For decades, when scientists and researchers focused on pandemics, they looked for vaccines and drugs. The mystery of what caused zoonotic pathogens to jump from animals to humans drew little attention.
"The idea was that something fundamental is going on in this era that is driving all these pandemics," says Peter Daszak, who has studied wildlife and human disease for more than two decades, "but no one was bringing the whole thing together."
Now, a series of studies, built upon research over the past two decades, provides increasing evidence that the loss of forest creates the conditions for a wide range of deadly diseases to jump from animals to humans.
"The fundamental change is what we’re doing to the planet," he says. "We’re not only driving global pollution, climate change and all the rest, but we’re driving the emergence and spread of all these new pathogens."
Daszak, the president of Ecohealth Alliance, a nonprofit focused on the intersection of conservation and global health, says he recently completed a re-analysis of more than 500 disease outbreaks over the past four decades, a study he and others originated published in Nature in 2008. (The new report recently was submitted for a future issue of Nature).
"Land use change is coming up as a significant driver of disease events, particularly from wildlife," he says. "This is a wake-up call. Deforestation and land conversion for agriculture is one of the biggest drivers of pandemics. We need to get on the case very quickly."
Deforestation may increase the prevalence of diseases including malaria, dengue fever, SARS, Ebola, schistosomiasis, leptospirosis (a bacterial disease that can lead to meningitis and liver failure), leishmaniasis (which causes skin lesions), and others, because it changes how and where the animals that transmit these diseases to humans live.
"These are all wildlife diseases, but a lot of them are caused by land use change," Daszak says. "It's all about human contact with high-risk reservoirs -- primates, bats, rodents."
Throughout history, diseases have moved from forests into humans through animal carriers. But the increasing proximity of humans to recently deforested areas magnifies the risk.
Clearing forests for agriculture increases sunlight exposure and often disrupts small streams, creating pools of warm water perfect for mosquito breeding.
Eventually, farming becomes unsustainable as the land becomes infertile and people depart, abandoning land to low-lying shrubbery, also conducive to mosquito breeding.
Research by Vittor and others show that the malaria-carrying species in a deforested area of Peru bit 278 times more frequently than the same species in an untouched forest.
In one region, after a road was built into pristine forest and people began clearing the land for agriculture, malaria cases rose from 600 to 120,000 per year.
Even small decreases in forest cover increase malaria exposure. Cutting four percent of a forest in Brazil, according to a 2010 study, was associated with a nearly 50 percent increase in human malaria cases.
The Zika virus, the cause of birth defects in Brazil, is another example. It emerged in mosquitoes in the Zika forest of Uganda in the 1940s, but there were few human cases until 2007. Aedes aegypti, the mosquito species that carries Zika and many other diseases, spread first to Asia where it likely mutated, then gained a foothold in the Brazilian Amazon, thanks to global travel.There, the mosquitoes carrying the disease flourished in the heat of places like Recife, a Zika hotspot and a city that had its hottest three months on record late last year.
Deforestation there has contributed to a record drought in Brazil, which leads to more people storing water in open containers. That leads to a rise in the mosquito population. Too, when temperatures go up, mosquitoes require more blood so they feed more often and reproduce faster.
Mosquitoes are just one vector for transmitting disease whose range and habits are affected by deforestation. Primates, snails, bats, sandflies, and rodents also carry death and deforestation increasingly brings them into contact with humans.
A study issued earlier this year found a dramatic increase in malaria cases in Malaysian Borneo followed rapid deforestation for the creation of palm oil plantations.
Researchers determined that macaques, the primates that carry one form of malaria, had been forced to concentrate in the remaining forest fragments, possibly increasing disease in their populations. As people moved in to work on the plantations along the forest edge, they were victims of mosquitoes thriving in that new habitat that transferred the disease from primate to human.
The evidence that land use change drives deadly emerging diseases grows with each new study.
The snails that carry flatworms that cause schistosomiasis prosper in warm, open areas created by deforestation. A 2015 Lancet Commission study concluded there is "circumstantial" evidence that changes in land use increased the likelihood of Ebola outbreaks. The prevalence of hantavirus, which can have a mortality rate as high as 30 percent, has increased in rodent populations in areas of Panama disturbed by human activity.
The nipah virus, a neurological disease with no known cure, emerged in the late 1990s in Malaysia in the aftermath of slashing and burning to create pig farms. Bats ate fruit in nearby orchards. Pigs ate the mangoes in those orchards and the virus made its way into humans. In the initial outbreak, 257 people were infected, killing 105.
Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, says the problem is complicated. Weather, climate change, human interaction, and the behavior of animal hosts contribute to the spread of a disease. To address the issue will require collaborations between environmental scientists, mammalian zoologists, vector biologists, social scientists, mathematicians and modelers. "We're realizing the problems are becoming more and more complex and no one field is going to be able to solve them," he says.
Vittor, who began studying deforestation and malaria in Peru in 1998, cautions against making generalizations. There is a species of mosquito in Africa, for instance, that thrives in the forest. "Each malaria mosquito behaves differently, has different breeding habits, and is adaptable," she says. Evidence suggests that other factors, including house structure, sanitation, and regular access to clean water also have an effect on malaria rates.
"These diseases are the product of rapid economic development in these tropical countries," Daszak says. "If we can manage that development, put in place structures to make sure we're ready for outbreaks then we're ahead of the curve. We know where the highest risk populations are."