The study raised alarm among U.S. health officials, who have ordered doctors to stop prescribing the antibiotic known as cefixime because lab cultures showed gonorrhea was starting to develop resistance to the drug.

That left U.S. doctors with only one effective treatment for most cases of gonorrhea, an injectible antibiotic known as ceftriaxone.

"We've been very concerned about the threat of potentially untreatable gonorrhea in the United States," Dr. Gail Bolan, director of the Centers for Disease Control and Prevention's division for sexually transmitted diseases, said in a telephone interview

There have been a number of cases in Europe, but "this is the first time we've had such a report in the actual North American continent," she said. "We feel it's only a matter of time until resistance will occur in the United States."

Until now, signs of antibiotic resistance in North America have been detected mostly through lab tests, which have shown a steady increase in the amount of antibiotic cefixime - marketed by Lupin Ltd as Suprax - that was needed to kill gonorrhea.

"We had seen one case beforehand, but this is the first published report, and it's also the first series of cases in North America," said Dr. Vanessa Allen of Public Health Ontario in Canada, who led the study published in the Journal of the American Medical Association.

Allen and colleagues studied nearly 300 individuals with gonorrhea between May 2010 and April 2011 who were treated with cefixime at a clinic in Toronto, looking for any patients who were still infected during a follow-up visit.

Of the initial 300, 133 returned for retesting. Of those, 13 were still infected, but only nine said they had not had sexual contact that might have reinfected them. That left a failure rate of 6.7 percent.

Allen said the study is a preliminary finding, but still important because it offers some confirmation that people treated with cefixime are not being cured.

It also points out a weakness of newer DNA-based tests commonly used to test for gonorrhea.

Previously, doctors would take fluid samples from patients and grow cultures of gonorrhea bacteria in lab dishes, which could then be used to identify drug resistance. More advanced DNA-based tests, such as nucleic acid amplification tests, cannot be used to test for antibiotic resistance.

"I do think reinvesting in culture-based methodologies is warranted," Allen said, adding that doctors should consider sending patients for retesting to make sure they have been properly treated.

If left untreated, gonorrhea can lead to pelvic inflammatory disease, ectopic pregnancy, stillbirths, severe eye infections in babies and infertility in both men and women.

In the United States, there are approximately 300,000 reported cases of gonorrhea each year. But because infected people often have no symptoms, the actual number of cases is likely closer to 600,000, Bolan said.

So-called "superbug" drug-resistant strains of gonorrhea accounted for almost one in 10 cases of sexually transmitted disease in Europe in 2010, more than double the rate of the year before, health officials from the Stockholm-based European Centre for Disease Prevention and Control said in June.

In addition to closely monitoring for resistance, Bolan said the CDC it is working with its partners at the National Institutes of Health and pharmaceutical companies to encourage the development of new antibiotics and test new combinations of existing drugs.

(Reporting by Julie Steenhuysen; editing by Christopher Wilson)

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