Researched by Jacqueline Tang, VRG Intern
The Covid-19 pandemic, caused by the novel coronavirus known as SARS-CoV-2,
presents many unique challenges to public health. People may be wondering how a
massive pandemic allegedly arose from one wet market—and how to prevent the
next crisis. Unfortunately, the answer is complicated. It involves our
relationships with animals (both wild and domestic) and with the environment.
Climate change further influences all of these interactions.
Recently wet markets (that sell
live animals for food) have come under scrutiny for the dangerous mixing of
wild animals and humans. The criticism of these markets is filled with
ethnocentrism and Western ideals. Although wet markets are undoubtedly perfect
places for viral multiplication and transmission, people fail to recognize two
other major factors:
1. The role of concentrated animal feeding operations (CAFOs) in spreading
infectious disease to people.
2. Humans’ relationships with the Earth’s ecosystems, both near and
distant.
THE ORIGINS OF ZOONOTIC DISEASES
Viruses that come from animals, also called zoonotic viruses, are a major
threat to society and public health. According to the Centers for Disease
Control and Prevention, (CDC), a division of the United States Department of
Health and Human Services, “… scientists estimate that more than six out of
every 10 known infectious diseases in people can be spread from animals, and
three out of every four new or emerging infectious diseases in people come from
animals.”1 Since humans come in contact with animals in many ways,
it’s important to consider different situations where zoonotic diseases can be
easily transmitted across species.
WET MARKETS
Recently, wet markets have come under scrutiny for the dangerous
commingling of wild animals and humans. Also, they bring an exotic combination
of animals, which normally are not found close to each other in the wild,
together in one place. At a wet market:
A. Wild animals can infect domesticated animals.
B. Viruses can spread between different species.
Birds and bats especially carry
many viruses. When they come in contact with an animal of a different species,
a virus can mutate and adapt, eventually gaining the ability to infect a new
host. In this manner, viruses spread widely.2
Some viruses are unable to
directly infect a human, including avian (from birds) forms of influenza and
some types of coronaviruses. These viruses require an intermediate host that
serves as the go-between such that the virus can enter a new species. In the
intermediate host, a virus changes sufficiently through mutations that enable
it to adapt to a new host. Later, when given the chance to infect a human at
close proximity, it may be able to do so by binding to specific cell receptors.3,4
The risk of wet markets stems
from being places that allow for many different types of species to come in
contact for the first time. This gives viruses numerous potential hosts and
greater possibility of genetic recombination through mutation. Without certain
mutations, a particular virus may not be able to exploit a new species.
The cramped conditions in wet markets lead to animals being under severe stress. As a result, wild animals, already carrying many viruses that they are immune to, will shed more viral particles in the market.5 This means that there is a greater chance of viral infection for humans and other animals that are in the wet market. Unfortunately, stressful conditions for animals is not unique to wet markets. Conventional animal agriculture also places extreme stress on food animals.
CONCENTRATED ANIMAL FEEDING OPERATIONS (CAFOs)
According to the Natural Resources Conservation Service, a division of the
United States Department of Agriculture (USDA), CAFOs “congregate animals,
feed, manure and urine, dead animals, and production operations on a small land
area.”6
CAFOs present many challenges for public
health. The risk lies in their scale and sanitation. Many individuals of the
same species, known as monocultures, are housed together in small cages, rooms,
or buildings. If a virus is able to infiltrate (either from wild animal, human,
or another method), it will easily spread between animals.
The lack of genetic diversity within
livestock also facilitates viral transmission between animals.7 The
Food and Agriculture Organization of the United Nations (FAO of the UN) stated
that “livestock health is the weakest link in our global health chain, and
disease drivers in livestock as well as wildlife are having increasing impacts
on humans.”8
THE WET MARKET-CAFO CONNECTION
Besides the contributions to pandemic risk that wet markets and CAFOs
present when considered individually, their connection is also problematic. For
example, consider the case of influenza, a common virus in poultry and swine. Influenza
is more common within CAFOs compared to wet markets. For example, in a
systematic review over a six-year period, researchers noted that of 364 avian
influenza outbreaks, 56.1% originated in poultry CAFOs while only 0.8% started
in wet markets.9
Scientists also report that many viruses
undergo genetic recombination that occur among individuals of the same species and
between species. In other words, if a wild animal comes in contact with
livestock, either directly or indirectly, it can easily spread certain viruses
to the domestic food animals.10
Typically, viruses are not very pathogenic in
the wild host. Once infected by a wild virus, domestic animals serve as
“amplifier hosts” in which that virus often becomes more pathogenic than it was
in the native species. Through genetic recombination and/or mutation, the wild
virus becomes adapted to the new host.11
The transport of both domesticated and wild
animals can also magnify zoonotic viruses. The animals being transported might
be exposed to viruses that their immune systems have never encountered. During
this exposure, foreign viruses have the potential to recombine and create completely
novel viruses.12
J.F. Chan and colleagues,
in a 2013 Trends in Microbiology article state that in the wild, bats
and birds are favorable reservoirs for most emerging viruses because of several
“unique ecological, biological, immunological, and genetic features.” The loss
of biodiversity and habitats for bats and birds leads to an increased
interaction with humans and other species, including domesticated animals. The
researchers report that the increased crowding of different wildlife species at
wet markets has made it easier for viruses to jump between species.13
In some countries, animal
agriculture also occurs in mixed species settings. For example, in China,
it’s common to raise pigs along with ducks and other avian species. 14
The high probability of viral transmission caused by interspecies mixing cannot
be overstated.
THE ROLE OF INTERMEDIATE HOSTS IN VIRAL TRANSMISSION
In a 2020 editorial by A. Rodriguez-Morales and fellow researchers
published in Le Infezioni in Medicina many viruses use intermediate
hosts like civets or camels because they cannot spread directly from the
original infected animal (bat or bird) directly to humans.15
M. Konda and fellow
investigators, in a 2020 review article in Cureus summarize the
viral process in intermediate hosts16:
1. Mutates and recombines in intermediate hosts, thus adapting to humans
2. Binds to the correct cell receptor in humans
3. Enters the human cell
4. Integrates its own genetic material into the human cell’s
5. Commands the human cell to reproduce virus.
6. Human cell continues to reproduce more virus.
Scientists hypothesize that
SARS-CoV-2, the virus that causes Covid-19, used an intermediate host between
bat and humans just like its viral cousins responsible for other recent
coronavirus outbreaks.17
CONCLUSIONS: PANDEMICS, WET MARKETS, AND CAFOs
Both wet markets and CAFOs present unique challenges for the future of
public health. CAFOs, with thousands of genetically similar animals in close
proximity, provide the perfect environment for a virus to spread.
Wet markets bring hundreds of
wild species together in one place. They allow for interspecies viral
transmission and the potential of recombination to form novel pathogens. Wet
markets also allow virus-infected animals, like wild bats and birds, to come
into contact with humans and domestic animals.
Both CAFOs and wet markets may have
very poor sanitation and living conditions for animals, creating stressed
animals that shed more virus. This situation facilitates rapid viral spread.
Wet markets and CAFOs jeopardize
public health and food security. Research should focus on prevention of
pandemics in both types of food systems. Ultimately, humans need to reevaluate
their role in the Earth’s ecosystems if we hope to prevent the next global
pandemic of a zoonotic virus like SARS-CoV-2.
Notes from the editors:
WHAT IS A WET MARKET?
According to researchers, “A
typical wet market is a partially open commercial complex with vending stalls
organized in rows; they often have slippery floors and narrow aisles along
which independent vendors primarily sell ‘wet’ items such as meat, poultry,
seafood, vegetables, and fruits.” See: https://link.springer.com/article/10.1007/s10460-019-09987-2
In this sense of the term, wet markets
exist all over the world, including the United States, where they are called “farmer’s
markets.”
In this paper, “wet market” refers to open
markets selling live animals, including wild, domestic, and/or domesticated
wild animals, as well as animal products (eggs, meat, bones, organs, etc.).
WHAT IS A CAFO (concentrated animal feeding operation)?
The
U.S. Environmental Protection Agency (EPA) defines Animal Feeding Operations (AFOs)
as “agricultural enterprises where animals are kept and raised in confined
situations. AFOs congregate animals, feed, manure and urine, dead animals, and
production operations on a small land area. Feed is brought to the animals
rather than the animals grazing or otherwise seeking feed in pastures, fields,
or on rangeland. There are approximately 450,000 AFOs in the United States.”
19
“A CAFO is another EPA term for a large
concentrated AFO. A CAFO is an AFO with more than 1,000 animal units (an
animal unit is defined as an animal equivalent of 1,000 pounds live weight and
equates to 1,000 head of beef cattle, 700 dairy cows, 2,500 swine weighing more
than 55 lbs, 125 thousand broiler chickens, or 82 thousand laying hens or
pullets) confined on site for more than 45 days during the year. Any size
AFO that discharges manure or wastewater into a natural or man-made ditch,
stream or other waterway is defined as a CAFO, regardless of size.”19
References
1. Zoonotic Diseases. cdc.gov. https://www.cdc.gov/onehealth/basics/zoonotic-diseases.html.
Published July 14, 2017. Accessed November 4, 2020.
2. Chan JF, To KK, Tse H,
Jin DY, Yuen KY. Interspecies transmission and emergence
of novel viruses: lessons from bats and birds. Trends Microbiol.
2013;21:544-555.
3. Johnson KEE, Song T,
Greenbaum B, Ghedin E. Getting the flu: 5 key facts about influenza virus
evolution. PLoS Pathog. 2017;13:e1006450. Published 2017 Aug 24.
4. Konda M, Dodda B, Konala
VM, Naramala S, Adapa S. Potential Zoonotic Origins of SARS-CoV-2 and Insights
for Preventing Future Pandemics Through One Health Approach. Cureus.
2020;12:e8932. Published 2020 Jun 30.
5. Brook CE, Boots M,
Chandran K, et al. Accelerated viral dynamics in bat cell lines, with implications
for zoonotic emergence. Elife. 2020;9:e48401. Published 2020 Feb 3.
6. Animal Feeding
Operations. NRCS.
https://www.nrcs.usda.gov/wps/portal/nrcs/main/national/plantsanimals/livestock/afo/.
Accessed November 4, 2020.
7. Jones BA, Grace D, Kock
R, et al. Zoonosis emergence linked to agricultural intensification and
environmental change. Proc Natl Acad Sci U S A. 2013;110:8399-8404.
doi:10.1073/pnas.1208059110
8. Food and Agriculture Organization of the
United Nations. World Livestock 2013 – Changing Disease Landscapes. Rome, IT: FAO; 2013.
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Arvanitidou M, Guitian J, et al. Global avian influenza outbreaks 2010-2016: a
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Systematic Reviews. 2018 Jan;7(1):17.
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Daszak P, et al. Impacts of biodiversity on the emergence and transmission of
infectious diseases. Nature. 2010;468:647-652.
11. Jones BA, Grace D, Kock
R, et al. Zoonosis emergence linked to agricultural intensification and
environmental change. Proc Natl Acad Sci U S A. 2013;110:8399-8404.
12. Liverani M, Waage J,
Barnett T, et al. Understanding and managing zoonotic risk in the new livestock
industries. Environ Health Perspect. 2013;121:873-877.
13. Chan JF, To KK, Tse H,
Jin DY, Yuen KY. Interspecies transmission and emergence
of novel viruses: lessons from bats and birds. Trends Microbiol.
2013;21:544-555.
14. Keck F. Livestock Revolution and Ghostly
Apparitions: South China as a Sentinel
Territory for Influenza
Pandemics. Current Anthropology.
2019:60:S20.
15. Rodriguez-Morales AJ,
Bonilla-Aldana DK, Balbin-Ramon GJ, et al. History is repeating itself:
Probable zoonotic spillover as the cause of the 2019 novel Coronavirus
Epidemic. Infez Med. 2020;28:3-5.
16. Konda M, Dodda B, Konala VM, Naramala S, Adapa S. Potential Zoonotic Origins of SARS-CoV-2 and Insights for Preventing Future Pandemics Through One Health Approach. Cureus. 2020;12:e8932. Published 2020 Jun 30.
17. Hu T, Liu Y, Zhao M,
Zhuang Q, Xu L, He Q. A comparison of COVID-19, SARS and MERS. PeerJ.
2020;8:e9725. Published 2020 Aug 19.
18. Wet Market. Merriam-Webster.
https://www.merriam-webster.com/dictionary/wet market. Accessed November 5,
2020.
19. Animal Feeding
Operations. NRCS.
https://www.nrcs.usda.gov/wps/portal/nrcs/main/national/plantsanimals/livestock/afo/.
Accessed
Jacqueline Tang did a Vegetarian
Resource Group internship while a pre-med and public health major at Johns
Hopkins University.
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