Boston's rats are carrying a deadly disease

The disease-causing bacteria thrive in warm, moist environments and are spread through the urine of infected animals, which can contaminate water or soil. They’re found worldwide, but as temperatures rise due to climate change, they’re becoming increasingly common in cooler areas, including the Northeast, said Marieke Rosenbaum, co-senior author of the study and assistant professor in the Department of Infectious Disease and Global Health at Cummings School of Veterinary Medicine at Tufts University.

Humans can contract the disease when infected water or soil gets into the eyes, nose, mouth, or a break in the skin. Those at highest risk are unsheltered adults, veterinarians, sanitary workers, and farmers. It can be treated with antibiotics when properly diagnosed.

Rosenbaum said it’s hard to say how many people have been infected with the disease in Boston, as many don’t get tested because they are asymptomatic or only receive a diagnosis if their illness is severe. But the study did track one human case of leptospirosis from 2018 in the city and found that it likely originated from a rat. The patient was hospitalized but ultimately recovered.

“There is a need for more awareness about this disease in this area, also among clinicians,” she said. “A lot of times it’s a disease that clinicians will test for in more extreme cases … but not everyone develops such serious illness.”

The paper is part of a larger project out of Tufts dubbed the Boston Urban Rat Study, which focuses on the role rodents play in spreading human diseases. Researchers hope the most recent study, which was published in April, will bring new attention to leptospirosis — one of the most common infections transmitted between humans and animals worldwide but largely considered neglected in terms of research — and give public health experts new ideas for how to control rat infestations.

“It’s a really unique population of people that are at high risk for contact with rats,” Rosenbaum said. “I hope this will raise some awareness about populations that could be targeted [with interventions], like people who are homeless or are engaged in outdoor injection drug use, in particular.”

In January, a study published in the journal Science showed that the rat population in Boston increased by 53 percent between 2011 and 2021, likely due to warming temperatures extending their foraging and breeding period. As climate change continues to intensify, so will the spread of disease, Rosenbaum said.

Ariana Eggleston, the director of rat education for the Rat City Arts Festival, an annual Allston music festival that raises awareness about the neighborhood’s large rodent population, said the results of the study are “very worrying.”

“Not only do you have to be careful, you also have to make sure your dog’s feet are clean, you have to be hypervigilant,” Eggleston said, adding that many residents are “exhausted” from mitigating their risk of catching other infectious diseases like bird flu and COVID-19.

Eggleston said a lot of her work with the festival revolves around educating people in Allston-Brighton — dubbed “Rat City” — about the importance of reporting broken trash cans, overflowing bins, and other items that can attract rats.

That may be the best bet against an expanding rodent population, experts said, even more so than using reactive measures like poison.

Last year, officials launched the “Boston Rodent Action Plan” a multiagency initiative to decrease the city’s rat population, with the help of a leading rodent expert. The strategies include getting rid of rats’ food sources, especially by making sure the public knows how to properly dispose of trash; targeting neighborhoods with the highest concentrations of rats, including downtown, Chinatown, and Allston/Brighton; and using technology to more aggressively seek out and get rid of their hiding places and escape routes.

“There’s a huge need for a concerted effort to sanitation,” Rosenbaum said, adding officials should focus on areas where humans have the most interactions with rodents. “And ultimately, if we restrict their access to food and water, they won’t be able to reproduce, and that will manage their populations.”

Dave Wagner, co-senior author of the study and executive director of the Pathogen and Microbiome Institute at Northern Arizona University, said establishing that local rat populations have their own strains of the disease could improve mitigation techniques.

Using information about distinct strains and populations, researchers tracked several rats that had migrated to other colonies, bringing their local strain of leptospirosis with them. That could mean common disruptions to rat populations, including construction, may lead to “intermingling of rat populations” and increased, not decreased, leptospirosis spread, Rosenbaum said.

“We need to start shifting our approach from, ‘Let’s try and kill them and eradicate them,’ to better understanding how to coexist with them. We’re not ever going to eradicate them,” Rosenbaum said.

Wagner said he hopes the study, which was published in the Public Library of Science of Neglected Tropical Diseases, will help the public appreciate that the disease is becoming more prevalent worldwide.

“It’s something we should be keeping an eye on,” he said.

Emily Spatz can be reached at emily.spatz@globe.com. Follow her on X @emilymspatz.