A Dangerous Diarrhea-Causing Superbug Is Spreading in Los Angeles

A diarrhea-causing superbug is stirring up trouble in Los Angeles. Researchers have tracked down a worrying new strain of Shigella bacteria in the area—one that can resist almost every antibiotic thrown at it.

Scientists at the University of California, Los Angeles, conducted the study, which looked at an unusual cluster of Shigella cases discovered between 2023 and 2024. Each of the cases were found to be carrying an extensively drug-resistant (XDR) strain of the bacteria. Though the victims all recovered, the researchers say their discovery is “alarming,” especially since it’s likely that the superbug is still actively spreading in LA and possibly elsewhere.

Shigella is a routine source of diarrhea and other gastrointestinal symptoms. While most cases only cause a week-long bout of misery that clears up on its own, the infection can rarely trigger severe, even fatal complications, particularly in people who have weakened or underdeveloped immune systems, such as very young children. Shigella kills about 200,000 people worldwide annually; in the U.S., it’s thought to infect about a half million people every year, with thousands hospitalized as a result. The germ usually spreads through contaminated food or drink, but it can also be transmitted sexually. This latter form of transmission tends to occur more frequently through anal contact among men who have sex with men.

Antibiotics are used to treat severe Shigella infections or to keep infections under control in those at higher risk of illness. But as with many other bacteria, Shigella bacteria have increasingly learned how to fend off the most common antibiotics used against it. The most concerning strains are extensively drug-resistant, meaning that they can overcome a wide variety of antibiotics. In their paper, published this month in the Journal of Infection Control, UCLA scientists detailed the discovery of a new XDR strain of Shigella sonnei in three of their patients.

According to the report, the three cases were detected within a three-month period between 2023 and 2024. All three infections involved men who had a history of having sex with other men, with one patient reporting that a recent sexual partner of theirs had been diagnosed with Shigella a week earlier. Initial lab tests revealed that they carried an XDR strain, formally defined as having resistance to the antibiotics azithromycin, ciprofloxacin, ceftriaxone, trimethoprim-sulfamethoxazole, and ampicillin.

The ULCA scientists conducted a genetic analysis of the samples collected from their patients. They found that all three patients carried a similar strain, though it was noticeably distinct from past XDR strains identified in other parts of the world. That likely means this version has been evolving and spreading in the area for some time, the researchers say.

“The discovery of a novel XDR S. sonnei actively spreading in Los Angeles is alarming,” they wrote.

Thankfully, the cases themselves were still treatable or otherwise cleared up on their own. One person did develop a severe infection and ended up in intensive care as a result. But the real-time detection of XDR resistance in this case prompted the doctors to switch to a different recommended drug that appeared to work and the patient eventually recovered fully.

Still, the emergence of XDR Shigella in the U.S. and worldwide is a serious and growing public health concern. These cases are obviously more difficult to treat than a typical Shigella infection, and the delay in finding the right drug that works against an XDR strain can potentially be fatal. Last year, scientists from the Centers for Disease Control and Prevention issued a public health advisory over the superbug. In the advisory, the CDC reported that about 5% of Shigella cases reported to the agency in 2022 were caused by XDR strains, up from 0% in 2015. Given these latest cases, the problem has likely only worsened since then. And the UCLA scientists say more has to be done to identify and curtail the spread of these dangerous bacteria.

“These cases highlight the rapid expansion of XDR Shigella in the U.S. and urgent need for appropriate detection and management,” they wrote.