Oral sex is producing dangerous gonorrhoea and a decline in condom use is helping it to spread, the World Health Organization has said.

It warns that if someone contracts gonorrhoea, it is now much harder to treat, and in some cases impossible.

The sexually transmitted infection is rapidly developing resistance to antibiotics.

Experts said the situation was "fairly grim" with few new drugs on the horizon.

About 78 million people pick up the STI each year and it can cause infertility.

The World Health Organization analysed data from 77 countries which showed gonorrhoea's resistance to antibiotics was widespread.

Dr Teodora Wi, from the WHO, said there had even been three cases - in Japan, France and Spain - where the infection was completely untreatable.

She said: "Gonorrhoea is a very smart bug, every time you introduce a new class of antibiotics to treat gonorrhoea, the bug becomes resistant."

Worryingly, the vast majority of gonorrhoea infections are in poor countries where resistance is harder to detect.

"These cases may just be the tip of the iceberg," she added.

Throat infection

Gonorrhoea can infect the genitals, rectum and throat, but it is the last that is most concerning health officials.

Dr Wi said antibiotics could lead to bacteria in the back of the throat, including relatives of gonorrhoea, developing resistance.

She said: "When you use antibiotics to treat infections like a normal sore throat, this mixes with the Neisseria species in your throat and this results in resistance."

Thrusting gonorrhoea bacteria into this environment through oral sex can lead to super-gonorrhoea.

"In the US, resistance [to an antibiotic] came from men having sex with men because of pharyngeal infection," she added.

A decline in condom use, which had soared because of fears of HIV/Aids, is thought to help the infection spread.

What is gonorrhoea?

Image copyright CAVALLINI JAMES/SCIENCE PHOTO LIBRARY

The disease is caused by the bacterium called Neisseria gonorrhoea.

The infection is spread by unprotected vaginal, oral and anal sex.

Symptoms can include a thick green or yellow discharge from sexual organs, pain when urinating and bleeding between periods.

However, of those infected, about one in 10 heterosexual men and more than three-quarters of women, and gay men, have no easily recognisable symptoms.

Untreated infection can lead to infertility, pelvic inflammatory disease and can be passed on to a child during pregnancy.

The World Health Organization is calling on countries to monitor the spread of resistant gonorrhoea and to invest in new drugs.

• BBC Radio 1 - Myth-busting STIs
• Pubic hair grooming 'STI risk from cuts'

Dr Manica Balasegaram, from the Global Antibiotic Research and Development Partnership, said: "The situation is fairly grim.

"There are only three drug candidates in the entire drug [development] pipeline and no guarantee any will make it out."

But ultimately, the WHO said vaccines would be needed to stop gonorrhoea.

Prof Richard Stabler, from the London School of Hygiene & Tropical Medicine, said: "Ever since the introduction of penicillin, hailed as a reliable and quick cure, gonorrhoea has developed resistance to all therapeutic antibiotics.

"In the past 15 years therapy has had to change three times following increasing rates of resistance worldwide.

"We are now at a point where we are using the drugs of last resort, but there are worrying signs as treatment failure due to resistant strains has been documented."

Follow James on Twitter.

Is oral sex more common now? By BBC World online

It's hard to say if more people around the world are having more oral sex than they used to, as there isn't much reliable global data available.

Data from the UK and US show it's very common, and has been for years, including among teenagers.

The UK's first National Survey of Sexual Attitudes and Lifestyles, carried out in 1990-1991, found 69.7% of men and 65.6% of women had given oral sex to, or received it from, a partner of the opposite sex in the previous year.

By the time of the second survey during 1999-2001, this had increased to 77.9% for men and 76.8% for women, but hasn't changed much since.

A national survey in the US, meanwhile, has found about two-thirds of 15-24 year olds have ever had oral sex.

Dr Mark Lawton from the British Association for Sexual Health and HIV said people with gonorrhoea in the throat would be unlikely to realise it and thus be more likely to pass it on via oral sex.

He recognises that while condoms would reduce the risk of transmission, many people wouldn't want to use them.

"My message would be to get tested so at least if you've got it you know about it," Dr Lawton said.