Gonorrhea has taken many forms over the last few decades. The strain that people acquire today isn't the same one that previous generations had to deal with. In fact, it might not be the same strain that infected people a little over 10 years ago. That's because gonorrhea, a sexually transmitted disease (STD), has become resistant to most of the antibiotics that we have used to combat it over the last three decades.

Today's gonorrhea patient has few options left

That's right: penicillin and various tetracyclines have all stopped working against the most prevalent strains. This means that today's gonorrhea patient has very few treatment options left. And with symptoms like burning, swelling of the testicles, vaginal discharge and anal itching, it's not exactly something that you want to leave untreated. Unfortunately, the US Center for Disease Control (CDC) thinks that emerging resistant strains will one day take the last remaining first-line treatment option away — a treatment that currently consists of a cephalosporin injection combined with an oral dose of either azithromycin or doxycycline. The government agency outlined how that scenario could unfold in a study released today.

By analyzing long-term surveillance data for 17 US cities between 1991 and 2006, researchers were able to trace how gonorrhea became resistant to ciprofloxacin, an antibiotic that lost its CDC recommendation in 2007 because studies demonstrated that 13.8 percent of patient samples were resistant to the drug.

The study's results are alarming, but not altogether surprising. The researchers found that increased resistance leads to an increase in gonorrhea cases. That's because being infected with a resistant strain lengthens the amount of time it takes to treat it, giving the infected party more time to pass it on to others.

Overall, the study suggests that if 10 percent of gonorrhea samples are resistant to the recommended treatment, then there will be a subsequent 7 percent increase in gonorrhea. Given that 820,000 cases are diagnosed in the US each year, a 10 percent cephalosporin-resistance rate would represent 57,400 additional cases — all of which would be extremely hard to treat. And this number could become even higher over time because of the cumulative effect of resistance. "Based on the experience with other drugs previously used to treat gonorrhea," said Sarah Kidd, a medical epidemiologist at the CDC who did not participate in the study, in an email to The Verge, "the emergence and spread of cephalosporin-resistant gonorrhea in the United States appears imminent."

cephalosporin-resistant gonorrhea in Japan, France and Spain

Gonorrhea is already adapting. Between 2007 and today, Kidd said, "the dose of cephalosporins required to stop gonorrhea from growing has increased." As a result, the CDC has had to change its recommended treatment twice, by increasing its dosage and changing the drug combination. So far, "no resistant cases have been identified in the United States," Kidd said, although several cases of cephalosporin-resistant gonorrhea have been reported in other countries, such as Japan, France and Spain.

For now, the CDC's latest recommended treatment is holding steady in the US. The percentage of gonorrhea samples that need to be treated more aggressively is lower than it was in 2012. And it's important to note that the US actually has a much lower gonorrhea incidence rate today than it did prior to the 1970s.

resistance to the drug persists in 15 percent of cases

But as the CDC points out in its report, it's important to keep monitoring emerging drug resistance. If that scenario were to happen again with cephalosporins, the health and economic consequences would be much larger than in previous recent outbreaks. And at the moment, there are no clear recommended treatment options for patients who have a cephalosporin-resistant infection, Kidd explained. Furthermore, even though ciprofloxacin is no longer widely used to treat gonorrhea, resistance to the drug persists in about 15 percent of cases. So, should the CDC's current treatment lose its effectiveness entirely, she said, "it won't be possible to return to ciprofloxacin as a first line treatment regimen for gonorrhea."