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The sicker a rat is, the more likely it is to carry bacteria that can cause disease in people.

That’s the conclusion of a led by Dr. Jamie Rothenburger, PhD, an assistant professor at the ��ݮ����Ƶ���� Faculty of Veterinary Medicine (UCVM) and a board-certified veterinary pathologist.

“Wild urban rats carry a number of bacteria that can cause disease in people, but these same bacteria do not typically cause illness in the rats themselves,” said Rothenburger. “That is why they are so good at harbouring and spreading these pathogens.”

The study’s findings, published in Zoonoses and Public Health, add to the complex understanding of zoonotic disease — those illnesses caused by infections that spread between animals and people.

Hundreds of inner-city rats studied

A team of researchers with the sampled hundreds of Norway rats from Vancouver’s Downtown Eastside, an impoverished neighbourhood whose residents face many challenges in addition to rats, including high rates of homelessness, intravenous drug use, and poverty.

As part of this larger study, Rothenburger documented signs of illness in each rat during a detailed autopsy examination. “If the rat had so much of a toenail out of place, we knew about it,” Rothenburger says.

An summarized the diseases found in these wild rats, including respiratory infections, heart disease, parasitic worms, and traumatic wounds. Rats were also tested for a number of bacteria that can cause illness in people. Using this data, Rothenburger analyzed whether unhealthy rats are more likely to carry germs that can make people sick.

“We found that rats with severe illnesses, from heart disease and respiratory infections to major lesions such as tumors and abscesses or broken bones, and those with bite wounds, were more likely to carry Leptospira bacteria,” she says. 

Leptospira is a bacterium that can cause a range of illnesses in people from mild fever to a fatal systemic condition known as Weil’s Disease. Rats and other animals can carry the bacteria in their kidneys without any overt signs of illness and pass the bacteria in their urine. People can be infected when they’re in contact with urine-contaminated water.

“Our results suggest that rats that are sick may be at higher risk of spreading these bacteria in the environment and possibly to people,” says Rothenburger, who conducted the research during her PhD studies at the University of Guelph.

First study of its kind to relate a rat’s health to its risk to people

The researchers also found that rats with parasitic worms were more likely to be infected with Bartonella bacteria. These blood-borne bacteria are transmitted by fleas and are associated with a variety of illnesses in people that can include fever, muscle pain, and heart inflammation. “It is possible that rats are picking up Bartonella and worms in similar environments,” says Rothenburger.  “The worms could also modify the immune system to increase Bartonella infections.” The data showed few rats were carrying both Leptospira and Bartonella pathogens, which is consistent with previous studies.

While other studies have looked at factors such as pregnancy and stress related to pathogens in species such as bats, Rothenburger’s is the first study of its kind to relate the relative health status of a rat to the risk it poses to people.

In an era where three out of every four new or emerging infectious diseases in people are spread from animals, as is the case with Ebola and avian influenza, understanding the factors that drive spillover infections in people is increasingly important. These factors are complex and challenging to study, making wild rats an ideal model species to investigate.