Zoonotic Diseases

With the growing human population, cities and towns popping up in previously untouched areas, farms and livestock encroaching on wildlife habitats, and rapid climate change, these opportunities abound. The good news is that zoonotic pathogens are making the leap from animals to humans under the spotlight of unprecedented scientific observation.

Some scientists seek to predict the future of outbreaks with sophisticated mathematical tools and programming. Others go directly to outbreak sites to track, and attempt to contain, the spread of the disease. Still others focus on the mechanisms that allow the pathogens to make jumps between seemingly disparate species. All seek to protect humans from the next unknown threat, whenever and wherever it inevitably emerges.

This emergence is a byproduct of contact with infected animals as well as the nature of viral evolution. Most viruses are programmed for survival within a certain host. This includes factors like the virus’ ability to attach to host cells. Like a microscopic lock-and-key mechanism, proteins on the surface of virus particles are shaped in specific ways that allow them to bind with receptors on host cells.

These interactions can be very specific—they determine which cells are bound (infections of cells in the respiratory tract may cause a cough, while those of the stomach can cause nausea and diarrhea) and which organisms the virus can infect.

 If that key changes shape because the virus has mutated, it may be able to open new locks, infect a new host, and cause an outbreak. “How an emerging pathogen spreads through a species tends to be ‘a black box’ until it causes an outbreak among people,” Gillespie says. The Zika virus, for instance, was first identified in monkeys in Uganda in 1947 but was not widely studied until recently, after it started sweeping through human populations.

Fliers and Fast Breeders

Many animal species harbor pathogens— some of which they live with and are largely unaffected by. A common example is bats.

“One quarter of mammal species overall are bats, and each of these myriad bat species carries a suite of different pathogens,” says Gillespie. “Bats are able to host different viruses without getting sick. So bats, and the pathogens that bats carry, are numerous. And bats and humans are both mammals. This relatedness means we’re more likely to get a pathogen from a bat than from a cricket, for instance.”

But bats are far from the only reservoirs of zoonotic pathogens. Influenza virus is housed primarily in aquatic birds, but in 2009 emerged from swine into humans, causing a global pandemic. The “swine flu,” caused by the H1N1 virus strain, which started in pigs killed between 150,000 and 575,000 people worldwide during the first year the virus circulated. Unlike COVID-19, 80 percent of H1N1- virus-related deaths were estimated to have occurred in people younger than 65 years of age—primarily children and young and middle-aged adults.

Rodents are another common host and make particularly good reservoirs, possibly because of their “live fast, die young” strategy.

Quantitative ecologist Sarah Bowden, an adjunct professor at Emory and a data scientist contractor with the Division of Global Migration and Quarantine at the Centers for Disease Control and Prevention, “fast-living species that reproduce quickly and frequently tend to be better reservoirs for pathogens.”

A case in point: Deer mice, which were linked to a hantavirus outbreak in 1993 that caused severe pulmonary disease.

“Previously hantavirus had not been associated with a pulmonary condition,” says infectious disease epidemiologist Robert Breiman, of Emory’s Global Health Institute. “It was a virus that hadn’t been identified before, and it appeared in these deer mice.”

Wild animals that migrate or have large territories also make effective vectors.

“If you cover a broader area, you're inherently more likely to encounter more species you can transmit that pathogen to,” Bowden says.

Zoonosis is a widespread, complex, and multifaceted phenomenon. It does not discriminate between borders, and by nature does not always discriminate between hosts.

Humans, agree scientists, have created conditions for these pathogens to thrive, and rapid human expansion will undoubtedly contribute to the emergence of new diseases.

In some places, it is common to hunt wildlife for food. Rural communities may rely on meat from local forests and savannas—whether chimpanzees, bats, rodents, or other species. These animals can be carriers of diseases, including Ebola, Marburg, HIV/AIDS, and anthrax.

“Anthrax is primarily zoonotic and wouldn’t cause human disease unless there was close contact [between animals and humans],” says Breiman.

Whether an individual is hunting, butchering, or consuming these animals, or is even a child playing near their habitats, this gives pathogens ample opportunity to cross over and wreak havoc.

Public health workers focus on educating and developing safer practices for hunting and butchering.

But domesticated and farm animals can transmit disease as well. Pigs, chickens, turkeys, and waterfowl transmit swine and avian flus. Cows and camels play host to bacterial species like Brucella.

“In parts of East Africa, people really love unpasteurized milk from both cows and camels,” said Breiman, who spent nine years in Kenya as an epidemiologist. “Both animals, if infected with Brucella, can excrete it into the milk,” which can infect those who drink it.

To add to the problem, livestock rarely show symptoms of infection with Brucella so it’s hard to know that the milk poses a risk.

Even if an animal appears sick, that is no guarantee it won’t be consumed. “There will be all sorts of clues that an animal was sick and died, but [people] will go ahead and eat it because it’s nutrition and it costs money,” Breiman says.

In “wet markets” around the world, animals alive and dead, including wild and exotic species, can be purchased for consumption or for use in traditional medicines.

 Civet cat, for instance, is a delicacy in some parts of China. “I was told people would eat it in the winter because they felt it increased their immunity to respiratory disease,” Breiman says.

The scales of the pangolin—an armadillo-like animal also sold in “wet markets” that may have played a role in the emergence of the new SARS-CoV-2 coronavirus in humans—are used in medicines. “The meat is thought to have all sorts of benefits,” Breiman says.

Animals in these markets are caged next to each other, which allows them to share pathogens. In the wild, many of these animals would never be close enough for this transfer to occur. The adaptations that happen during infection of a new host can have the unwanted side effect of making them able to infect people, too.

Humans have created an environment in which this is possible, and multiple outbreaks have occurred as a result. The most recent is the 2019 coronavirus.

Connectivity and Climate Change

Sometimes, however, our encounters with animals are unintentional—a result of rapid human expansion and the resultant narrowing of animal habitats.

“We are in an extremely connected world, a world that's vastly more connected than it was even back in the early 2000s when the SARS epidemic happened,” Bowden says. “That is just going to inherently increase the likelihood of new people coming into contact with new things that can make them sick.”

While there are many species known to transmit diseases to humans, there are plenty that we haven’t encountered yet. That is changing as we travel more and expand our reach into untouched areas with remote species and pathogens.

“Things are already there and primed to go, and the storm sort of comes together at the perfect moment,” Bowden says.

But that doesn’t stop researchers from trying to predict outbreak likelihoods from what we do know. Researchers like Bowden have adapted mathematical modeling techniques to better understand where risk of an outbreak lies. “Prediction allows us to look at the relative risk or the likelihood of a pathogen emerging in different areas of the world,” she says.

She looks at different factors—the risk landscape on a spatial scale (where on a map is infection most likely), different known reservoirs (which animals are common carriers, and where are they located), and different pathogens (what germs/diseases are commonly found in an area).

“We can drill through these and look for places where risk is high on all three planes. That allows us to say, ‘Relative to everywhere else in the world, given the large players in a disease outbreak, here’s where we think the risk is highest.’ ”

Many of these factors are changing rapidly, however, and models will have to change to account for that.

Climate change, Bowden says, is “a huge consideration in disease ecology. It is causing species’ geographic ranges to expand or shift. They can bring new pathogens with them or come into contact with ones they hadn’t encountered before. In both cases, we’re likely to experience unforeseen outbreaks.”

Written by Megan Hockman. Illustrations by Brian Stauffer. Design Peta Westmaas. Editor Mary Loftus