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Black Death aside, we know surprisingly little about rodents and disease

Banner image of a rat by Rod Wilson via Flickr.

  • Rodents make up 40% of all mammal species on the planet, and an estimated 10.7% of them are known hosts of zoonotic diseases, such as cat scratch disease, bartonella, hantavirus, Lyme disease, leishmaniasis, leptospirosis, and the plague.
  • A recent letter in the journal Conservation Biology calls for more attention and funding to be directed toward studying small rodents, “the wildlife species most likely to be abundant, come into contact with humans, and be potential reservoirs in future zoonotic outbreaks.”
  • Controlling and mitigating the risk of zoonotic diseases through rodent control is another area that lacks research, with the current approach of killing and poisoning rodents in urban areas actually posing the risk of causing more disease.
  • Experts call for evidence-based, whole-system approaches to control rodents and champion the One Health approach to address zoonotic disease, acknowledging that human, environmental, and animal health are all interconnected.

Rodents live among us, but we rarely see them. Nondescript, tiny, and often nocturnal, they slip through the cracks of society, largely unnoticed. However, the COVID-19 pandemic has brought the connection between animals and human diseases into the limelight, and because they are the most plentiful and pervasive mammal group on Earth, we may have to reckon with rodents.

An incredible 40% of all mammal species on the planet are rodents, with approximately 2,600 known species — more than just the familiar rats and mice. An estimated 10.7% of those species are known hosts of zoonotic diseases (those that can pass from animals to people) such as cat scratch disease, bartonella, hantavirus, Lyme disease, leishmaniasis, leptospirosis, and the bubonic plague.

A recent laboratory study found that SARS-CoV-2, the virus that causes COVID-19, was able to infect mouse cells and replicate. In fact, viral richness in rodents is much higher than in their oft-feared cousin, the bat.

Most people are familiar with mice and rats, but there are more than 2,600 species of small rodents. Image of a wood mouse by Hanna Knutsson via Flickr (CC BY-NC-ND 2.0).

At the same time, rodents are immensely important to the entire web of life. They disperse seeds, fertilize the soil, and are critical components in the food chain for predatory birds, reptiles and many other mammals.

“They’re a key part of the functioning of all global terrestrial ecosystems,” Thomas Lacher, professor emeritus in the Department of Ecology and Conservation Biology at Texas A&M University, told Mongabay. “They are the abundant organism everywhere.”

“We live alongside rodents more than any other wildlife species, and yet, we know so little about them,” Chelsea Himsworth, regional director of the Canadian Wildlife Health Cooperative, told Mongabay. “I think people assume because we’ve lived with them for so long that we must know about them … There’s sort of a complacency.”

A recent letter by Lacher and colleagues in the journal Conservation Biology makes the case for less complacency and calls for more attention and funding to be directed toward studying the ecology, conservation and zoonotic diseases of small rodents: “the wildlife species most likely to be abundant, come into contact with humans, and be potential reservoirs in future zoonotic outbreaks.”

Understanding the ecology of rodents is not just a matter of scientific interest, Lacher said, “it’s a sort of preventative medicine…We really don’t know many aspects of the basic ecology associated with many small mammals, all of which have potential consequences in terms of their ability to vector diseases.”

In regards to many rodent species, Lacher said, we are still asking basic questions such as: “What are their habitat requirements? How tolerant are they [to] potential human disturbance? What things would result in potential explosive population growth? … If we alter [their] dynamics, does that impede or facilitate the transmission of zoonotic disease?”

“There are just really, really basic things that we don’t know about, you know, 40% of the mammals on the planet,” Lacher said, “which is kind of amazing.”

An Ord’s Kangaroo rat (Dipodomys ordii) in Alberta, Canada. Photo by Andy Teucher via Flickr (CC BY-NC 2.0)..

When it comes to rodent-borne zoonotic diseases, Himsworth said, there are two ways to think about the risks. One is to consider the risk of ongoing, low-level transmission between animals and people. The other is to consider the risk that one pathogen could be spread into the human population and then propagated rapidly, like the viral respiratory illness SARS.

“One of the things we’ve learned from SARS is it only took one transmission event from an animal to human to set off a global pandemic,” Himsworth said. “So the risk of that one event happening is like a needle in a haystack, but then it then precipitated this chain reaction.”

It’s clear rodents can have real consequences for human health, but controlling and mitigating that risk is another area we know little about.

“The question is, what do we do about that health risk? ” Himsworth said. “And that’s when we start to get the crickets chirping … There’s pretty much no empirical research on the interventions that will decrease that risk.”

The current dogma on rodent control seems to be “kill as many as possible.” This is backed by the simple logic that fewer rats or mice mean less disease. But actually, in some cases, the opposite can occur.

“Where you go in and just do our common approach, which is haphazardly trapping, poisoning, and killing members of the colony,” Himsworth said, “for the rats that are left, we go back and look and say ‘Oh, my gosh! They actually have more disease than when we started.’”

From an ecological perspective, she said, this makes sense. Most rodents live in tightly knit family groups with well-established social hierarchies. When you remove animals, that sends everything into chaos. “You’re going to have fighting and mingling and reestablishment of who’s boss, and all of that promotes disease transmission.”

Peppering the landscape with rat poisons, a common method of rodent control, can also have negative consequences for other, non-target animals. Besides, Himsworth said, “I think most people would agree they don’t want their city to be laced with toxic poison.”

The red squirrel (Sciurus vulgaris) is a species of tree squirrel common throughout Eurasia. Image by Zweer de Bruin via Flickr (CC BY-NC-ND 2.0).

We also know from looking at cities around the world and their ongoing rodent problems, despite investments into control, that current approaches aren’t really working. No city is rat-free.

All of this, Himsworth said, points to the idea that we need to be more strategic about rodent management; using whole system and evidence-based approaches, instead of focusing solely on getting rid of them.

A systems-based approach employs experts ranging from biologists to urban planners to architects who ask, “What increases the overall resilience of the system?” This approach takes into account all of the built environment of the city and considers how garbage works, what people are concerned about, who’s most vulnerable, budget, politics, and other strategies that the city is trying to accomplish in order to create healthier, more impactful programs.

For instance, researchers have found that people in impoverished inner-city areas are more at risk of coming into contact with rats. At the same time, they often don’t have the financial resources to do pest management themselves and are more likely to have concurrent health problems, which makes them more susceptible to getting diseases. “So, can we prioritize those specific areas for a campaign to reduce public health risks associated with rats?” Himsworth asks.

For example, many of the factors that make buildings more energy-efficient also make them rodent-proof. When a home is properly sealed to keep out rats and mice, that also seals in the heat, lowering energy bills. This type of approach is a win-win for health and finances.

“Everything is interconnected,” Nikki Roach, a conservation scientist at the new Global Center for Species Survival and co-author of the letter in Conservation Biology, told Mongabay. “It’s not just about bringing in research on the environment, but it’s also peoples’ social status and equity … The more we can get people to understand that we’re interconnected, hopefully, the better off we will be in the future.”

Humans are not only coming into closer contact with rodents on our turf but also on theirs. A recent study found that as people destroy forests and other habitats, the small mammals within them become more stressed out, making them more susceptible to disease. With more humans living in close proximity to animals in forest fragments worldwide, there is “the potential of hotspots for all sorts of emerging and/or zoonotic diseases,” Noé de la Sancha, associate professor of biology at Chicago State University, told Mongabay.

A mouse opossum (Gracilinanus agilis), from a deforested area of the Atlantic Forest in eastern Paraguay. Shrinking habitats can stress out small mammals making them more likely to catch diseases. Image © Noé U. de la Sancha (2021).

“The future is going to likely be unpredictable,” Himsworth said. “But wherever there are disruptions, rats will capitalize on those opportunities … You just look to Hurricane Katrina or a garbage strike or a homeless encampment or anything where services are disrupted, and you suddenly get a buildup of refuse and more places for rats to live … Rats capitalize on chaos.”

As the interface between humans and animals increases, we can’t be sure what potential threats that might pose to human health, Roach said. And because humans can also pass diseases on to animals, we don’t know what effects we might have on them.

“I think that line is becoming so much smaller,” Roach said, “and we’re crossing it so much more frequently, and that’s putting us in a lot of risk for potential future outbreaks and pandemics.”

All of this underscores the importance of turning to new approaches, both Lacher and la Sancha say, such as the One Health approach that acknowledges that human, environmental, and animal health are interconnected. One Health has been gaining momentum among health professionals, veterinarians, scientists and policymakers, especially in light of the COVID-19 pandemic, and may serve as a breeding ground for more systems-based approaches and ideas.

But regardless of the approach we take, Himsworth said, “We’re never going to get rid of rodents. We’re going to have to learn to live with them.”

Citations:

Lacher Jr., T. E., Kennerley, R., Long, B., McCay, S., Roach, N. S., Turvey, S. T., & Young, R. P. (2021). Support for rodent ecology and conservation to advance zoonotic disease research. Conservation Biology35(4), 1061-1062. doi:10.1111/cobi.13763

Rothenburger, J. L., Himsworth, C. H., Nemeth, N. M., Pearl, D. L., & Jardine, C. M. (2017). Environmental factors and zoonotic pathogen ecology in urban exploiter species. EcoHealth14(3), 630-641. doi:10.1007/s10393-017-1258-5

Banner image of a rat by Rod Wilson via Flickr (CC BY-SA 2.0).

Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter: @lizkimbrough_

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