Some 88 per cent of Dublin syphilis victims sampled in a US study are carrying a new antibiotic-resistant strain of the disease.
The figure is far above the 17.5 per cent average for three US cities also sampled. Most of those infected with the new fast-spreading form are believed to be homosexual men.
The results of the findings are published in the latest New England Medical Journal, based on research organised by the University of Washington and conducted in Dublin through St James's Hospital.
"In Dublin, a very high proportion (88 per cent) of samples contained the mutation, suggesting the introduction of a mutant strain that spread rapidly within a defined sexual network," the report found.
The study is receiving considerable attention in the US, where the incidence of syphilis among gay and bisexual men has increased 12-fold in recent years. The University of Washington study focused on azithromycin, an antibiotic pill used to cure syphilis in the US and Ireland.
Dr Christina Marra, a researcher at the University of Washington, said the high instance of the strain in Dublin may be caused by a more concentrated use of sexual partners.
"I can't prove it, but I believe that's exactly the thinking that people have. You have a core group who share sexual partners and had a resistant strain introduced into that group," she said.
Dr Fiona Mulcahy, consultant in genito-urinary medicine at St James's, said: "Ireland would have a very pure MSM (men who have sex with men) group. It's concentrated around a small number of venues."
She added that part of the reason for the emergence of a resistant strain of syphilis could be the increased use of antibiotics generally in society.
Dr Marra noted that the upsurge in syphilis in the developed world was almost exclusively through gay sex.
The report found that the new strain is spreading rapidly in San Francisco, where it jumped from 4 per cent of syphilis patients in 1999-2002 to 37 per cent in 2003.
In the Republic, 595 cases of syphilis were notified to the National Disease Surveillance Centre between January 2000 and December 2002. Some 292 related to gay men, and 52 to bisexual men.
Apart from San Francisco and Dublin, two other cities were studied: Baltimore, Maryland, where the incidence of the new strain among syphilis sufferers was 11 per cent, and Seattle, Washington, where the incidence was 13 per cent.
The research team said the findings showed that patients treated with azithromycin must have follow-up tests to be sure they are cured. Many patients had preferred azithromycin pills to penicillin, which is administered by injection.