A recent study published in Clinical Infectious Diseases has uncovered an increase in severe multidrug-resistant (MDR) Shigella sonnei infections, particularly among people experiencing homelessness (PEH) in Vancouver, Canada. The findings shed light on an important and emerging infectious disease issue, with cases shifting from men who have sex with men (MSM) to infections disproportionately impacting vulnerable populations living in precarious conditions.
“We are seeing a significant and concerning change in the number and severity of these infections,” said Dr. Marc Romney, clinical professor in the department of pathology and laboratory medicine at the University of British Columbia and senior author of the study. “The increasing severity of Shigella infections among homeless individuals likely reflects a combination of microbiological, epidemiological and patient factors. Importantly, increased severity of illness recently coincided with the emergence of a highly resistant bacterial strain of Shigella sonnei.”
S. sonnei is a bacterium that causes shigellosis, a highly contagious infection of the digestive system. Transmitted through contaminated food, water, or contact with infected individuals, it requires only a tiny number of bacteria to cause illness. Transmission can also occur through sexual contact, and in settings where environmental contamination is high. Symptoms range from mild diarrhea to severe conditions such as dysentery, which includes abdominal pain, fever and bloody stools. In some cases, the infection can lead to complications like dehydration, colitis (inflammation of the colon) or sepsis (a life-threatening infection in the bloodstream).
Shift in infections
Between 2015 and 2022, researchers analyzed 163 cases of S. sonnei infections in Vancouver, noting a striking rise in severe cases. While infections initially centered on the MSM community, recent years have seen a shift, with 77 per cent of cases now occurring in PEH. The study highlights that these patients face significantly greater health risks, with higher hospitalization rates and more severe outcomes compared to other groups.
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The culprit behind this alarming trend is a specific clone of S. sonnei—identified as genotype 3.6.1.1.2—which is resistant to nearly all first- and second-line antibiotics. This strain likely originated in the MSM community but has since spread to people experiencing homelessness, where environmental contamination and close living conditions have created a perfect storm for transmission.
Vancouver also saw a 33-per-cent increase in people experiencing homelessness between 2020 and 2023, according to an annual count.
Escalating severity and resistance
Infections among PEH were significantly more severe than those in other groups. While only 14 per cent of cases were classified as severe between 2015 and 2020, this figure jumped to 61 per cent by 2022. These infections often required longer hospital stays, referrals to specialist services, intensive care, and were marked by a high rate of bacteremia and sepsis—a dangerous condition where bacteria spread into the bloodstream.
The study also found that the strain of S. sonnei affecting these populations has developed resistance to nearly all oral antibiotics commonly used for treatment, leaving healthcare providers with few treatment options.
Addressing the crisis
The researchers argue that improving access to hygiene and sanitation facilities, as well as providing targeted outreach to PEH, is crucial to stemming the tide of these infections. They also call for accelerated development of vaccines and new antibiotics to combat multidrug-resistant S. sonnei.
In the meantime, Dr. Romney and his team recommend improved diagnostics and community-level interventions to mitigate further spread. “This is not just a problem for Vancouver,” he warned. “Other cities and hospitals in North America—and beyond—are likely to face similar challenges if we do not act swiftly.”
As healthcare systems grapple with the aftermath of the COVID-19 pandemic, this study serves as a stark reminder of the continued vulnerability of marginalized communities to emerging health threats.
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