A Human Outbreak of Shigellosis Reveals the Variety of Infectious Organisms Carried in Wild Urban Rat Feces

A rat darts down your grimy city sidewalk. While rats have notoriously spread the Black Death across Medieval Europe, what other infectious organisms do rats carry that can make you sick?

In 2021, there was an outbreak of shigellosis, a gastrointestinal disease caused by the bacteria Shigella flexneri, among residents of a low-income neighbourhood in downtown Vancouver, British Columbia. Disease more often occurs in areas with poor sanitation, and patients had reported living on the streets, in shelters, or in single room occupancy hotels with minimal access to washrooms.

Public health agencies investigated the outbreak, but could not determine the source of exposure. However, patients had reported contact with wild rats on the streets. Could rats be the culprit? And what other infectious organisms do rats carry in their poop?

A team of researchers across human and animal health fields worked together to answer this question. Researchers from the British Columbia regional node of CWHC and the Vancouver Rat Project, led by Dr. Chelsea Himsworth, trapped 22 brown rats within the outbreak area. Fecal samples were collected from each rat.  Scientists at the University of British Columbia, Providence Health Care, and the British Columbia Centre for Disease Control then tested the samples for Shigella flexneri and other infectious organisms.

Two rats were indeed carrying a small amount of Shigella flexneri that was closely related to those isolated from the human outbreak. These rats were also infected with other bacteria that can cause gastrointestinal disease in people, including Clostridioides difficile, Yersinia enterocolitica, Campylobacter spp., as well as enteroaggressive and enteropathogenic Escherichia coli. Some rats were also infected with Rotavirus A, Cryptosporidium spp., and Giardia duodenalis. Strikingly, rats were most often carrying more than one potentially infectious organism in their feces.

While this study alone cannot determine if rats were the source of infection to the patients, the relatedness between Shigella flexneri found in the rats and people suggest that transmission was possible. The low amount of bacteria in the rats’ feces may indicate that rats could have been exposed to human feces contaminated with Shigella flexneri in alleyways, and that they could have had a role in spreading the bacteria.

So overall, what infectious organisms do rats carry in their poop that can make you sick? The answer is many! Our work highlights that continued rat management and surveillance is needed to minimize the risk of disease transmission to humans, especially in lower socioeconomic areas where there may be increased exposure to rats.

Submitted by Lisa Lee, CWHC BC

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