PARIS, France (AFP) Middle-aged swingers -- heterosexual couples who swap partners at sex parties -- have been identified as an "at-risk" group for chlamydia and gonorrhoea, says a Dutch study published on Thursday.

Researchers in South Limburg, a district of 630,000 people that includes Maastricht city, sounded the alarm after looking at statistics from three sexual disease clinics which have

been registering swingers as a patient category since 2007.

Of 8,971 consultations for chlamydia and gonorrhea in 2007 and 2008, almost one in eight were for swingers, whose average age was 43, the investigators found.

Swingers placed third after young heterosexuals and gays in the disease tally. Female swingers had higher infection rates than male swingers.

"Swingers, like other groups with risk behaviours, need to be identified and treated as a risk group in STI (sexually transmitted infection) prevention and care," according to the study, headed by Nicole Dukers-Muijrers of the South Limburg Public Medical Health Service.

Exact figures on the Netherlands' population of swingers are unclear but the number "is probably large", says the paper, published in the journal Sexually Transmitted Infections.

One popular website alone has 15,000 couples, it says.