This Stomach Bug Isn’t Responding to Antibiotics. Scientists Are Worried.

Multiple strains of the diarrhea-causing bacteria Shigella have acquired resistance to the five antibiotics most commonly used to treat it, a health alert released by the Centers for Disease Control and Prevention revealed last month. The first case of extensively drug-resistant Shigella was discovered in the United States in 2016; by 2022, the strains accounted for 5 percent of Shigella infections.

The drug-resistant bacteria have been found in 29 states, experts at the C.D.C. told The New York Times, and are probably even more widespread. The fact that several strains are circulating indicates that the drug resistance emerged “in different species of Shigella, in different parts of the country, in different parts of the world,” said Dr. Louise Francois Watkins, a medical officer at the C.D.C.

The gastrointestinal illness that Shigella causes is unpleasant and occasionally requires treatment but is rarely fatal. The bigger concern, Dr. Watkins said, is that the drug-resistant Shigella strains could be a predictor of what’s to come with other bacterial infections.

What is Shigella, and how is it treated?

Shigella (pronounced “shi-GEH-luh”) is a stomach bug that causes dysenteric diarrhea, in which the lining of the intestines becomes inflamed and damaged. The telltale signs of dysenteric diarrhea are stools that are not only watery but also have blood and mucus in them. Nausea, stomach cramps and fever often occur with Shigella infections, but vomiting is less common.

“The difference with Shigella is that it has this ability to trigger this effect whereby you get a real aggressive inflammation of the gut,” said Stephen Baker, a professor of molecular microbiology at the University of Cambridge. That “is why you get blood and mucus in the stool, which E. coli and salmonella generally don’t do,” he added.

Most Shigella infections resolve on their own in a few days with bed rest and fluids. Once the infection passes, the gut will heal, and there is usually no long-term damage. If symptoms linger for more than a week, or if you have signs of severe dehydration like lightheadedness or confusion, you should see a doctor. Infants, older adults and people who have weakened immune systems can be at higher risk of severe illness and may be more likely to require treatment.

Five different antibiotics have historically been used to treat Shigella: azithromycin, ciprofloxacin, ceftriaxone, trimethoprim-sulfamethoxazole and ampicillin. It is not uncommon for strains of the bacteria to not respond to one or two of the medications, but resistance to all five is new. Other antibiotics exist that may work for Shigella, but they have not yet been tested against the bacteria. As a result, the C.D.C. does not currently have a recommended treatment option for the drug-resistant strains.

“This is not what we would call pan-resistance,” when a bacterial strain is resistant to everything, leaving no more treatment options, Dr. Watkins said. “But we have gone beyond the boundaries of what’s been studied for Shigella.”

How common is Shigella, and how is it transmitted?

According to the C.D.C., there are approximately 450,000 cases of Shigella annually in the United States, roughly 6,400 of which require hospitalization. The majority of infections occur in children under the age of 5.

For reasons that are still unclear, most of the drug-resistant cases have been seen among adults, particularly men who have sex with men, people experiencing homelessness and people with H.I.V. “We’re seeing drug-resistant Shigella infections among these vulnerable populations, and that’s concerning,” said Dr. Naeemah Logan, also a medical officer at the C.D.C. Drug-resistant strains have also been identified in people who recently traveled internationally.

Shigella spreads very easily, with only a tiny amount of the bacteria required to trigger illness. People are exposed when bacteria present in fecal matter come into contact with the nose or mouth. Common routes of transmission are unwashed hands, sexual activity and contaminated food. (Food transmission most often stems from an infected person preparing or serving a meal, not Shigella in the food supply.)

To limit the spread of the bacteria, Dr. Logan urged people to practice good hand hygiene, washing with soap and warm water for at least 20 seconds after using the bathroom or changing diapers, before preparing food, and before and after sex. “Hand-washing hygiene really can’t be dismissed. It’s such a crucial strategy to curb transmission,” she said.

People can remain infectious even after their symptoms have resolved, so Dr. Logan advised that anyone with diarrhea avoid swimming pools and abstain from sex for two weeks afterward.

Why is antibiotic-resistant Shigella suddenly on the rise?

The use of antibiotics is what drives antibiotic resistance in bacteria. The microbes that are susceptible to the medications die off, while the ones that have evolved to evade them will survive and spread.

“The more we use antibiotics, the more bacteria become resistant to them — that’s a kind of inevitable evolutionary process,” Dr. Baker said. “It’s quite common that once it goes on this trajectory, the resistant organisms get selected for, and therefore they take over very quickly within a short space of time.”

Bacteria can acquire drug resistance through random DNA mutations or by swapping genes with other strains or species. Shigella is particularly good at picking up new genes, so antibiotic resistance often emerges in Shigella before it does in other types of bacteria. But DNA sharing goes both ways.

“The resistance that develops in Shigella, it can spread into those other organisms,” Dr. Watkins said. “So what we’re seeing in Shigella now, this resistance is something that we could be seeing in other types of bacteria like E. coli or salmonella in the future.”

Dr. Watkins added that the Shigella itself is not necessarily going to be responsible for spreading resistance to every other pathogen, “but the Shigella is maybe an early warning sign that these selective pressures are out there and that they’re acting on other bacteria also.”

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