Coronavirus won't be the last outbreak. Climate change, waning habitat will drive disease

Researchers aren't clear yet about how the novel coronavirus began infecting people, but say it seems to have originated in bats, making it the latest in a string of emerging human disease that spread from animals.

Disease and wildlife experts say they expect to see a rise in diseases transmitted among animals, plants and humans, and in particular, more frequent outbreaks in humans of diseases that come from animals. Such "zoonotic" diseases already make up the majority of new infections in humans.

But animals aren't to blame for the increase in new outbreaks, according to Arizona State University virologist Arvind Varsani. Humans are.

Varsani researches the emergence of diseases around the globe and he said two major factors likely to impact the dynamics of virus spread are climate change and changes in land use due to urbanization or agriculture.

Both factors put animals under strain, forcing them to compete for resources in shrinking habitats and making them opportune hosts for zoonotic diseases, which more easily infect stressed animals. Shrinking suitable habitat amid urbanization and the rapid expansion of agriculture also cluster animals closer together with humans and domestic animals. That allows the viruses to jump from one host to the next.

“You stress these animals and that means you’re creating an environment for viruses to succeed, where in most cases the viruses would’ve been put to check," he said. "And with that, everyone is in very, very close proximity to each other, so what you’ve got is one event — and you only need one event to take off — and the next thing you know, it's spilling over.”

And sometimes, the viruses spill over into humans.

According to a 2016 report from the United Nations Environment Programme, around 60 percent of all infectious diseases in humans are zoonotic, as well as 75 percent of all emerging infectious diseases. Researchers also detected an increase in the rate of new, emerging infectious diseases, according to the report.

In the case of the novel coronavirus, the source is believed to be a market in Wuhan, China, that sold both dead and live animals. Arizona Game and Fish wildlife veterinarian Anne Justice-Allen said that veterinarians and disease experts strongly recommend against having markets like these, which can be densely packed, difficult to keep clean, and pose a higher risk of viruses jumping from animals into humans.

She said the current COVID-19 outbreak is an example of how human actions to mix together animals that weren't normally together can cause diseases to jump into new hosts.

"The more we increase the interaction between people and animals, especially wildlife and especially livestock as well, or animals that are raised for food, the more likely we are to see that kind of situation crop up," she said.

The COVID-19 outbreak is an example of the risk zoonotic diseases pose. Other well-known examples of zoonotic diseases that impact humans are West Nile Virus, HIV, Ebola, Zika, bird flu, Middle East respiratory syndrome, or MERS, Rift Valley fever, and sudden acute respiratory syndrome, or SARS. On average, one new infectious disease emerges in humans every four months, according to the UNEP report, but not all are this severe.

“While it is very uncommon for diseases in animals to suddenly jump and become so transmissible between people as this one did, the more we do that sort of thing, mixing animals together ... then the more likely that disease is going to be able to jump into people and become transmissible between people," Justice-Allen said.

With climate change effects such as drought, increasing temperatures and wildfires, Justice-Allen said animals will be forced to move to new areas. That would lead to the same type of crowding and mixing seen in live animal markets and possibly expose animals to new diseases.

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Warming helps disease-carriers like mosquitos, ticks

Climate change affects human exposure, too.

Warming global temperatures are increasing the habitat ranges of some insects that transmit diseases from one organism to another, such as mosquitoes or ticks. Warming is also lengthening the season in which these insects can survive in some areas, leading to a boom in their population. In certain areas, these insects, also known as disease vectors, are now able to survive year-round where they weren't able to before.

The increased range and abundance of vectors could lead to more widespread cases of mosquito- and tick-borne diseases such as malaria, West Nile Virus, dengue fever, chikungunya, Zika, and Lyme disease, according to Heidi Brown, an ASU researcher who studies vector-borne diseases.

Brown is one of the researchers who works with the Arizona Department of Health Services on a Building Resilience Against Climate Effects plan, which investigates the impacts of climate change on human health, projects future scenarios and suggests solutions to mitigate those effects. Arizona is one of 16 state grant recipients creating a BRACE plan as part of the CDC's Climate-Ready States and Cities Initiative.

"We have to be prepared for the known, which is increases in the stuff that we're already seeing, but we also have to be prepared for the unknown, which is novel introductions," she said.

In Arizona, known diseases such as West Nile Virus are likely to become a bigger concern, and may spread into new areas of the state outside of Maricopa County, Brown said.

"Flagstaff doesn't typically think about or worry too much about West Nile Virus, and we don't usually have a lot of cases up there, but there's evidence that suggests that we might need to start making sure that we're watching for it," she said.

Arizona is also worried about other mosquito-born diseases like chikungunya and dengue fever, which aren't currently part of Arizona's ecosystem but could become permanently established in the future.

Factors like drought can also cause vectors to cluster around water sources alongside of animals, leading to further mixing and further possibilities for new diseases spread in vectors.

Domestic animals act as 'disease bridge'

Another bridge between humans and animal diseases is domestic animals, according to Bernard Bett, a scientist at International Livestock Research Institute.

Around the world, Bett said agriculture is increasingly encroaching on wild lands, bringing humans and domestic animals closer to wild ones and disturbing the natural ecosystem. But domestic animals have been bred in such a way that they lack the genetic diversity needed for disease resilience and can then pass the infection on to humans.

One example is the bird flu, which moved from wild birds into domestic poultry before being passed to humans.

Demand for milk and meat "driven mainly by fast-growing populations of urban consumers in developing countries," is projected to double by 2050, according to the UNEP report, meaning that the problem is likely to get worse as agricultural demand expands.

Urbanization also can encroach on wild lands, Varsani said, and that means humans are coming into increased contact with animals — creating another potential avenue for disease transmission.

Outbreaks could eradicate some species

Increased zoonotic disease spread could also have devastating effects on animals, especially those already at risk from other climate change effects.

“We have a number of endangered species here in Arizona which are kind of on the edge when it comes to being able to respond to new stressors," Justice-Allen said.

One is the Mount Graham red squirrel, whose habitat was severely damaged by a wildfire in 2017. While the population seems to be recovering, Justice-Allen said, "another stressor might tip them over the edge."

Other populations on the brink include the California condor, the black-footed ferret, and Arizona's indigenous snakes and amphibians such as frogs. Some are already being challenged by disease.

Black-footed ferrets feed on prairie dogs, but the plague, spread by fleas, can wipe out entire colonies of prairie dogs, decimating the ferrets' food supply. This disease isn't native to North America, currently has a small incident rate in humans and is curable in most cases. But cases of plague could surge in coming years due to climate change, according to Justice-Allen.

"The dynamics of plague outbreaks is changing, or will change," she said. "This spring is setting us up for a pretty significant plague outbreak."

Arizona's frogs and other amphibians are battling a fungal disease called chytridiomycosis, which was first seen in Africa and interferes with the ability of their skin to absorb moisture.

These diseases, along with mosquito- and tick-borne diseases, are just a few known diseases that Arizona experts are monitoring. But there's always the possibility for unknowns — for new diseases to show up unexpectedly — especially in tight living conditions.

In more clustered spaces, Varsani said there's a greater chance for an animal to become infected with two different diseases at the same time, opening the possibility for the birth of chimeras, or mash-ups, of two different, related viruses.

If two viruses are infecting the same cell, when those viruses replicate, they can combine into a new hybrid virus that could jump into new animals that were previously unaffected.

"You mix components and what you’re then doing is you’re allowing for these viral genomes to evolve very rapidly and adapt to a new host," he said.

Varsani also works to find and study unknown viruses in remote areas, such as the polar regions. He has traveled to Antarctica nearly every year since he first visited about seven years ago, and said viruses he studies there may have been around for a while, but were only discovered recently.

One example is a circovirus that has only recently been noticed in penguins and that causes lethal feather loss.

Viruses in the polar regions are just as abundant as they are in other parts of the world, he said, and since the polar regions are expected to see some of the most dramatic changes due to climate change, these regions and their viruses are crucial to understand for the future.

Unknowns pose challenge for preparedness

As seen with the COVID-19 outbreak, vaccines for new diseases take time to develop, as do tests to screen for them.

Arizona and other states have come up with plans to deal with increased outbreaks and have vector-control measures in place, but Brown acknowledged that it can be hard to prepare for new emerging diseases.

"It's a long process to be able to come up with tests and identify new pathogens, especially when they're not the ones you're used to looking for.”

The Arizona Game and Fish Department monitors for new disease emergences by surveilling animal populations and investigating animal deaths to ensure that the cause of death wasn't an unknown disease, according to Justice-Allen.

Bett said health care officials can take other measures to ensure that there is a structured, strong health system in place with contingency plans for disease outbreaks and centers where patients can be isolated or receive supportive treatment.

“I wonder to what extent the U.S. anticipated this spread. If you have good anticipation, that’s a good first line of defense," Bett said.

Even with further study of climate effects on disease, there can be unpredicted challenges in trying to anticipate and address new diseases, said Ben Beard, the deputy director of the CDC’s Division of Vector-Borne Diseases in Fort Collins, Colorado.

"While long term trends in climate change can be modeled, specific weather events or disasters such as hurricanes, floods, or fires are very difficult to predict and may have a big impact on disease outbreaks," he wrote in an email to The Republic.

To prepare and respond to future outbreaks, he said, a strong public health framework that prioritizes public health surveillance, state and local preparedness, and long-term research would be necessary.

In addition to being more conscious of how people affect the environment and existing ecosystems, Varsani said identifying and understanding the viruses present in the world currently can help society better prepare to control them. This is no small feat, considering that viruses are among the most abundant entities on the planet.

"Viruses are understudied," he said. “We have no idea what the next entity will be, nobody expected this outbreak to take place and it's now consumed most of the world in a very short period.”

Amanda Morris covers all things bioscience, which includes health care, technology, new research and the environment. Send her tips, story ideas, or dog memes at amorris@gannett.com and follow her on Twitter @amandamomorris for the latest bioscience updates.

Independent coverage of bioscience in Arizona is supported by a grant from the Flinn Foundation.

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