You don’t have to be a pioneer on the Oregon Trail to catch the bug that causes dysentery.

Nowadays, the infection is spreading more through sex — and evolving into something that’s “close to untreatable,” one expert said.

Shigella bacteria infect nearly half a million people in the US every year — causing diarrhea with blood or mucus (the hallmark of a dysentery infection), stomach pain, cramps, fever and vomiting.

Scientists are learning more about why the disturbing disease is spreading so quickly, and why it’s developing a resistance to treatment.

Spreading through sex

Shigella is a nasty illness that causes more than uncomfortable doo-doos. It can become life-threatening by dehydrating and depleting bodily fluids including blood, causing bacterial and septic shock, neurological complications, and internal bleeding and infection.

The bacteria spreads through infected poop that gets into the mouth because of ill-washed hands, or swallowing tainted food or water. A minuscule amount — as few as 10 organisms — is enough to get you sick.

The population most likely to get infected are children under 5, who catch it at day care and schools from other kids who are just starting to learn about keeping themselves clean. Travelers to places where sanitation standards are lower, and people experiencing homelessness are also at higher risk.

But the population that’s experiencing one of the fastest rises in shigella infection are gay and bisexual men, and other men who have sex with men (GBMSM). Experts say shigella is “endemic” to these communities, meaning it’s constant and regularly present.

A study in July showed that the spread of the disease through sex grew 15% faster year-over-year between 2015 and 2020 compared to spread through travel, food, or nursery outbreaks.

Emerging evidence suggests that sex may even be the major route of transmission now, but it’s hard to tell when it’s not asked about when patients show up for medical care.

Experts say many people don’t realize Shigella can spread through sex — and that lack of awareness may be fueling infections.

“Many men who have sex with men are unaware of the serious and increasing risk posed by sexually transmitted Shigella,” said Cambridge geneticist Kate Baker, a lead author on the July study.

Becoming a superbug

As shigella spreads rapidly, it’s also becoming harder to manage. The CDC reports that drug-resistant strains of the infection skyrocketed from 0% in 2011 to 8.5% in 2023.

“This disease has gone from being relatively treatable with off-the-shelf drugs to being close to untreatable over the course of the last decade or so,” Baker said.

Why is this happening? Some experts believe “bystander resistance” might be at play. This happens when the wrong antibiotic is used for a pathogen, not only failing to kill the disease, but also accidentally giving the disease the opportunity to develop resistance to that antibiotic.

The CDC reports that most drug-resistant shigella cases affected adult men. The strains of shigella spread through sex are serious. Research shows that those who contracted shigella through sex were hospitalized for four to five days, on average.

“This isn’t just one form of sexually transmissible diarrhea,” Baker told Science Alert.

“This is multiple overlapping variants emerging that are all quickly becoming resistant to the drugs we use to treat them. It’s highly likely that if you contracted your Shigella through sex, you require different treatment to someone who contracted it through travel.”

Treating and preventing shigella

Many cases of shigella resolve on their own within a week, with the help of rest and fluids. But for serious infections, antibiotics might be used not only to shorten the illness but to reduce contagiousness, as well.

This is becoming harder as the strains become more resistant to antiobiotics. That’s why the CDC is urging more attention and development of prevention strategies.

They recommend:

• Washing hands, genitals, and anus with soap and water before and after sexual activity. This applies to everyone having anal, oral, and vaginal sex — not just men who have sex with men.
• Washing sex toys with soap and water
• Using barriers like condoms and dental dams

The California Department of Public Health also adds simply, “don’t have sex with someone who has had diarrhea in the last two weeks.”