“…Research suggests that
outbreaks of animal-borne and other infectious diseases such as Ebola, Sars,
bird flu and now Covid-19, caused by a novel coronavirus, are on the rise.
Pathogens are crossing from animals to humans, and many are
able to spread quickly to new places. The US Centers for Disease Control and
Prevention (CDC) estimates that three-quarters of new or emerging diseases that
infect humans originate in animals.
“Some, like rabies and plague, crossed from animals centuries
ago. Others, such as Marburg, which is
thought to be transmitted by bats, are still rare. A few, like Covid-19, which
emerged last year in Wuhan, China, and Mers, which is linked to camels in the Middle East, are new to
humans and spreading globally.
“Other diseases that have crossed into humans include Lassa fever, which was first
identified in 1969 in Nigeria; Nipah from Malaysia; and Sars from China, which killed more than 700 people and
travelled to 30 countries in 2002–03. Some, like Zika and West Nile virus, which emerged in Africa, have
mutated and become established on other continents.
“Kate Jones, chair of ecology and biodiversity
at UCL, calls emerging animal-borne infectious diseases an ‘increasing and very
significant threat to global health, security and economies.’
Amplification effect
“In 2008, Jones and a team of
researchers identified 335 diseases that emerged
between 1960 and 2004, at least 60% of which came from animals. Increasingly,
says Jones, these zoonotic diseases are linked to environmental change and
human behaviour. The disruption of pristine forests driven by logging, mining, road building through remote places, rapid
urbanisation and population growth is bringing people into closer contact with
animal species they may never have been near before, she says.
“The resulting transmission of disease from
wildlife to humans, she says, is now ‘a hidden cost of human economic
development. There are just so many more of us, in every environment. We are
going into largely undisturbed places and being exposed more and more. We are
creating habitats where viruses are transmitted more easily, and then we are
surprised that we have new ones.’
“Jones studies how changes in land use
contribute to the risk. ‘We are researching how species in degraded habitats
are likely to carry more viruses which can infect humans,’ she says. ‘Simpler
systems get an amplification effect. Destroy landscapes, and the species you
are left with are the ones humans get the diseases from.’
“‘There are countless pathogens out there
continuing to evolve which at some point could pose a threat to humans,’ says
Eric Fevre, chair of veterinary infectious diseases at the University of
Liverpool’s Institute of Infection and Global Health. ‘The
risk [of pathogens jumping from animals to humans] has always been there.’
“The difference between now and a few decades
ago, Fevre says, is that diseases are likely to spring up in both urban and
natural environments. ‘We have created densely packed populations where
alongside us are bats and rodents and birds, pets and other living things. That
creates intense interaction and opportunities for things to move from species
to species,’ he says.
Tip of the iceberg
“‘Pathogens do not respect species
boundaries,’ says disease ecologist Thomas Gillespie, an associate professor in
Emory University’s department of environmental sciences, who
studies how shrinking natural habitats and changing behaviour add to the risk
of diseases spilling over from animals to humans. ‘I am not at all surprised
about the coronavirus outbreak,’ he says. ‘The majority of pathogens are still
to be discovered. We are at the very tip of the iceberg.’
“Humans, says Gillespie, are creating the
conditions for the spread of diseases by reducing the natural barriers between
host animals – in which the virus is naturally circulating – and themselves.
‘We fully expect the arrival of pandemic influenza; we can expect large-scale
human mortalities; we can expect other pathogens with other impacts. A disease
like Ebola is not easily spread. But something with a mortality rate of Ebola
spread by something like measles would be catastrophic,’ Gillespie says.
“Wildlife everywhere is being put under more
stress, he says. ‘Major landscape changes are causing animals to lose habitats,
which means species become crowded together and also come into greater contact
with humans. Species that survive change are now moving and mixing with
different animals and with humans.’
“Gillespie sees this in the US, where suburbs
fragment forests and raise the risk of humans contracting Lyme disease.
‘Altering the ecosystem affects the complex cycle of the Lyme pathogen. People
living close by are more likely to get bitten by a tick carrying Lyme
bacteria,’ he says.
“Yet human health research seldom considers the surrounding
natural ecosystems,’ says Richard Ostfeld, distinguished senior scientist at
the Cary Institute of Ecosystem Studies in Millbrook, New York. He and others
are developing the emerging discipline of planetary health, which looks at the
links between human and ecosystem health. ‘There’s
misapprehension among scientists and the public that natural ecosystems are the
source of threats to ourselves. It’s a mistake. Nature poses threats, it is
true, but it’s human activities that do the real damage. The health risks in a
natural environment can be made much worse when we interfere with it,’ he says.
“Ostfeld points to rats and bats, which are strongly linked with the
direct and indirect spread of zoonotic diseases. ‘Rodents and some bats thrive
when we disrupt natural habitats. They are the most likely to promote
transmissions [of pathogens]. The more we disturb the forests and habitats the
more danger we are in,’ he says.
“Felicia Keesing, professor of biology at Bard
College, New York, studies how environmental changes influence the probability
that humans will be exposed to infectious diseases. ‘When we erode
biodiversity, we see a proliferation of the species most likely to
transmit new diseases to us, but there’s also good evidence
that those same species are the best hosts for existing diseases,’
she wrote in an email to Ensia, the nonprofit media outlet that reports on our
changing planet.
The market connection
“Disease ecologists argue that viruses and
other pathogens are also likely to move from animals to humans in the many
informal markets that have sprung up to provide fresh meat to fast-growing
urban populations around the world. Here, animals are slaughtered, cut up and
sold on the spot.
“The ‘wet market’ (one that sells fresh
produce and meat) in Wuhan, thought by the Chinese government to be the
starting point of the current Covid-19 pandemic, was known to sell numerous wild animals, including live wolf
pups, salamanders, crocodiles, scorpions, rats, squirrels, foxes, civets and
turtles.
“Equally, urban markets in west and
central Africa sell monkeys, bats, rats, and
dozens of species of bird, mammal, insect and rodent slaughtered and sold close
to open refuse dumps and with no drainage. ‘Wet markets make a perfect storm
for cross-species transmission of pathogens,’ says Gillespie. ‘Whenever you
have novel interactions with a range of species in one place, whether that is
in a natural environment like a forest or a wet market, you can have a
spillover event.’
“The Wuhan market, along with others that sell
live animals, has been shut by the Chinese authorities, and last month
Beijing outlawed the trading and eating of wild animals except
for fish and seafood. But bans on live animals being sold in urban areas or
informal markets are not the answer, say some scientists.
“‘The wet market in Lagos is notorious. It’s
like a nuclear bomb waiting to happen. But it’s not fair to demonize places
which do not have fridges. These traditional markets provide much of the food
for Africa and Asia,’ says Jones. ‘These markets are essential sources of food
for hundreds of millions of poor people, and getting rid of them is
impossible,’ says Delia Grace, a senior epidemiologist and veterinarian with
the International Livestock Research Institute, which is based in Nairobi,
Kenya. She argues that bans force traders underground, where they may pay less
attention to hygiene…
Changing behaviour
“So, what, if anything, can we do about all of
this? Jones says that change must come from both rich and poor societies.
Demand for wood, minerals and resources from the global north leads to the
degraded landscapes and ecological disruption that drives disease, she says.
‘We must think about global biosecurity, find the weak points and bolster the
provision of health care in developing countries. Otherwise, we can expect more
of the same,’ she adds.
“‘The risks are greater now. They were always
present and have been there for generations. It is our interactions with that
risk which must be changed,’ says Brian Bird, a research virologist at the
University of California, Davis School of Veterinary Medicine One Health
Institute, where he leads Ebola-related surveillance activities in Sierra Leone
and elsewhere.
“‘We are in an era now of chronic emergency,’
Bird says. ‘Diseases are more likely to travel further and faster than before,
which means we must be faster in our responses. It needs investments, change in
human behaviour, and it means we must listen to people at community levels.’
“Getting the message about pathogens and disease to hunters,
loggers, market traders and consumers is key, Bird says. ‘These spillovers
start with one or two people. The solutions start with education and awareness.
We must make people aware things are different now. I have learned from working
in Sierra Leone with Ebola-affected people that local communities have the hunger
and desire to have information,’ he says. ‘They want to know what to do. They
want to learn.’
“Fevre and Tacoli advocate rethinking urban
infrastructure, particularly within low-income and informal settlements.
‘Short-term efforts are focused on containing the spread of infection,’ they write. ‘The longer term – given that new
infectious diseases will likely continue to spread rapidly into and within
cities – calls for an overhaul of current approaches to urban planning and
development.’
“The bottom line, Bird says, is to be
prepared. ‘We can’t predict where the next pandemic will come from, so we need
mitigation plans to take into account the worst possible scenarios,’ he says.
‘The only certain thing is that the next one will certainly come’” (The
Guardian).
