(Reuters) - Drug-resistant strains of gonorrhoea have spread to countries across the world, the United Nations health agency said on Wednesday, and millions of patients may run out of treatment options unless doctors catch and treat cases earlier.

Scientists reported last year finding a superbug strain of gonorrhoea in Japan in 2008 that was resistant to all recommended antibiotics and warned then that it could transform a once easily treatable infections into a global health threat.

The World Health Organisation (WHO) said those fears are now reality with many more countries, including Australia, France, Norway, Sweden and Britain, reporting cases of the sexually transmitted disease resistant to cephalosporin antibiotics - normally the last option for drugs against gonorrhoea.

Gonorrhoea is becoming a major public health challenge, said Manjula Lusti-Narasimhan, from the WHO's department of reproductive health and research. She said more than 106 million people are newly infected with the disease every year.

The organism is what we term a superbug - it has developed resistance to virtually every class of antibiotics that exists, she told a briefing in Geneva. If gonococcal infections become untreatable, the health implications are significant.

Gonorrhoea is a bacterial sexually transmitted infection which, if left untreated, can lead to pelvic inflammatory disease, ectopic pregnancy, stillbirths, severe eye infections in babies, and infertility in both men and 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.

The WHO called for greater vigilance on the correct use of antibiotics and more research into alternative treatments for so-called gonococcal infections.

The emergence of drug-resistant or superbug strains of gonorrhoea is caused by unregulated access to and overuse of antibiotics, which helps fuel natural genetic mutations within the bacteria.

Experts say an added problem with gonorrhoea is that its strains tend to retain their genetic resistance to previous antibiotics even after their use has been discontinued.

Major producers of antibiotics for gonorrhoea include global drugmaking giants GlaxoSmithKline, Pfizer and Abbott, as well as Indian firms like Cipla.

The WHO said it is not yet clear how far or wide drug resistance in gonorrhoea has spread, as many countries lack reliable data. The available data only shows the tip of the iceberg, said Lusti-Narasimhan.

Without adequate surveillance we won't know the extent of resistance...and without research into new antimicrobial agents there could soon be no effective treatment for patients.

LIKE PASSING RAZOR BLADES

Francis Ndowa, formerly the WHO's lead specialist for sexually transmitted infections, said gonorrhoea has not only adapted to elude antibiotics but developed less painful symptoms, increasing its survival chances.

They used to say that if you have urethral gonorrhoea you go to the toilet to pass urine, it would be like passing razor blades. It was that painful, he explained. Now people with gonorrhoea sometimes...only notice the discharge if they look when they pass urine, it's not that painful anymore.

So the organism has readjusted itself to provide fewer symptoms so that it can survive longer. It's an amazing interaction between man and pathogen.

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 infections like tuberculosis in an attempt to make it more difficult for the bacteria to learn how to conquer the drugs.

Gonorrhoea can be prevented through safer sexual intercourse. The WHO said early detection and prompt treatment, including of sexual partners, is essential to control sexually transmitted infections.

(Additional reporting by Stephanie Nebehay in Geneva, editing by Alessandra Rizzo and Mark Heinrich)

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