A nasty diarrheal bug that can spread through sex, infect someone with as few as 10 bacteria and shrug off some of doctors’ go-to antibiotics is surging across the United States.
Shigella, the highly contagious bacteria that causes the diarrheal illness shigellosis, is nothing new to Wyoming — but an extensively drug-resistant strain known as XDR Shigella is making itself at home next door.
A Centers for Disease Control and Prevention study released this year found 60 XDR Shigella isolates in Colorado between 2016 and 2023. California had 111 and Oregon 21.
Ten years ago, XDR Shigella was basically a statistical oddity. Now about 1 in every 12 Shigella samples tested is XDR.
And the fact that 84% of all identified XDR infections piled up in the final two years tells you this isn't just 10 years of slow accumulation — it accelerated sharply in recent years.
Symptoms can include diarrhea — sometimes bloody and sometimes prolonged — stomach pain, fever and a condition called tenesmus, the persistent and often painful feeling that a person needs to have a bowel movement even when the bowel is empty.
“It's a little bit difficult to distinguish it from your typical gastroenteritis,” said Dr. Mark Drumheller, chief medical officer and emergency medicine physician at Cheyenne Regional Medical Center, adding that there are clues.
“Most of the time, some blood in the stool as well as mucousy stools,” Drumheller said.
It's not just a problem in the United Staes.
European health officials reported this year that more than 2,300 multidrug-resistant or extensively drug-resistant Shigella infections were identified across 14 countries since 2023, with seven resistant strains circulating across Europe as of April.
England has seen a particularly sharp rise. Laboratory diagnoses of potentially sexually transmitted Shigella reached 2,560 in 2025, and more than half of one type of Shigella tested among a population used to track sexual transmission was extensively drug resistant.
The emergence has produced headlines warning about “sexually transmitted diarrhea” and a new “superbug," but Wyoming microbiologists and doctors say that isn't the most troubling part.
It’s that Shigella is getting harder to kill.
So What Is It?
Shigella isn't a new sexually transmitted disease. It is an old, extremely contagious intestinal bacterium that has historically spread through contaminated food and water, dirty hands and close-contact settings such as daycares, schools and other places where people share bathrooms or sanitation can be difficult.
Sex is simply another way it can travel.
Drumheller said Wyoming recorded 13 shigellosis cases in 2024, compared with just one in 2020.
Those figures represent all Shigella infections, not XDR infections.
For Wyoming physicians, however, what's happening nationally matters because resistant bacteria don't respect state lines.
And XDR Shigella is already well established immediately south of Wyoming.
University of Wyoming microbiologist Bledar Bisha said the rise has his attention.
“This is definitely a rising risk,” he told Cowboy State Daily.
'Extensively Drug Resistant'
It doesn't take much.
“Ingesting 10 cells can lead to infection,” Bisha said.
The CDC estimates about 450,000 Shigella infections occur every year in the United States.
Most people recover without antibiotics.
A healthy person's immune system can generally clear even an XDR infection, Bisha said.
That distinction is important.
“Extensively drug resistant” describes the bacteria's ability to withstand antibiotics. It does not mean every person who catches it will become critically ill or that the human immune system is powerless against it.
For many otherwise healthy patients, Drumheller said, treatment can amount to supportive care, particularly staying hydrated while the infection runs its course.
It's when a patient actually needs antibiotics that XDR Shigella becomes troublesome, he said.
Medicine Cabinet
Scientists generally don't use the popular term “superbug,” Bisha said.
The CDC defines XDR Shigella as resistant to five antibiotics commonly recommended for treating the infection: Azithromycin, ciprofloxacin, ceftriaxone, ampicillin and trimethoprim-sulfamethoxazole.
There is no FDA-approved oral antimicrobial specifically available to treat XDR Shigella.
For a patient who is severely ill or particularly vulnerable to complications, that can leave physicians reaching deeper into the medicine cabinet.
“Once we identify it as Shigella and we can see what it's resistant to and sensitive to, that helps tailor the antibiotics,” Drumheller said.
Sometimes that means a pill at home, but not always.
“If it is a highly drug-resistant form, then, yeah, it would require IV antibiotics and treatment in the hospital,” Drumheller said.
The CDC found roughly one-third of XDR patients with available hospitalization information had been hospitalized.
That's one reason the increase has drawn the attention of microbiologists.
Primarily Affect Children
Shigella spreads through what scientists call the fecal-oral route.
Put plainly, a tiny amount of fecal material from an infected person has to make its way into another person's mouth.
“Shigella, it's an interesting bug because it does not come from animals,” Bisha said. “It comes from humans.”
Sexual transmission has become particularly important in the current rise of resistant Shigella.
Of XDR patients in the CDC study with demographic information available, more than 96% were adults and 86% were men.
That pattern has also emerged overseas.
England reported a sharp rise in Shigella infections potentially acquired through sexual contact in 2025, and European disease officials say resistant strains are circulating through sexual networks across multiple countries.
That does not mean Shigella is exclusively — or inherently — sexually transmitted.
Historically, shigellosis in the United States primarily affected children and was usually caused by strains susceptible to antibiotics.
The bacterium has simply found another highly efficient route between humans.
Borrowed Resistance
The human intestinal tract contains a teeming population of microorganisms, some of which already possess genetic instructions for surviving particular antibiotics, and shigella can acquire those instructions.
“Bacteria don't exist in a void,” Bisha said. “They're always interacting with other bacteria in the gut, in the environment and in other types of situations.”
Some resistance genes are carried on small, mobile circles of DNA known as plasmids.
Those pieces of genetic material can move between bacteria in a process called horizontal gene transfer.
“Shigella is really good at picking up genes, resistance genes, from other bacteria in the gut,” Bisha said.
That means resistance doesn't necessarily have to appear through a succession of lucky mutations.
A bacterium can acquire genetic instructions that another organism has already developed.
Daniel Wall, professor and chair of molecular biology at the University of Wyoming, said a bacterium such as E. coli can carry a plasmid loaded with resistance genes and transfer genetic material another bacterium can use.
“All of a sudden now it's resistant to one, two or three new antibiotics,” Wall said.
It's evolution occurring on a scale and at a speed difficult to see with the naked eye.
And humans help apply the pressure.
80-Year Arms Race
Antibiotics transformed medicine in the middle of the 20th century, Wall said, turning infections that routinely killed people into something a doctor could often treat with a prescription.
But bacteria adapt. Every time antibiotics wipe out susceptible bugs, the ones capable of surviving have an advantage.
And if that continues another five or 10 years?
“It’s going to become a lot more problematic,” Wall said. “The routine antibiotics that people have used are becoming less and less effective.”
That can mean drugs with worse side effects, IV treatment and hospital stays.
And eventually, doctors can run out of options.
“In the worst-case scenario, sometimes we don't have any antibiotics that can treat an infection,” Wall said. “And that can lead to serious illness or death.”
Finding replacements isn't easy. Wall said developing a new antibiotic can take about a decade and cost around $1.5 billion.
Then comes a strange catch: When a promising new antibiotic finally arrives, doctors don't want to use it unless they have to. The more it's used, the more chances bacteria get to develop resistance.
So a drug that took years and a fortune to develop might spend much of its life sitting on the shelf — not exactly an enticing business model for pharmaceutical companies.
Many have backed away from antibiotic development, Wall said.
“The pipeline is drying up.”
What People Can Do
For all the complexity of plasmids, antibiotic development and bacterial evolution, some of the defenses against Shigella remain remarkably simple.
The CDC has several recommendations. Soap and water is one of them.
Someone sick with diarrhea should not prepare food for other people and should avoid swimming.
People with bloody diarrhea, severe stomach cramping, fever or diarrhea lasting more than three days should contact a health-care provider. A stool test can identify Shigella, and additional testing can determine which antibiotics the organism will respond to.
People with bloody diarrhea also shouldn't simply reach for medications such as Imodium that slow the gut without talking to a medical professional because those medications can make shigellosis worse.
People diagnosed with Shigella should avoid sex while sick and wait at least two weeks after diarrhea ends before resuming sexual activity because the bacteria can continue to be shed after symptoms disappear.
None of that means Wyomingites should assume the next stomach bug is an untreatable superbug.
For now, most healthy people with Shigella will recover without antibiotics. The concern is what happens as more bacteria learn to survive the drugs doctors have left.
Kolby Fedore can be reached at kolby@cowboystatedaily.com.
