Marissa Milstein: Preventing ’spillover’ in the Amazon

From plague and rabies to Lyme disease and Ebola, nasty pathogens have a long history of jumping from their usual animal hosts to people. A future “spillover” could spark a pandemic to rival COVID-19.

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One area at risk for this is the vast Amazon rainforest, which harbors countless species—including diverse wildlife, fish, and even domestic dogs—that could pick up a disease and pass it to humans. That’s a scenario U of M-trained wildlife veterinarian Marissa Milstein is out to short-circuit. She works on emerging infectious diseases at the interface of wildlife and an Indigenous community of Waiwai people, in the far south of Guyana.

“My interest is in looking at pathogen spillover from the hunting and consumption of wildlife,” says Milstein, who is now studying for a PhD in veterinary population medicine in the College of Veterinary Medicine (CVM).

“Millions of tons of wild meat are extracted every year from tropical rainforests,

and wild meat is incredibly important for the subsistence and food security of Indigenous Amazonians.”

Hers is one of relatively few such studies in the neotropics; most current data comes from Africa and Asia. But given the Amazon’s great size and the fact that much of the Brazilian forest is being destroyed for cattle grazing, soybean farming, dam construction, and mining, “this stable ecosystem is being knocked off balance,” she says. Her remote study area in Guyana, however, has not suffered these disturbances.

And as the holder of a master’s degree in biological anthropology (from Washington University, St. Louis), Milstein takes an ethnographic approach.

“Something that’s really important to me, especially as I work with Indigenous and historically marginalized populations, is to be an advocate and ally for the communities that I work with,” she says.

Doing as the Waiwai do

How or whether a disease organism might jump depends a lot on how people go about their activities, especially those that involve food. So Milstein uses interviews and “participant observation”—joining in food-related activities—to gain insight into the Waiwai’s cultural background.

She collects tissue samples from hunter-harvested wildlife and has trained hunters to take samples themselves. Analyzing the samples, Milstein has found, for example, preliminary evidence for the pathogens that cause Chagas disease and toxoplasmosis. Those are endemic diseases in the area and major public health concerns, so it’s important to know what’s circulating in the region.

“There’s also the opportunity for the discovery of pathogens we didn’t know about,” Milstein explains. But, she adds, finding a pathogen doesn’t mean it will cause problems.

Since the Waiwai also depend on fish, Milstein has collected tissue samples for future examination. And she looks forward to collecting and analyzing mosquitoes.

The culture connection

By studying Waiwai customs, Milstein adds another dimension to her work to promote health. For instance, she looks at how they handle the eight species of monkeys and the large rodents that are key to their subsistence way of life.

“The Waiwai have cultural mechanisms to prevent disease," she says. "They always wash their hands after butchering, and they avoid animals that are acting strangely or have a lot of ectoparasites."

A scrawny game animal is also a bad sign.

“The project I’m involved in serves as a good demonstration site for the concept of One Health, and we need more examples of this type of work to see how One Health can be practiced on the ground,” Milstein says.

Going to the dogs

For her baseline study, Milstein also researches domestic dogs in the village and how they may serve as bridges for pathogen transmission from wildlife to the human population.

Milstein and her dog — *Milstein with her dog, Miles*

“While I was living in the village," she recalls, "I would see consistently that no matter where the Waiwai discarded the entrails of a butchered animal, domestic dogs would always find the entrails and consume them, which could be a way that dogs are exposed to wildlife pathogens.”

Surveying the Waiwai, Milstein and her colleagues found that they take good care of the dogs.

“The Waiwai regularly pick up their dogs' feces and remove ectoparasites like fleas and ticks, and all dogs are named and fed,” she says. “The Waiwai traditionally have been known across Amazonia for having the best hunting dogs. The dogs were traded throughout Amazonia among different Indigenous groups—and still are sometimes—so culturally, dogs are super important for the Waiwai.”

There’s plenty of room for young researchers in work like hers, says Milstein, who works with “the best advisers”: CVM Assistant Professor Tiffany Wolf and Assistant Professor Peter Larsen.

“Whether it’s through traditional lab-based methods, or if they’re interested in understanding the social/cultural context of how diseases get spread … there’s a ton of scientific discoveries and different avenues that people can get involved in,” Milstein says.