City rats do not harbor more diseases than their country cousins

In a surprising new study led by Georgetown University, scientists have contradicted the theory that city rats and other urban animals host a greater number of communicable diseases. The research suggests that urban wildlife has less potential to spark the next pandemic than expected. 

More than two years after the COVID-19 pandemic first emerged, scientists are urgently investigating the risks of future pandemics. It has long been assumed that city rats and urban animals are likely to harbor more infectious diseases.

The researchers set out to analyze the pathogens hosted by nearly 3,000 mammal species. They found that urban animals could host roughly ten times as many types of diseases. On the other hand, the experts noted that urban species had been the focus of about 100 more scientific studies.

“There are plenty of reasons to expect urban animals to host more diseases, ranging from their food to their immune systems to their close proximity to humans,” said study lead author Dr. Greg Albery. “We found that urban species do indeed host more diseases than non-urban species, but the reasons for this appear to be largely associated with the way we study the ecology of disease. We’ve looked more at animals in our cities, so we’ve found more of their parasites – and we’ve started to hit diminishing returns.”

Once the sampling bias was accounted for, the researchers were surprised to find that urban animals do not host more viruses than animals in rural areas.

“Stunningly, although urban-adapted species have 10 times as many parasites, more than 100 times as many studies have been published on them,” said Dr. Albery. “When you correct for this bias, they don’t have more human pathogens than expected – meaning that our perception of their novel disease risk has been overinflated by our sampling process.”

The results suggest that city rats and other urban animals are not the disease reservoirs that they were expected to be. However, that doesn’t mean cities are disease-free, said Dr. Albery.

“This probably means that urban animals aren’t hiding as many important novel pathogens as we might think – those pathogens that might cause the next ‘Disease X.’ But they are still incredibly important carriers of many pathogens that we do know about. Rats, raccoons, and rabbits are still good at coexisting alongside us, and they still spread a lot of diseases to humans living in urban areas.”

Study co-author Dr. Colin Carlson said that the study highlights the value of scientific data. “If we take the time to build better datasets, and look more closely at the patterns in them, we might keep overturning long-standing assumptions about who’s at risk from emerging diseases and why.”

According to v the researchers, further research is needed to explore how many pathogens any given animal has, as well as how city living may influence the transmission of those diseases. 

“What this really accentuates is that we need to design more evenly distributed, more equitable sampling regimes if we want to find novel pathogens of humans; sampling needs to be more focussed in wild areas of the world, but also in urban areas in less well-studied places,” said Dr. Albery. “Not only will this help us to find the new ones, but will improve surveillance of the old ones, and will ultimately help to address ages-old geographic biases in ecology.”

The study is published in the journal Nature Ecology & Evolution.

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By Chrissy Sexton, Earth.com Staff Writer