(CNN) For patients with gonorrhea, a two-drug antibiotic punch is the usual recommended treatment, but new research found the sexually transmitted infection is less vulnerable to one of these drugs: azithromycin.
Because the bacteria causing gonorrhea are already resistant to penicillin, tetracycline and fluoroquinolones, scientists at the Centers for Disease Control and Prevention worry that the sexually transmitted infection may outsmart another antibiotic. Left untreated, gonorrhea can cause serious health problems, including pelvic inflammatory disease in women (which may lead to infertility) and a painful condition in the tubes attached to the testicles in men (which may lead to sterility).
"So far we've had no treatment failures in the United States," said Dr. Jonathan Mermin, the director of CDC's National Center for HIV/AIDS, Viral Hepatitis, STD and TB Prevention. The center's treatment recommendation, then, continues to be an oral dose of azithromycin combined with a single shot of ceftriaxone.
The CDC has been monitoring the threat of antibiotic resistance since 1986, Mermin explained. Listening to the "chatter" is a matter of the CDC collecting bacterial samples from 25 men each month. Then, the CDC conducts laboratory tests to see how weak the bacteria are in comparison to the various antibiotic drugs intended to destroy them.
Between 2013 and 2014, the CDC discovered the percentage of bacteria fighting against the drug azithromycin increased by more than 400%. These results, published Thursday in the Morbidity and Mortality Weekly Report, indicate that gonorrhea is in the earliest stages of resisting the antibiotic.
Still, the numbers are small. Thousands of bacteria samples were collected in 2014 but only 2.5% showed resistance to azithromycin in 2014 --- compared to 0.6% in 2013.
The CDC also has been monitoring resistance to cephalosporins, the class of antibiotics that includes ceftriaxone, the other key drug currently recommended for treatment. These, too, have begun to demonstrate early warning signs of possible ineffectiveness, said Mermin, with a recent study conducted in England and Wales suggesting the same.
When will these drugs stop working?
"We can't predict that," said Mermin. "The speed of development of resistance is unpredictable."
Following the development of antimicrobial agents in the first half of the 20th century, scientists believed these drugs would end, once and for all, infectious diseases caused by bacteria. That, however, never happened for a simple reason: resistance. Over time, bacteria adapt to the strong drugs intended to kill them and eventually mutate or acquire genes capable of interfering with -- or even destroying -- the antibiotic itself. Neisseria gonorrhoeae, the bacteria that causes gonorrhea, is following this familiar narrative of adapting in order to survive.
A growing threat of potentially untreatable gonorrhea, combined with increasing rates of disease, makes it more important than ever to prevent new infections, Mermin said.
When it comes to gonorrhea -- and all STDs -- there are two basic methods of reducing the risk of infection, he explained. Treat immediately and effectively with the recommended antibiotics to limit the spread of infection from person to person. And work to prevent gonorrhea and other STDs. The CDC recommends being in a long-term mutually monogamous relationship with an uninfected partner and using latex condoms.
The numbers he quoted are striking: After being on the decline for several years, total reported cases of gonorrhea increased to more than 350,000 in 2014. Yet, the CDC estimates more than 800,000 gonorrhea infections occur each year, with less than half diagnosed. The reason? Most men and women do not show any symptoms, only some experience a burning sensation while peeing, or green or whitish discharge.
In the end, Mermin said, we need to acknowledge the ability to cure people with antibiotics will fade over time. We must deal with that, he said.
"By preventing gonorrhea," he said, "we can also prevent resistant gonorrhea."