As more and more forest is cleared around the world, scientists fear that the next deadly pandemic could emerge from what lives within them.
By Katarina Zimmer - 22.
In 1997, clouds of smoke hung over the rainforests of Indonesia as an area roughly the size of Pennsylvania was burned to make way for agriculture, the fires exacerbated by drought. Smothered in haze, the trees couldn’t produce fruit, leaving resident fruit bats with no other option than to fly elsewhere in search of food, carrying with them a deadly disease.
Not long after the bats settled on trees in Malaysian orchards, pigs around them started to fall sick—presumably after eating fallen fruit the bats had nibbled on—as did local pig farmers. By 1999, 265 people had developed a severe brain inflammation, and 105 had died. It was the first known emergence of Nipah virus in people, which has since caused a string of recurrent outbreaks across Southeast Asia.
It’s one of many infectious diseases usually confined to wildlife that have spilled over to people in areas undergoing rapid forest clearing. Over the past two decades, a growing body of scientific evidence suggests that deforestation, by triggering a complex cascade of events, creates the conditions for a range of deadly pathogens—such as Nipah and Lassa viruses, and the parasites that cause malaria and Lyme disease—to spread to people.
As widespread burning continues today in tropical forests in the Amazon, and some parts of Africa and Southeast Asia, experts have expressed concern about the health of people living at the frontiers of deforestation. They’re also afraid that the next serious pandemic could emerge from our world’s forests.
“It’s pretty well established that deforestation can be a strong driver of infectious disease transmission,” says Andy MacDonald, a disease ecologist at the Earth Research Institute of the University of California, Santa Barbara. “It’s a numbers game: The more we degrade and clear forest habitats, the more likely it is that we’re going to find ourselves in these situations where epidemics of infectious diseases occur.”
A direct link
Malaria—which kills over a million annually due to infection by Plasmodium parasites transmitted by mosquitoes—has long been suspected of going hand in hand with deforestation. In Brazil, while control efforts have dramatically reduced malaria transmission in the past—bringing 6 million cases a year in the 1940s down to just 50,000 by the 1960s—cases have since been steadily rising again in parallel with rapid forest clearing and expansion of agriculture. At the turn of the century, there were over 600,000 cases a year in the Amazon basin.
Work in the late 1990s by Amy Vittor, an epidemiologist at the University of Florida’s Emerging Pathogens Institute, and others, suggested a reason why. Clearing patches of forest appears to create ideal habitat along forest edges for the mosquito Anopheles darlingi—the most important transmitter of malaria in the Amazon—to breed. Through careful surveys in the Peruvian Amazon, she found higher numbers of larvae in warm, partially shaded pools, the kind that form beside roads cut into forests and puddles behind debris where water is no longer taken up by trees.
“Those were the [places] that Anopheles darlingi really enjoyed being,” Vittor recalls.
A man sprays to kill the Aedes mosquito that carries the yellow fever virus in Matadi, Democratic Republic of the Congo. Photograph by William Daniels, Nat Geo Image Collection
In a complex analysis of satellite and health data published recently in the journal Proceedings of the National Academy of Sciences, MacDonald and Stanford University’s Erin Mordecai reported a significant impact of deforestation across the Amazon basin on malaria transmission, in line with some previous research.
Between 2003 and 2015, on average, they estimated that a 10 percent yearly increase in forest loss led to a 3 percent rise in malaria cases. For example, in one year of the study, an additional 618-square-mile (1,600-square-kilometer) patch of cleared forest—the equivalent of nearly 300,000 football fields—was linked to an additional 10,000 cases of malaria. This effect was most pronounced in the interior of the forest, where some patches of forest are still intact, providing the moist edge habitat that the mosquitoes like.
“I am concerned about what’s going to happen with transmission following the end of the fires,” MacDonald says.
It’s hard to generalize about mosquito ecology, which varies depending on species and region, Vittor stresses. In Africa, studies have found little association between malaria and deforestation—perhaps because the mosquito species there like to breed in sunlit bodies of water and favor open farmland over shady forest areas. But in Sabah, a part of Malaysian Borneo, malaria outbreaks also occur in tandem with bursts of forest clearing for palm oil and other plantations.
Fever from the jungle
Mosquitoes aren’t the only animals that can transmit deadly scourges to people. In fact, 60 percent of new infectious diseases that emerge in people—including HIV, Ebola, and Nipah, all of which originated in forest-dwelling animals—are transmitted by a range of other animals, the vast majority of them wildlife.
In a 2015 study, researchers at Ecohealth Alliance, a New York-based non-profit that tracks infectious diseases globally, and others found that “nearly one in three outbreaks of new and emerging disease[s] are linked to land-use change like deforestation,” the organization’s president Peter Daszak tweeted earlier this year.
Many viruses exist harmlessly with their host animals in forests, because the animals have co-evolved with them. But humans can become unwitting hosts for pathogens when they venture into or change forest habitat.
“We are completely changing the structure of the forest,” notes Carlos Zambrana-Torrelio, a disease ecologist at Ecohealth Alliance.
Diseases can also occur when new habitats draw disease-carrying species out of the forest.
For instance, in Liberia forest clearings for palm oil plantations attract hordes of typically forest-dwelling mice, lured there by the abundance of palm fruit around plantations and settlements. Humans can contract Lassa virus when they come into contact with food or objects contaminated with feces or urine of virus-carrying rodents or bodily fluids of infected people. In humans, the virus causes hemorrhagic fever—the same kind of illness triggered by Ebola virus—and in Liberia killed 36 percent of infected people.
Virus-carrying rodents have also been spotted in deforested areas in Panama, Bolivia, and in Brazil. Alfonso Rodriguez-Morales, a medical researcher and tropical disease expert at Colombia’s Universidad Tecnológica de Pereira, fears that their ranges will increase following the resurgence of fires in the Amazon this year.
Such processes aren’t limited to tropical diseases. Some of MacDonald’s research has revealed a curious association between deforestation and Lyme disease in the Northeastern United States.
Borrelia burgdorferi, the bacterium that causes Lyme disease—is transmitted by ticks that rely on forest-dwelling deer to breed and obtain enough blood to survive. However, the bacterium is also found in the white-footed mouse, which happens to thrive in forests fragmented by human settlements, MacDonald says.
Spillovers of infectious diseases to people is more likely to occur in the tropics because overall wildlife and pathogen diversity is higher, he adds. There, a number of diseases transmitted by a wide range of animals—from blood-sucking bugs to snails—have been linked to deforestation. On top of known diseases, scientists fear that a number of yet-unknown deadly diseases are lurking in forests that could be exposed as people encroach further.
Zambrana-Torrelio notes that the likelihood of spillovers to people may increase as the climate warms, pushing animals, along with the viruses they carry, into regions where they’ve never existed before, he says.
Whether such diseases stay confined to forest fringes or if they gain their own foothold in people, unleashing a potential pandemic, depends on their transmission, Vittor says. Some viruses, like Ebola or Nipah, can be transmitted directly between people, theoretically allowing them to travel around the world as long as there are humans.
Zika virus, which was discovered in Ugandan forests in the 20th century, could only cruise the world and infect millions because it found a host in Aedes aegpti, a mosquito that thrives in urban areas.
“I’d hate to think that another or several other pathogens could do such a thing, but it’d be foolish not to think of that as a possibility to prepare for,” says Vittor.
A new service
Ecohealth Alliance researchers have proposed that containing diseases could be considered a new ecosystem service, that is, a benefit that humans freely gain from natural ecosystems, just like carbon storage and pollination.
To make that case, their team has been working in Malaysian Borneo to itemize the exact cost of malaria, down to each hospital bed, and syringe that doctors use. On average, they found that the Malaysian government spends around $5,000 to treat each new malaria patient in the region—in some areas much more than they spend on malaria control, Zambrana-Torrelio says.
Over time, that adds up, outweighing the profits that could be gained by cutting forests down and making a compelling financial argument to leave some forests standing, Daszak says.
He and his colleagues are beginning work with the Malaysian government to incorporate this into land use planning, and are undertaking a similar project with Liberian officials to calculate the cost of Lassa fever outbreaks there.
MacDonald sees value in this idea: “If we can conserve the environment, then perhaps we can also protect health,” he says. “That I think is the silver lining that we should keep in mind.”
In Borneo, an island shared by Indonesia and Malaysia, some of the world’s oldest tropical forests are being cut down and replaced with oil palm plantations at a breakneck pace. Wiping forests high in biodiversity off the land for monoculture plantations causes numerous environmental problems, from the destruction of wildlife habitat to the rapid release of stored carbon, which contributes to global warming.
But deforestation is having another worrisome effect: an increase in the spread of life-threatening diseases such as malaria and dengue fever. For a host of ecological reasons, the loss of forest can act as an incubator for insect-borne and other infectious diseases that afflict humans. The most recent example came to light this month in the Journal of Emerging Infectious Diseases, with researchers documenting a steep rise in human malaria cases in a region of Malaysian Borneo undergoing rapid deforestation.
This form of the disease was once found mainly in primates called macaques, and scientists from the London School of Tropical Medicine and Hygiene wondered why there was a sudden spike in human cases. Studying satellite maps of where forest was being cut down and where it was left standing, the researchers compared the patchwork to the locations of recent malaria outbreaks. They realized the primates were concentrating in the remaining fragments of forest habitat, possibly increasing disease transmission among their own populations. Then, as humans worked on the new palm plantations, near the recently created forest edges, mosquitoes that thrived in this new habitat carried the disease from macaques to people.
Such phenomena are not uncommon. “In years when there is a lot of land clearance you get a spike in leptospirosis [a potentially fatal bacterial disease] cases, and in malaria and dengue,” says Peter Daszak, the president of Ecohealth Alliance, which is part of a global effort to understand and ameliorate these dynamics. “Deforestation creates ideal habitat for some diseases.”
The Borneo malaria study is the latest piece of a growing body of scientific evidence showing how cutting down large swaths of forests is a major factor in a serious human health problem — the outbreak of some of the world’s most serious infectious diseases that emerge from wildlife and insects in forests. Some 60 percent of the diseases that affect people spend part of their life cycle in wild and domestic animals.
The research work is urgent — land development is rapidly taking place across regions with high biodiversity, and the greater the number of species, the greater the number of diseases, scientists say. They are deeply concerned that the next global pandemic could come out of the forest and spread quickly around the world, as was the case with SARS and Ebola, which both emerged from wild animals.
Mosquitoes are not the only carriers of pathogens from the wild to humans. Bats, primates, and even snails can carry disease, and transmission dynamics change for all of these species following forest clearing, often creating a much greater threat to people.
The risk of disease outbreaks can be greatly magnified after forests are cleared for agriculture and roads.
Throughout human history pathogens have emerged from forests. The Zika virus, for example, which is believed to be causing microencephaly, or smaller than normal heads, in newborns in Latin America, emerged from the Zika forest of Uganda in the 1940s. Dengue, Chikungunya, yellow fever, and some other mosquito-borne pathogens likely also came out of the forests of Africa.
Forests contain numerous pathogens that have been passed back and forth between mosquitoes and mammals for ages. Because they evolved together, these viruses often cause few or no symptoms in their hosts, providing “a protective effect from a homegrown infection,” says Richard Pollack of the T.H. Chan School Public Health at Harvard. But humans often have no such protection.
What research is demonstrating is that because of a complex chain of ecological changes, the risk of disease outbreaks, especially those carried by some mosquitoes, can be greatly magnified after forests are cleared for agriculture and roads.
A flood of sunlight pouring onto the once-shady forest floor, for example, increases water temperatures, which can aid mosquito breeding, explained Amy Vittor, an assistant professor of medicine at the University of Florida. She is an expert in the ecology of deforestation and malaria, which is where this dynamic is best understood.
Deforestation creates other conditions conducive to mosquito breeding. Leaves that once made streams and ponds high in tannins disappear, which lowers the acidity and makes the water more turbid, both of which favor the breeding of some species of mosquito over others. Flowing water is dammed up, deliberately and inadvertently, and pools. Because it is no longer taken up and transpired by trees, the water table rises closer to the forest floor, which can create more swampy areas.
As agriculture replaces forest, “re-growth of low lying vegetation provides a much more suitable environment” for the mosquitoes that carry the malaria parasite, Vittor says.
A man sleeps inside a mosquito net in his home in West Papua, occupied by Indonesia. Ulet Ifansasti/Getty Images
The link between deforestation and increases in malaria has been known for some time, but research in the last two decades has filled in many of the details. Much of the work has been done in Peru, where in one region in the 1990s cases of malaria went from 600 per year to 120,000, just after a road was built into virgin forest and people began clearing land for farms.
The cascade of human-induced ecological changes dramatically reduces mosquito diversity. “The species that survive and become dominant, for reasons that are not well understood, almost always transmit malaria better than the species that had been most abundant in the intact forests,” write Eric Chivian and Aaron Bernstein, public health experts at Harvard Medical School, in their book How Our Health Depends on Biodiversity. “This has been observed essentially everywhere malaria occurs.”
Mosquitoes can adapt fairly quickly to environmental change. In response to a push to use bed nets to prevent nighttime bites in malaria-prone regions of the world, for example, researchers are seeing a change in the time of day mosquitoes bite — many now target their human quarry in the hours before bed.
A study by Vittor and others found that one malaria-carrying mosquito species, Anopheles darlingi, in a deforested area in Peru was radically different than its cousins in intact forests; the Anopheles darlingi in deforested areas bit 278 times more frequently than in an intact forest, according to a study published in the American Journal of Tropical Medicine and Hygiene in 2006.
“In the forest, we found almost no breeding whatsoever, and no biting by the adult mosquitoes,” Vittor said. That’s probably because the ecology of the deforested landscape — short vegetation and deep water — favored their breeding, and they need human blood to grow their eggs.
The types of mosquitoes that do well in this radically altered ecosystem are more “vector competent,” which means their systems are particularly good at manufacturing a lot of the pathogen that causes malaria. A study in Brazil, published in the Journal of Emerging Infectious Diseases in 2010, found that clearing four percent of the forest resulted in a nearly 50-percent increase in human malaria cases.
The ecology of the viruses in deforested areas is different. As forests are cut down, numerous new boundaries, or edges, are created between deforested areas and forest. A mosquito called Aedes africanus, a host of the yellow fever and Chikungaya viruses, often lives in this edge habitat and bites people working or living nearby. Other primates, which are also reservoirs for the pathogens, gather in the borders of these different ecosystems, providing an ongoing source of virus for the insects.
Insects are not the only way that deforestation can exacerbate infectious diseases. For some unknown reason, the species of snails that can better adapt to warm open areas that occur after a forest is cut down are better hosts for parasites called flatworms, some of which cause schistosomiasis, a disease which damages human organs.
Scientists are concerned that these outbreaks exacerbated by human alteration of landscapes could cause the next pandemic. The Roman Empire once stretched from Scotland to Africa and lasted for more than 400 years. No one knows exactly why the empire collapsed, but one contributing factor may have been malaria. A mass grave of babies from that era, excavated in the 1990s, found, through DNA analysis, that many of them had died from malaria, according to a study published in 2001 in the journal Ancient Biomolecules. Some researchers speculate that the malaria outbreak may have been exacerbated by deforestation in Rome’s surrounding Tiber River Valley to supply timber to the growing city.
One piece of the puzzle is to know what pathogens might come out of the forest in the future.
Once a disease has left a forested region, it can travel in human beings, crossing the world in a matter of hours by airplane before the person even shows symptoms. How well it does in its new homes depends on several factors. Once Zika traveled to Brazil from Africa, for example, it flourished because Aedes aegypti mosquitoes hang out around people and love to lay their eggs in small containers of water. Many people in Brazil’s large slums store water in buckets, and standing water also collects in tarps, old tires, and trash.
A key question about the Zika virus is whether it will enter the primate populations in South America, which means it might become a permanent resident and an ongoing source of infection. “Is it going to set up shop there?” asks Vittor. “We don’t know.”
Mosquitoes aren’t the only creatures that bring fever out of the forest. Angolan free-tailed bats were believed to harbor the Ebola virus that broke out and killed more than 11,000 people last year. And AIDS, which has killed more than 25 million people worldwide, came from people eating bush meat, likely chimpanzees.
A wild card in this disease scenario is the rapidly changing climate. If spring comes early, mosquitoes hatch earlier and summer populations are larger. In Southeast Asia, the spike in temperatures during El Niño weather cycles correlates with dengue fever outbreaks, because the warmer weather allows mosquitoes to breed faster and expand the population, which spreads the virus further, according to a study last year in the Proceedings of the National Academy of Sciences.
Part of the solution is to recognize and understand these connections and teach people that keeping nature intact has protective effects. And where people do cut down forests or build roads, numerous steps can be taken to lessen the chance of mosquito-borne disease outbreaks — education campaigns, more clinics, health training, and medical monitoring.
Another piece of the puzzle is to know what pathogens the world might be up against in the future as they come out of the forest. Ecohealth Alliance is cataloging wildlife-borne viruses in wild places where there is new encroachment into undisturbed nature and health care is poor or non-existent. The goal is to better understand how these viruses might spread and to potentially develop vaccines.
“If we could deal with the trade in wildlife and deforestation we wouldn’t need to stop an outbreak,” like Zika or Ebola, said Daszak, the organization’s president. “We would have already dealt with it.”