HQ Team
May 6, 2025: Rats living in urban areas spread a deadly disease called Leptospirosis as they migrate within the cities, according to Tufts University researchers, who also discovered a method to test the kidneys of the mammals.
Leptospirosis is a disease caused by a type of bacteria often found in rats. It’s spread through their urine into soil, water, or elsewhere in the environment, where it becomes a source of infection and contamination for humans, dogs, and other species.
Some people may get infected, not develop any symptoms, and never know they were infected. Others might develop a mild fever or other nonspecific symptoms before their immune system clears the infection.
A small percentage of people will go on to develop a more serious case of the disease, which can affect different organs and, ultimately, cause multi-organ failure and death.
“Human exposure to rats is not very common,” said Marieke Rosenbaum, assistant professor in the Department of Infectious Disease and Global Health at Cummings School of Veterinary Medicine at Tufts University, Rosenbaum.
Unhoused people at high risk
“But certain populations might be at higher risk, like unhoused people or people who engage in outdoor injection drug use, which are situations that lead to more direct contact with rats,” she said.
The researchers concentrated on Boston, where leptospirosis persists in local rat populations, and different strains of the bacteria move around the city as groups of rats migrate.
Genetic analysis of a 2018 human leptospirosis case in Boston strongly suggested a link to rats as the source, according to the study that tracked rats between 2016 to 2022.
The researchers analysed DNA from 328 rat kidney samples collected from 17 sites in Boston, and 59 rats representing 12 of 17 sites were positive for Leptospira bacteria, which causes Leptospirosis.
“The primary way to get a full genomic sequence of a virus or bacteria is to culture it, which was a challenge in this case because Leptospira is considered a fastidious organism,” Rosenbaum said.
‘Frozen rodent kidneys’
“It has specific requirements for temperature, pH, and nutrients. But our United States Department of Agriculture collaborators cultured the bacteria from not only fresh but also frozen rodent kidneys, which has never been reported in the literature before, to get isolates,” she said.
The Northern Arizona University collaborators at the Pathogen & Microbiome Institute (PMI) used targeted DNA capture and amplification to pick out and enhance leptospirosis DNA in the samples, which resulted in a lot of fine-scale genomic information about the isolates.
Dave Wagner, PhD, professor of biological sciences and executive director of the PMI at Northern Arizona University said: “The new genetic, and tools that we developed and used in this study are real game changers for leptospirosis research, as we can now use the power of the whole genome to look for relatedness among samples, something that just wasn’t possible before.”
Leptospirosis disease is prevalent globally, and about one million cases in people occur worldwide each year, with nearly 60,000 deaths.
Humans and environment
“Rats have a high degree of genetic structure, which means there are distinct rat populations throughout the city that are highly related to each other,” Rosenbaum said.
“It doesn’t look like they’re intermingling with other populations a lot, and that’s contributing to a stable population over time, but when they do disperse, they can take leptospirosis with them.”
When it comes to pest control, it is important to understand how pest management interventions impact rat migrations and their population structure, as well as how they impact humans and the environment, Rosenbaum said.
The researchers hope their findings will help guide rat control and human leptospirosis mitigation efforts in urban settings. The study was published in the journal PLOS Neglected Tropical Diseases.
