LONDON (Reuters) - Scientists studying swingers -- straight couples who regularly swap sexual partners and indulge in group sex at organized meeting -- say they have higher rates of sexually transmitted infections (STIs) than prostitutes.

A female guest enjoys the atmosphere in a swingers club The Secession art gallery in Vienna late evening February 23, 2010. REUTERS/Herwig Prammer

Dutch researchers publishing their work in the British Medical Journal showed that older swingers -- those over the age of 45 -- are particularly vulnerable and yet are a group largely ignored by healthcare services.

With estimates that the swinger population could be many millions across the world, the scientists said there was a risk this untreated group could act as an STI “transmission bridge to the entire population.”

“Although exact estimates are unavailable, the swingers’ population is probably large,” wrote Anne-Marie Niekamp, who worked on the study with colleagues from Maastricht University.

The Dutch study analyzed the numbers of patients seeking treatment in 2007 and 2008 at three sexual health clinics in South Limburg in the Netherlands.

The clinics have recorded whether a patient is a swinger since the start of 2007, in an attempt to track infection rates among this group.

During the study period, there were just under 9,000 consultations at the three clinics. One in nine of the patients was a swinger, with an average age of 43.

Overall, combined rates of Chlamydia and gonorrhea were just over 10 percent among straight people, 14 percent among gay men, just under 5 percent in female prostitutes, and 10.4 percent among swingers, they found. And female swingers had higher infection rates than male swingers.

One in 10 older swingers had Chlamydia and around one in 20 had gonorrhea.

Chlamydia is the most common sexually transmitted disease among women and in 70 percent of cases causes no symptoms. The bacterial infection can lead to pelvic inflammatory disease, ectopic pregnancy and infertility. Gonorrhea is another bacterial infection which can also lead to infertility if left untreated.

Niekamp said that while other high risk groups, such as young straight people, gay men and prostitutes, were relatively easy for healthcare service to identify and target for advice and help, swingers were generally a hidden community.

“That makes them very hard to reach,” she said in a telephone interview. “Because they are so hidden and in some ways also stigmatized, it is hard for them to come forward for STI testing and treatment.”