We all know that deforestation is bad for everyone and everything. It leads to global warming, climate change, extreme weather, habitat loss, and more. But apparently there’s one more negative impact, which is the spread of a disease.
In 1997, the need of agricultural land sacrificed hectares of rainforest of Indonesia. The fire was made worse by drought and as a result, trees which were smothered in haze couldn’t produce fruit. Fruit bats didn’t have a choice but to fly elsewhere in search of food, carrying with them a deadly disease.
Shortly after bats have settled on trees in Malaysian orchards, pigs and local pig farmers started to fall sick. Best guess is because they ate fruits which were nibbled on by the bats. In 1999, 265 people had developed a severe brain inflammation, and 105 had died. Nipah virus made its way to humans and since then there’s been a string of recurrent outbreaks across Southeast Asia.
This disease had been only affecting the wildlife most of the time, but thanks to the rapid forest clearing, it spilled over people who live around the forest. Over the past two decades, a growing body of scientific evidence suggests that deforestation creates the conditions for a range of deadly pathogens. Nipah and Lassa viruses are included, as well as parasites that cause malaria and Lyme disease.
But since humanity seems to not be able to stop clearing forests, like the ones in the Amazon, some parts of Africa, and Southeast Asia, the health of people living around the forests are at risk of developing diseases. Experts are 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. 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,” Andy MacDonald, a disease ecologist at the Earth Research Institute of the University of California.
Plasmodium parasites, which are transmitted by mosquitoes and are responsible for killing over a million annually, has long been suspected of going hand in hand with deforestation. In Brazil, around 6 million people were diagnosed with the disease in the 1940s. But since the country’s efforts to reduce malaria transmissions, they’ve managed to bring it down to just 50,000 in the 1960s.
Now, the pandemic in Brazil has 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.
Amy Vittor, an epidemiologist at the University of Florida’s Emerging Pathogens Institute did a study in the late 1990s and they suggested a reason why deforestation appears to create ideal habitat along forest edges for the mosquito Anopheles darlingi to breed. This species is the most important transmitter of malaria in the Amazon.
Vittor carefully surveyed the Peruvian Amazon and she found higher numbers of larvae in warm, partially shaded pools. These areas are the kind that form beside roads cut into forests and puddles behind debris where water is no longer taken up by trees. She said, “Those were the [places] that Anopheles darlingi really enjoyed being.”
Jungle fever
Mosquitoes have a bad reputation already, and the fact that they can bring deadly diseases to humans makes it worse. However, they aren’t the only animals that spread out viruses. 60% of new infectious diseases that emerge in people such as HIV, Ebola, and Nipah come from forest-dwelling animals and they transmit them to a range of other animals.
In 2015, a study done by researchers at Ecohealth Alliance, a New York-based non-profit that tracks infectious diseases globally, found that “nearly one in three outbreaks of new and emerging disease[s] are linked to land-use change like deforestation.”
Now, animals feel nothing and are unfazed by the viruses because they’ve co-evolved with them. However, it’s different with humans who become unwitting hosts for pathogens when they venture into or change forest habitat. “We are completely changing the structure of the forest,” said Carlos Zambrana-Torrelio, a disease ecologist at Ecohealth Alliance.
Animals need food to live, that’s for sure. That’s why, there are some circumstances in which they are drawn out of the forest into whatever places they’re interested in.
For example, in Liberia forest clearings for palm oil plantations attract hordes of typically forest-dwelling mice. They were attracted by the abundance of palm fruit around plantations and settlements and they began to party like there was no tomorrow.
However, their urine and feces contaminate food and other objects, and humans who come into contact with it (or bodily fluids of infected people) can contract Lassa virus. This virus causes hemorrhagic fever, the same kind of illness triggered by Ebola virus. And in Liberia, it has killed 36% of infected people.
These virus-carrying rodents have also been spotted in deforested areas in Panama, Bolivia, and in Brazil. An expert of tropical disease and a medical researcher Alfonso Rodriguez-Morales fears that their ranges will increase following the resurgence of fires in the Amazon this year.
Not just rainforests
This nightmare, apparently, isn’t exclusive to tropical areas. Some of MacDonald’s research has revealed a curious association between deforestation and Lyme disease in the Northeastern United States.
Lyme disease is caused by a bacterium called borrelia burgdorferi. And this disease is transmitted by ticks that rely on forest-dwelling deer to breed and obtain enough blood to survive. But, according to MacDonald, the bacterium is also found in the white-footed mouse, which happens to thrive in forests fragmented by human settlements.
MacDonald added that it’s true that spillovers of infectious diseases to people is more likely to occur in the tropics because overall wildlife and pathogen diversity is higher. Transmission of diseases can be done by a wide range of animals from blood-sucking bugs to snails, which have all been linked to deforestation.
However, climate change and global warming can change that. Zambrana-Torrelio said 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. Scientists fear that
Vittor stated that 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. Ebola and Nipah can be transmitted directly between people, theoretically allowing them to travel around the world as long as there are humans. Rainforests or not, it’s going to spread.
Moreover, Zika virus, which became a deadly pandemic quite recently, could spread and infect millions because it found a host in Aedes aegypti. Familiar with this name? Yes, it’s a type of 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,” said Vittor.
We can remain hopeful with this fact, though. Because if we keep preventing deforestation, there’s a chance that we can also reduce the spread of many deadly, infectious diseases. “If we can conserve the environment, then perhaps we can also protect health. That I think is the silver lining that we should keep in mind,” said MacDonald.
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