A new federal health surveillance study says gonorrhea is becoming resistant to azithromycin – one of the duo of antibiotics recommended for treating the sexually transmitted disease in the United States.

The study, from the Centers for Disease Control and Prevention (CDC), is published in the agency’s Morbidity and Mortality Weekly Report (MMWR).

Gonorrhea is a sexually transmitted disease (STD) caused by a bacterium called Neisseria gonorrhoeae that infects the genitals, rectum, and throat.

Gonorrhea occurs in both men and women, and is one of the most common STDs in the U.S. It is especially common among young people of 15-24 years of age.

The CDC estimate that every year, around 800,000 gonorrhea infections occur in the U.S., although more than half are undiagnosed. The number of American men diagnosed with gonorrhea has gone up in recent years.

While some people experience symptoms with gonorrhea infection – such as a burning sensation when urinating, or green or whitish discharge – most do not.

The CDC currently recommend that gonorrhea be treated with a combination of two antibiotics – azithromycin as an oral dose, and ceftriaxone as a single injection.

The CDC say the combination therapy “still works,” but there is a “distressing sign that the future of current treatment options may be in jeopardy.”

The new evidence of emerging resistance suggests azithromycin will be next in the long line of antibiotics to which gonorrhea has become resistant.

The STD is already resistant to penicillin, tetracycline, and fluoroquinolones.

Azithromycin resistance up 400 percent in 2013-2014

The latest information from the agency’s surveillance system for monitoring the threat of antibiotic-resistant gonorrhea shows reduced susceptibility (weaker response) to azithromycin has risen by over 400 percent between 2013 and 2014.

The figures come from nationwide routine testing of “isolates,” or samples taken from patients diagnosed with the STD.

In 2013, only 0.6 percent of isolates showed a weaker response to azithromycin. In 2014, this figure had shot up to 2.5 percent, an increase of over 400 percent.

If the trend continues, the CDC say it is not clear how long the azithromycin and ceftriaxone combination will remain effective.

The 2014 STD surveillance report also finds that gonorrhea and other reportable STDs are on the rise. Dr. Jonathan Mermin, director of the CDC’s National Center for HIV/AIDS, Viral Hepatitis, STD, and Tuberculosis Prevention, says:

“The growing threat of untreatable gonorrhea, combined with increasing rates of disease, makes it more important than ever to prevent new infections.”

“We are running just one step ahead in order to preserve the remaining treatment option for as long as possible,” he adds.

The CDC say there is a need to keep pushing on several fronts to tackle the problem and ensure successful treatment reaches those who need it.

In 2016, Congress approved $160 million in extra funds for the CDC to implement its role in the National Strategy for Combating Antibiotic-Resistant Bacteria. The strategy outlines interventions to reduce the emergence and spread of antibiotic-resistant pathogens, including N. gonorrhoeae.

“There is a critical need to strengthen STD prevention services. Gonorrhea and other STDs can currently be prevented, diagnosed and treated – action today is essential to prevent worse outcomes tomorrow.”

Dr. Jonathan Mermin