A new study published in the U.S. Centers for Disease Control and Prevention’s (CDC’s) Emerging Infectious Diseases describes an unusual Escherichia coli outbreak that occurred in 2025 in France, causing multiple adult cases of hemolytic uremic syndrome (HUS) and three deaths. The outbreak was caused by an emerging, genetically unique hybrid strain of E. coli.

All Patients Were Adults, 90 Percent Developed HUS

First detected by the E. coli National Reference Center of France, the outbreak included 18 confirmed cases, as well as five probable and possible infections determined by in-house polymerase chain reaction (PCR) and two additional cases from Scotland and Belgium.

The illnesses occurred between December 2024 and April 2025. Infections primarily affected older adults, with a median age of 72.

More than 90 percent of identified patients developed HUS—one of the highest-ever reported among STEC outbreaks internationally—and three deaths were reported. Typical STEC outbreaks result in HUS in 1–56 percent of infected children and 3–24 percent of infected adults.

Unlike typical HUS outbreaks, no pediatric cases were detected. STEC infections, particularly HUS, are approximately ten times less frequent in adults than in children, per the study.

Raw Cow’s Milk Cheese Identified as Vehicle of Illness

Epidemiological and traceback investigations, including food questionnaires and supermarket loyalty card record analyses, determined that 88 percent of confirmed patients and half of probable case patients had consumed the same brand of raw cow’s milk cheese. The outbreak strain was confirmed in a cheese sample taken from a patient’s refrigerator, prompting a product recall in January 2025.

Characteristics and Global Emergence of the Unique STEC/ETEC Hybrid Strain

Whole genome sequencing (WGS) showed the strain to be a LEE-negative Shiga toxin-producing E. coli (STEC) of serotype O77 g:K92:H18, carrying the stx2d toxin variant and additional enterotoxin genes commonly associated with enterotoxigenic E. coli (ETEC). Researchers described the organism as a hybrid STEC/ETEC pathotype with enhanced virulence characteristics.

The study noted that the strain’s unusual genetic profile, including multiple toxin genes and a rare capsule type (K92), may have contributed to the severity of illness and the exclusive impact on adults. The researchers hypothesized that the mandatory meningococcal C vaccination of infants in France may have conferred cross-protection against K92 E. coli infections in children, as experimental cross-immunogenicity between the two has been demonstrated in previous literature.

Additionally, through a search of a global bacterial genome database (EnteroBase), the researchers identified 31 closely related strains spanning three continents with cases dating back to 2005, suggesting the pathogen has been emerging internationally.

Implications for Foodborne Pathogen Surveillance

Overall, the outbreak underscores the serious health impact and outbreak potential of STEC in adults and the risk presented by atypical emerging strains.

Just before the 2025 outbreak, HUS surveillance in France for adults was formally implemented to better capture the characteristics of STEC infections in this population, enabling detection of the outbreak. Additionally, in-house PCR can help detect outbreaks, investigate their sources, and identify reservoirs.

The authors emphasized the need for robust surveillance and detection to monitor this STEC/ETEC hybrid pathotype, especially given its outbreak potential, severity, and global presence.