of the six women ''became ill while visiting Chiang Mai and one developed symptoms three days before arriving there.''

However, extensive epidemiological investigation ''has not revealed any common exposures across the three groups.''

The media release says autopsies on two of the dead patients (American and French) were done by forensic medical experts from Chiang Mai University.

The joint investigation team found four clinically-confirmed cases of myocarditis (inflammation of the heart muscle) and another two patients with mild symptoms.

''The autopsies found nothing abnormal except for inflammation of the heart muscle,'' it says.

''The Department of Disease Control has shared this information with the World Health Organisation and US CDC offices in Bangkok, as well as the New Zealand Embassy and the International Health Regulation focal points of France and the European Community.''

The report carefully distinguishes between the cases involving the six young women and the other cases.

''In a separate episode, since 3 February, there were three other deaths in the same hotel where the three New Zealanders stayed,'' it says.

''This included an elderly British couple and a 47 year old Thai woman.

''As these three deaths occurred outside the hospital, the police took charge of the investigation.

''The autopsies of the two elderly Britons found a high degree of coronary occlusion while the examination of the Thai woman found no inflammation of the heart muscle or any other clear evidence to explain the cause of her death.''

One recent report on the Chiang Mai cases also claimed there had been 50 ''unexplained deaths'' among expats on Phuket in an eight-month period last year.

Germany's honorary consul for Phuket, Dirk Naumann, said today: ''That's absolute rubbish. If it was true, all the honorary consuls would have been up in arms about it.''

Neither of the online sites that carried the misleading article - Asia Sentinel or Irrawaddy - has responded to requests that they correct the falsehood.

Two young women - Norwegian Julie Bergheim, 22, and American Jill St Onge, 27 - died within hours of falling sick at a guesthouse on the holiday island of Phi Phi, off Phuket, in May 2009 in a baffling case that failed to produce a cause for their deaths.