Scientists have found a "superbug" strain of gonorrhoea in Japan that is resistant to all recommended antibiotics and say it could transform a once easily treatable infection into a global public health threat.

The new strain of the sexually transmitted disease -- called H041 -- cannot be killed by any currently recommended treatments for gonorrhoea, leaving doctors with no other option than to try medicines so far untested against the disease.

Magnus Unemo of the Swedish Reference Laboratory for Pathogenic Neisseria, who discovered the strain with colleagues from Japan in samples from Kyoto, described it as both "alarming" and "predictable".

"Since antibiotics became the standard treatment for gonorrhoea in the 1940s, this bacterium has shown a remarkable capacity to develop resistance mechanisms to all drugs introduced to control it," he said.

In a telephone interview Unemo, who will present details of the finding at a conference of the International Society for Sexually Transmitted Disease Research (ISSTDR) in Quebec, Canada on Monday, said the fact that the strain had been found first in Japan also followed an alarming pattern.     

"Japan has historically been the place for the first emergence and subsequent global spread of different types of resistance in gonorrhoea," he said.   

The team's analysis of the strain found it was extremely resistant to all cephalosporin-class antibiotics -- the last remaining drugs still effective in treating gonorrhoea. 

Gonorrhoea is a bacterial sexually transmitted infection and if left untreated can lead to pelvic inflammatory disease, ectopic pregnancy and infertility in women.

It is one of the most common sexually transmitted diseases in the world and is most prevalent in south and southeast Asia and sub-Saharan Africa. In the United States alone, according to the Centers for Disease Control and Prevention (CDC), the number of cases is estimated at around 700,000 a year.

British scientists said last year that there was a real risk of gonorrhoea becoming a superbug -- a bacteria that has mutated and become resistant to multiple classes of antibiotics -- after increasing reports of gonorrhoea drug resistance emerged in Hong Kong, China, Australia and other parts of Asia.

Experts say the best way to reduce the risk of even greater resistance developing -- beyond the urgent need to develop effective new drugs -- is to treat gonorrhoea with combinations of two or more types of antibiotic at the same time.

This technique is used in the treatment of some other diseases like tuberculosis in an attempt to make it more difficult for the bacteria to learn how to conquer the drugs.

Unemo said however that experience from previous degrees of resistance acquired by gonorrhoea suggested this new multi-drug resistant strain could spread around the world within decades.

"Based on the historical data ... resistance has emerged and spread internationally within 10 to 20 years," he said.

Asked whether a class of drugs called carbapenems -- known as the most powerful antibiotics yet devised -- might be a last ditch option for treating this new gonorrhoea strain, Unemo said there would first need to be trials to assess their potential.   

"Carbapenems have never been used for the treatment of gonorrhoea so we cannot interpret the data in any reliable or quality-assured way at the moment," he said.

The World Health Organisation estimates there are at least 340 million new cases of curable sexually transmitted infections -- including syphilis, gonorrhoea, chlamydia and trichomoniasis -- every year among people aged 15 to 49. - Reuters