In June, Kaylee Byers, a Ph.D. student, was examining a live trap in Vancouver’s V6A postal code, a neighborhood noted for its poverty. One of her initial catches was near a blue dumpster in front of a small apartment complex. She found a rat and took notes, planning to transport it to her van parked nearby, where it would undergo a series of procedures including anesthesia, photography, flea checking, disease testing, and tagging before being released back into the area within 45 minutes.

Byers is under the supervision of Chelsea Himsworth, a veterinary pathologist at the University of British Columbia. Himsworth has gained recognition for her "Vancouver Rat Project," initiated to address concerns regarding the city’s rapidly growing rat population, which has not been systematically counted.

Prior to Himsworth's efforts, knowledge about wild rats in Canada was limited to a single study on a mere 43 rats in a Richmond landfill back in 1984. Over the past six years, Himsworth has captured over 700 rats in V6A to analyze their DNA and understand the bacteria they carry.

Her findings have prompted a reevaluation of the conventional perspective that rats are invasive pests to be eradicated. Instead, they may simply be integral members of our urban ecosystems. In many cases, it would be wiser to leave them undisturbed.

Rats flourish in urban environments largely due to human activities. As cities expand and populations shift, rats have discovered new habitats. "Rats are true specialists in adapting to disturbances," says biologist Ken Aplin, who has been studying these rodents for years. "Few wild species have adjusted so well to human-dominated areas without being domesticated." Rats tend to thrive in environments that have been disrupted. They require basic necessities: a burrowing site, access to clean water, and approximately 50 grams of food daily. The distribution of these resources is often linked to human behavior.

While it's common for people to view rats as parasites benefiting from human existence, this notion isn't entirely accurate. "I have to remind myself that rats have adapted to our cities," states Matthew Combs, a doctoral student studying the genetic history of rats in New York City. In fact, rats have existed on Earth for millions of years, predating modern humans. They have always been opportunistic feeders, likely consuming food from other species long before humans arrived. Even in isolated areas like New Guinea, rats tend to inhabit regions affected by natural disturbances. In contrast, they are quite rare in untouched forests. Thus, it’s not that they have become parasites of urban life; rather, they exploit the chaos and waste that humans generate.

This raises questions about the human desire to eradicate rats. Despite their success in chaotic environments, Byers notes that rats can establish stable colonies. They typically live in tight family groups confined to single city blocks and rarely overlap with neighboring families. The Rat Project hypothesizes that when pest control displaces a rat, it may cause its family to disperse, potentially spreading diseases that were previously contained.

The diseases carried by rats are not limited to those they harbor. Himsworth often compares Vancouver's rats to sponges, as their waste-based diets allow them to absorb a variety of bacteria from their surroundings. "The harmful bacteria are not inherent to the rats," she explains. They acquire these pathogens from their environment and carry them when they migrate.

When unfamiliar rats meet, territorial conflicts can arise. "They may urinate due to fear and injure each other," Byers notes, facilitating the transfer of more bacteria. During these territorial disputes, bacteria can mix and create new strains of disease. "The rat gut acts like a mixing bowl," Himsworth explains, allowing previously unconnected bacteria to exchange genetic material.

One alarming discovery was a strain of methicillin-resistant Staphylococcus aureus (MRSA) found in V6A's rats, which contained genetic material from another superbug associated with domestic animals. This indicates that rats may be picking up MRSA from urban environments and mixing it with bacteria they encounter from pets.

In V6A, the litter is evident. Trash spills out from bins, and discarded syringes surround one of the traps. Byers, along with two student assistants, is now in the second phase of the project, tracking rat movements in real-time using ear tags. Once mapped, she will begin euthanizing some rats to study how their families respond. This part of her research aims to understand how human-induced disruptions, especially pest control, affect rat movements in V6A.

A significant finding from the initial phase is that not all rats in V6A harbor the same diseases. Rat families tend to be localized to specific blocks, and while one block may be entirely infected, adjacent blocks can be disease-free. "The risk of disease is more about which family you come into contact with than the sheer number of rats," notes Robbin Lindsay, a researcher from Canada's National Microbiology Lab.

If true, city rat policies should consider an unconventional approach: leaving local populations intact. "Maintaining established rat communities that have reached some equilibrium with local residents could be beneficial," Aplin suggests. Many diseases shared with rats have been part of human health cycles for centuries. Viewed this way, rats are "an unstoppable force of nature, an undeniable aspect of our lives." Instead of pursuing eradication, the focus should shift to stabilizing their populations and managing new arrivals.

Established rat communities in a neighborhood can deter newcomers, especially those arriving through shipping ports. Exotic rats pose a greater risk than local populations, as each community has adapted to its specific pathogens. The introduction of new rats raises concerns about the emergence of new diseases. "What happens when these diverse pathogens interact?" Aplin wonders, emphasizing that actively reducing local rat populations may open the door for foreign rats and their diseases.

In Vancouver, this reality is unavoidable. "One crucial aspect we have is Canada’s largest shipping port," Byers points out, indicating the Port of Vancouver. Himsworth previously discovered mites on rats near the port that were not found in other areas, indicating that the local rat population may act as a buffer against these exotic species.

Disruption is inherent to urban life. Vancouver's population is growing rapidly, leading to increased construction and waste. Even well-intentioned efforts, like feeding birds, can attract rats. For instance, a community garden in East Vancouver became infested after rats were lured by dropped birdseed, earning it the nickname "Rat Park." The city responded by urging residents to avoid feeding birds and hired exterminators, perpetuating further disruption.

Himsworth hopes that her research will influence Vancouver's rat policies, which she describes as "essentially nonexistent." This lack of attention troubles her. "Vancouver Coastal Health seems to operate under the assumption that since they don't see diseases in people, they aren't an issue," she laments. While residents can report rat infestations, this reactionary approach does not address prevention. "Rats are pests, and we don’t allocate healthcare resources to tracking them," explains a spokesperson from Vancouver Coastal Health. Himsworth believes this perspective is shortsighted, particularly since rats typically infest poorer neighborhoods like V6A, which lack political influence.

Inside one of Byers' traps in V6A, a rat is identified. "It's a black rat," she notes, aware of its historical association with the Black Death. Although she doesn't worry about bubonic plague, which is primarily linked to prairie dogs in North America, she acknowledges that rats have been implicated in outbreaks globally. "New diseases are constantly evolving," she warns.

Becca Cudmore is a science journalist.