From Animal Hosts to Humans
Researcher studies how diseases jump from other creatures to people
By Raequel Roberts
With its doe-like eyes, soft ears and furry white belly, the diminutive deer mouse has a cartoon quality that puts it closer to gerbil than rat on the rodent scale of adorability.
UM disease ecologist Angela Luis knows deer mice well, having spent countless hours studying these tiny rodents in fields across western Montana to learn about environmental impacts on their lifecycle and, more importantly, how they carry — or serve as the reservoir for –Sin Nombre hantavirus, which causes hantavirus pulmonary syndrome.
This respiratory disease, while not a player on the same pandemic stage as COVID-19, is one of the most serious illnesses carried by rodents, killing between 30% to 40% of humans who contract it.
“We have a high incidence of the virus here in mice, but we don’t see as much disease because Montana’s human population is so small,” Luis says, noting that up to 20% of deer mice carry the virus and that several Montanans contract it each year. “Still it is definitely an emerging disease.”
Deer mice, then, may be cute, but they are no Mickey Mouse.
Luis and her research team at UM’s Population and Disease Ecology Lab focus on the mechanisms of host ecology in determining disease dynamics in wildlife and the transmission of diseases between species, including humans. Such information can be a critical component to predicting when an outbreak will occur and ultimately mitigating its spread or stopping it altogether.
Although deep into hantavirus research, Luis has spent a significant amount of 2020 lending her experience and expertise to discussions of COVID-19, appearing in newspaper articles and on webinars.
“I was teaching a disease ecology class this spring when we had to switch to remote instruction. My students were certainly engaged,” says Luis of the unique circumstances of teaching a subject making front-page news.
The COVID-19 pandemic may have caught governments and the public off guard, she adds, but it came as no surprise to disease ecologists.
“And, it will not be the last pandemic in my in my lifetime,” Luis says.
The reasons for this are many.
Bacteria, fungi and viruses exist everywhere and are a natural part of our environment –sometimes for the better (the aging of cheese and fermenting of beer) and sometimes for the worse (Ebola, MERS, HIV/AIDS, COVID-19).
In the case of viruses, the switch — or mutation — from harmless to deadly, and the mechanisms for how this happens, are at the crux of research taking place in the bowels of bat caves, remote jungles, Montana fields and research facilities around the world. The advent of COVID-19 has given this often-unheralded work a global platform.
Viruses are slippery subjects, and much remains to be learned about their modus operandi, but scientists comfortably estimate that between 60% to 70% of emerging diseases in humans come from wildlife. This spillover to humans is called zoonosis. Those viruses that make the leap share similar traits, including the ability to mutate quickly, adapt to a wide range of animal hosts (like bats, pigs, cows, camels or rodents) and spread easily through saliva, fecal droppings or blood.
And when they make the leap, even the most effective virus — one that penetrates host cells and highjacks its genetic material to replicate — can have widely ranging impacts on its host, from no symptoms to imminent death.
While animal-to-human spread is a millennia-old phenomenon, the emergence of zoonotic diseases has accelerated during the past few decades as humans clear forests for farmland, recreate in remote areas, and hunt and trade wildlife for food and other resources. The impact of our ever-increasing interactions with nature and the destruction to biodiversity that often results drives much of Luis’ research.
“Montana is great place to do this research because we have some of the most intact ecosystems in the country,” Luis says.
As you might expect, biodiversity, overall, is a good thing. Luis points to Lyme disease as the perfect example — one that, in this case, involves another rodent, the white-footed mouse. It serves as the reservoir species for Borrelia burgdorferi, the bacterium that causes Lyme disease. The dreaded deer tick transports the virus to humans in its bite.
“These mice grow the bacteria really well,” says Luis, “and if a tick feeds on a mouse that has the bacteria and then feeds on another mouse, it spreads the bacteria. If the population in the area is diverse and the tick feeds on a raccoon or a vole, the virus doesn’t spread. So, with Lyme disease, biodiversity leads to less disease.”
But increased animal density also can stress animals, weakening their immune systems and leading to more disease in animals. Deer mice, Luis says, transmit the hantavirus through grooming and biting. A stressed mouse, fighting for food, may be more likely to bite and more likely to spread a virus or bacteria.
This interplay, then, between density and biodiversity is a dynamic and delicate balance, one that Luis finds fascinating.
“Some researchers spend their whole career studying two genes,” she says. “I like the bigger picture. How are diseases transmitted? How do we stop them? How do we prevent them?”
Luis now works with the Commission on Ecosystem Management, part of the International Union for Conservation of Nature, studying sustainable management, conservation and ecosystem restoration. Established in 1948, IUCN is an organization of scientists, government agencies and NGOs working in the field of nature conservation on a variety of fronts — from water resources to environmental law.
Luis and collaborators are interested in how loss of biodiversity, for example from habitat loss, can lead to new diseases emerging from wildlife. “And, are there habitats that are already damaged that we can restore and possibly reduce viruses?” she says.
Cultural issues, too, must be addressed, she says, if we want to preserve the environment and slow the transmission of diseases from animals to humans.
“We can’t just say let’s get rid of wet markets because they’ll just turn into black markets,” Luis says, referring to the suspected arena of COVID-19 outbreak in Wuhan, China.
It’s important in any discussion of zoonosis, Luis says, not to vilify animals who carry disease viruses, but to understand how the viruses they carry spill over to humans. Bats, for example, are prestigious in their ability to harbor viruses including, many scientists suspect, the coronavirus that causes COVID-19.
“Bats are fascinating creatures and are really important in the ecosystem. They eat a lot of insects, they pollinate tequila (agave) plants, they contribute to our economy,” says Luis, who has also done extensive research on bats. “But they also carry disease. So, we have a lot to learn from them and how their immune systems work.”
And about those deer mice? There is a lot to learn from them too, Luis says, “And, yes, they are very cute.” •
