Gonorrhea, one of the most common sexually transmitted diseases in the United States, is increasingly showing resistance to one of the last known effective antibiotic treatments, leading researchers from the Centers for Disease Control to "sound the alarm" about potentially untreatable forms of the disease.

"During the past three years, the wily gonococcus has become less susceptible to our last line of antimicrobial defense, threatening our ability to cure gonorrhea," Gail Bolan, director of the CDC's sexually transmitted disease prevention program, wrote in The New England Journal of Medicine last week.

According to the CDC, gonorrhea has a long history of developing immunity to antibiotics, but doctors have always had a stronger medicine up their sleeves to treat patients. Not anymore—about 1.7 percent of gonorrhea is now resistant to cephalosporins, the last line of defense against gonorrhea. That might not seem like much, but it's a 17-fold increase since 2006, when about one tenth of one percent of gonorrhea was believed to have resistance to cephalosporins.

According to Bolan, the strains are showing up most often in the western states, where 3.6 percent of gonorrhea has shown resistance to cephalosporins, and in men who have sex with men, with nearly 5 percent of gonorrhea showing resistance.

The disease has been estimated to affect 600,000 Americans annually, causing burning with urination, abdominal pain, itching, and genital discharge.

Nikki Mayes, a spokesperson for the CDC, wrote in an email that by using a combination of cephalosporins and other antibiotics, American doctors have been able to prevent anyone from getting a completely untreatable case of gonorrhea. But she says it's only a matter of time.

"The trends in decreased susceptibility that we're seeing, coupled with the history of emerging resistance and reported treatment failures in other countries point to the likelihood of treatment failures on the horizon," she writes.

Not much help is on the way, according to both Mayes and Nicole Mahoney, senior officer of the antibiotics and innovation project at PEW Charitable Trusts.

"As far as gonorrhea goes, I'm not aware of any new drugs in the pipeline," says Mahoney. "This is just one more example of a bigger problem—bacteria are developing resistance faster than we're inventing new medicines to fight them."

Mahoney says Congress and the Food and Drug Administration should encourage and reward pharmaceutical companies to devise new antibiotics. According to a PEW report, only two new classes of antibiotics have been introduced since 1968 because antibiotics are difficult to produce and are less profitable than other drugs.

Bolan writes in the medical journal that a vaccine to prevent gonorrhea "remains key to prevention and control," but that it is a "distant goal."