How Deforestation Helps Deadly Viruses Jump from Animals to Humans

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

Small-scale slash-and-burn agriculture is one of the
deforestation problems in Brazil’s Amazon jungle. Credit: Mario
Osava/IPS

By External Source
Jun 26 2020 (IPS)

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

specializing 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

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.
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 favored 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
.

 

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.

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 harbor 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.

Amy Y.
Vittor
, Assistant Professor of Medicine,
University of Florida
;
Gabriel Zorello Laporta
, Professor of biology and infectious
diseases,
Faculdade de Medicina do ABC
, and
Maria Anice Mureb Sallum
, Professor of Epidemiology,
Universidade de São Paulo

This article is republished from The Conversation under a
Creative Commons license. Read the
original article
.

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How Deforestation Helps Deadly Viruses Jump from Animals to
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How Deforestation Helps Deadly Viruses Jump from Animals to
Humans