In this file photo taken on June 30, 2017, a juvenile Sunda pangolin feeds on termites at the Singapore Zoo. The endangered pangolin may be the link that facilitated the spread of the novel coronavirus across China, Chinese scientists said on February 7, 2020. Researchers at the South China Agricultural University have identified the scaly mammal as a "potential intermediate host," the university said in a statement, without providing further details. — AFP

How deforestation helps deadly viruses jump from animals to humans

Humans need to balance the production of food, forest commodities and other goods with the protection of tropical forests.

Updated 03 Jul, 2020 11:40am

The coronavirus pandemic, suspected of originating in bats and pangolins, has brought the risk of viruses that jump from wildlife to humans into stark focus.

These leaps often happen at the edges of the world's tropical forests, where deforestation is increasingly bringing people into contact with animals' natural habitats. Yellow fever, malaria, Venezuelan equine encephalitis, Ebola — all of these pathogens have spilled over from one species to another at the margins of forests.

As doctors and biologists specialising in infectious diseases, we have studied these and other zoonoses as they spread in Africa, Asia and the Americas. We found that deforestation has been a common theme.

More than half of the world's tropical deforestation is driven by four commodities: beef, soy, palm oil and wood products. They replace mature, biodiverse tropical forests with monocrop fields and pastures. As the forest is degraded piecemeal, animals still living in isolated fragments of natural vegetation struggle to exist. When human settlements encroach on these forests, human-wildlife contact can increase, and new opportunistic animals may also migrate in.

The resulting disease spread shows the interconnectedness of natural habitats, the animals that dwell within it, and humans.

Yellow fever: Monkeys, humans and hungry mosquitoes

Yellow fever, a viral infection transmitted by mosquitoes, famously halted progress on the Panama Canal in the 1900s and shaped the history of Atlantic coast cities from Philadelphia to Rio de Janeiro. Although a yellow fever vaccine has been available since the 1930s, the disease continues to afflict 200,000 people a year, a third of whom die, mostly in West Africa.

The virus that causes it lives in primates and is spread by mosquitoes that tend to dwell high in the canopy where these primates live.

In the early 1990s, a yellow fever outbreak was reported for the first time in the Kerio Valley in Kenya, where deforestation had fragmented the forest. Between 2016 and 2018, South America saw its largest number of yellow fever cases in decades, resulting in around 2,000 cases, and hundreds of deaths. The impact was severe in the extremely vulnerable Atlantic forest of Brazil — a biodiversity hotspot that has shrunk to 7% of its original forest cover.

A girl carries a monkey inside her house in their village at Xingu national park in Mato Grosso, Brazil, October 2, 2015. — Reuters
A girl carries a monkey inside her house in their village at Xingu national park in Mato Grosso, Brazil, October 2, 2015. — Reuters

Shrinking habitat has been shown to concentrate howler monkeys — one of the main South American yellow fever hosts. A study on primate density in Kenya further demonstrated that forest fragmentation led a greater density of primates, which in turn led to pathogens becoming more prevalent.

Deforestation resulted in patches of forest that both concentrated the primate hosts and favoured the mosquitoes that could transmit the virus to humans.

Malaria: Humans can also infect wildlife

Just as wildlife pathogens can jump to humans, humans can cross-infect wildlife.

Falciparum malaria kills hundreds of thousands of people yearly, especially in Africa. But in the Atlantic tropical forest of Brazil, we have also found a surprisingly high rate of Plasmodium falciparum (the malaria parasite responsible for severe malaria) circulating in the absence of humans. That raises the possibility that this parasite may be infecting new world monkeys. Elsewhere in the Amazon, monkey species have become naturally infected. In both cases, deforestation could have facilitated cross-infection.

We and other scientists have extensively documented the associations between deforestation and malaria in the Amazon, showing how the malaria-carrying mosquitoes and human malaria cases are strongly linked to deforested habitat.

A worker looks for holes in mosquito netting at a textile mill in Arusha, Tanzania, February 18, 2008.  — Reuters
A worker looks for holes in mosquito netting at a textile mill in Arusha, Tanzania, February 18, 2008. — Reuters

Another type of malaria, Plasmodium knowlesi, known to circulate among monkeys, became a concern to human health over a decade ago in Southeast Asia. Several studies have shown that areas sustaining higher rates of forest loss also had higher rates of human infections, and that the mosquito vectors and monkey hosts spanned a wide range of habitats including disturbed forest.

Venezuelan equine encephalitis: Rodents move in

Venezuelan equine encephalitis is another mosquito-borne virus that is estimated to cause tens to hundreds of thousands of humans to develop febrile illnesses every year. Severe infections can lead to encephalitis and even death.

In the Darien province of Panama, we found that two rodent species had particularly high rates of infection with Venezuelan equine encephalitis virus, leading us to suspect that these species may be the wildlife hosts.

One of the species, Tome’s spiny rat, has also been implicated in other studies. The other, the short-tailed cane mouse, is also involved in the transmission of zoonotic diseases such as hantavirus and possibly Madariaga virus, an emergent encephalitis virus.

While Tome's spiny rat is widely found in tropical forests in the Americas, it readily occupies regrowth and forest fragments. The short-tailed cane mouse prefers habitat on the edge of forests and abutting cattle pastures.

As deforestation in this region progresses, these two rodents can occupy forest fragments, cattle pastures and the regrowth that arises when fields lie fallow. Mosquitoes also occupy these areas and can bring the virus to humans and livestock.

Ebola: Disease at the forest’s edge

Vector-borne diseases are not the only zoonoses sensitive to deforestation. Ebola was first described in 1976, but outbreaks have become more common. The 2014-2016 outbreak killed more than 11,000 people in West Africa and drew attention to diseases that can spread from wildlife to humans.

The natural transmission cycle of the Ebola virus remains elusive. Bats have been implicated, with possible additional ground-dwelling animals maintaining "silent" transmission between human outbreaks.

A common vampire bat (Desmodus rotundus) flies during its nocturnal hunt in Belize in this handout photo released on February 20, 2018. — Reuters
A common vampire bat (Desmodus rotundus) flies during its nocturnal hunt in Belize in this handout photo released on February 20, 2018. — Reuters

While the exact nature of transmission is not yet known, several studies have shown that deforestation and forest fragmentation were associated with outbreaks between 2004 and 2014. In addition to possibly concentrating Ebola wildlife hosts, fragmentation may serve as a corridor for pathogen-carrying animals to spread the virus over large areas, and it may increase human contact with these animals along the forest edge.

What about the coronavirus?

While the origin of the Sars-Cov-2 outbreak hasn't been proved, a genetically similar virus has been detected in intermediate horseshoe bats and Sunda pangolins.

The range of the Sunda pangolin — which is critically endangered — overlaps with the intermediate horseshoe bat in the forests of Southeast Asia, where it lives in mature tree hollows. As forest habitat shrinks, could pangolins also experience increased density and susceptibility to pathogens?

In fact, in small urban forest fragments in Malaysia, the Sunda pangolin was detected even though overall mammal diversity was much lower than a comparison tract of contiguous forest. This shows that this animal is able to persist in fragmented forests where it could increase contact with humans or other animals that can harbour potentially zoonotic viruses, such as bats. The Sunda pangolin is poached for its meat, skin and scales and imported illegally from Malaysia and Vietnam into China. A wet market in Wuhan that sells such animals has been suspected as a source of the current pandemic.

Preventing zoonotic spillover

There is still a lot that we don’t know about how viruses jump from wildlife to humans and what might drive that contact.

Forest fragments and their associated landscapes encompassing forest edge, agricultural fields and pastures have been a repeated theme in tropical zoonoses. While many species disappear as forests are cleared, others have been able to adapt. Those that adapt may become more concentrated, increasing the rate of infections.

Given the evidence, it is clear humans need to balance the production of food, forest commodities and other goods with the protection of tropical forests. Conservation of wildlife may keep their pathogens in check, preventing zoonotic spillover, and ultimately benefiting humans, too.

The header image is a file photo taken on June 30, 2017, in which a juvenile Sunda pangolin feeds on termites at the Singapore Zoo. — AFP

This article originally appeared in The Conversation and has been reproduced with permission.


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Dr Amy Vittor is an Assistant Professor in the Division of Infectious Diseases and Global Medicine at the University of Florida, where she studies the interface between vector-borne disease, land use and host immunity.

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Gabriel Zorello Laporta is Professor of biology and infectious diseases in Faculdade de Medicina do ABC. His research involves understanding long-term effects of anthropogenic activities in the ecology and evolution of zoonotic infectious diseases within a multi-scale framework encompassing bio-social and physical processes in ecosystems, biomes and biosphere.

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Maria Anice Mureb Sallum is Professor of Epidemiology at Universidade de São Paulo.

The views expressed by this writer and commenters below do not necessarily reflect the views and policies of the Dawn Media Group.

Comments (13) Closed

Jul 02, 2020 12:58pm
It's not deforestation but undemocratic hunger.
Recommend 0
Jul 02, 2020 01:23pm
Go, tell it to the deforesters, where ever they be, in this land of the pure.
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Jul 02, 2020 01:35pm
Please request your to stop eating everything and destroying nature. And next time when they again are the source of Pandemic like SARS and COVID-19 then keep it limited within their own border or share it only with their iron brothers.
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Jul 02, 2020 02:22pm
Vegetarianism is the way to go. Especially starting small gardens at homes will help a lot too in bringing humans in equilibrium with nature.
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Asif A. Shah
Jul 02, 2020 03:09pm
Thank you for this public service article.
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Dr. Salaria, Aamir Ahmad
Jul 02, 2020 03:29pm
Deforestation must be declared a humanitarian crime by the United Nations and other powerful and influential global institutions like the European Union, NATO, IMF, World Bank and the Gulf Cooperative Council in the Middle East.
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M. Saeed
Jul 02, 2020 05:38pm
Deforestation is the result of inhuman increasing cooking gas prices every other day, forcing people to go for wood as the kitchen fuel.
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Charlie west
Jul 02, 2020 11:09pm
@M. Saeed, nope. Not at all
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Jul 02, 2020 11:27pm
Iron brother eats these animals by catching and caging them. That is how virus spreads!
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Afshan sohail Ahmed
Jul 03, 2020 01:06am
Unprecedented land clearance once curtailed and reforestation reinforced will absolutely give longevity to planet earth
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Jul 03, 2020 04:53am
You are touching only symptoms and not addressing root cause. The explosion of population and carnivorism is the mother of deforestation. The simple solution is making one-child policy and vegetarian (that too renewables) only diet for the next 50 years.
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R. Kannan
Jul 03, 2020 11:02am
The article starts by talking about reasons for deforestation but does not talk about ways to reduce deforestation. Human population increased from 1Bn at 0 AD to 2 Bn in 1900. It has since increased nearly four fold. As a result, a lot of more humans need to be fed. Second, development of refrigeration technologies has resulted in more storage of meat and giant food processing facilities, especially the meat factories. As a result, there has been much greater per capita meat consumption all across the globe. While noise has been made about the wet markets , the fact is all slaughetr houses are places where the virus can jump species. Mankind needs to question its choices and think about better eating habits and also population control.
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Jul 03, 2020 11:36am
Vegetarianism cannot be a solution. Glueten-free Veganism is the only alternative.
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