A common cause of diarrhea and vomiting is rapidly evolving into a difficult-to-treat sexually transmitted infection, prompting renewed warnings from health agencies. Shigella bacteria, which cause shigellosis, infect approximately 450,000 people in the United States each year and are becoming increasingly resistant to important antibiotics.
Shigella has traditionally spread through contaminated food or water and close contact in childcare settings, particularly among young children. Since the early 2000s, however, outbreaks associated with sexual contact have been documented, especially among gay, bisexual and other men who have sex with men.
Growing threat of drug resistance
Data from the US Centers for Disease Control and Prevention (CDC) show a sharp increase in extensively drug-resistant Shigella, known as XDR strains. An analysis of nearly 17,000 US isolates collected between 2011 and 2023 found that the proportion resistant to at least five commonly used antibiotics rose from virtually zero to 8.5 percent.
Most XDR infections were identified in adult men, many of whom had not recently traveled internationally, indicating sustained transmission within the United States. At a major Los Angeles hospital, clinicians now report approximately one XDR Shigella case per month, compared with just one case throughout 2022.
According to the CDC, roughly one-third of US patients with XDR shigellosis have required hospitalization. Among patients with known HIV status, nearly half were living with HIV, highlighting the disproportionate impact on vulnerable and immunocompromised populations.
Sexual transmission takes hold
Sexually transmissible shigellosis was first described in 1974 but represented a minority of cases for decades. Recent studies suggest that sexual contact has become a major transmission route within some urban networks, although incomplete sexual histories mean the true scale may be underestimated.
In England, researchers have documented sustained outbreaks of sexually transmitted Shigella among men who have sex with men. The UK Health Security Agency has warned that although many infections are mild, severe cases increasingly require antibiotics that may no longer work reliably.
By 2025, surveillance in the United Kingdom showed resistance in 86 percent of tested Shigella sonnei samples and 94 percent of Shigella flexneri samples, the two main species circulating sexually among men who have sex with men. Earlier research found that up to one-third of patients with sexually acquired infections required four to five days of hospitalization.
Why sexual cases are harder to treat
Scientists explain that sexually transmitted Shigella does not represent a single strain but several overlapping lineages circulating internationally. These lineages are rapidly accumulating antibiotic resistance genes, often carried on mobile genetic elements capable of moving between bacteria.
One proposed explanation is so-called bystander resistance, which occurs when antibiotics prescribed for other sexually transmitted infections unintentionally encourage Shigella in the gut to develop resistance. This can happen when medications are administered at doses or for durations that do not fully eliminate other bacterial species present in the body.
An analysis from the United Kingdom found that the increase in azithromycin-resistant Shigella slowed after national guidelines stopped recommending azithromycin for gonorrhea. The finding suggests that STI treatment practices can indirectly influence antibiotic resistance patterns in intestinal pathogens.
Public health response and prevention
Health officials in the United States and United Kingdom now advise clinicians to consider sexual transmission when assessing adults with dysentery, particularly men who have sex with men. Recommendations include asking about recent sexual activity, ordering stool cultures with antibiotic susceptibility testing and screening for other STIs and HIV.
Most Shigella infections are self-limiting and resolve within a week with rest and adequate hydration. However, patients with severe illness, those at higher risk of complications or people with suspected XDR infections may require last-resort antibiotics selected according to laboratory findings to avoid encouraging further resistance.
Public health agencies emphasize that prevention requires attention to both sexual health and hygiene. They recommend avoiding sexual contact with anyone who has experienced diarrhea during the previous two weeks, using barriers during oral and anal sex, and washing hands, genitals and sex toys before and after sexual activity.
Experts also stress the importance of early testing for people who develop persistent diarrhea, bloody stools or fever following sexual exposure. Prompt diagnosis can help guide treatment, reduce transmission between partners and strengthen local surveillance for emerging drug-resistant strains.
As researchers continue monitoring the global spread of drug-resistant Shigella, many warn that the trend reflects the broader crisis of antimicrobial resistance. They argue that responsible antibiotic use, improved sexual health education and investment in new treatments are essential to prevent previously manageable infections from becoming increasingly difficult to treat.
