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EPIDEMIOLOGY
The infectious disease trap of animal agriculture
Matthew N. Hayek
Infectious diseases originating from animals (zoonotic diseases) have emerged following deforestation from agricul-
ture. Agriculture can reduce its land use through intensification, i.e., improving resource use efficiency. However,
intensive management often confines animals and their wastes, which also fosters disease emergence. Therefore,
rising demand for animal-sourced foods creates a “trap” of zoonotic disease risks: extensive land use on one hand
or intensive animal management on the other. Not all intensification poses disease risks; some methods avoid
confinement and improve animal health. However, these “win-win” improvements alone cannot satisfy rising
meat demand, particularly for chicken and pork. Intensive poultry and pig production entails greater antibiotic
use, confinement, and animal populations than beef production. Shifting from beef to chicken consumption mit-
igates climate emissions, but this common strategy neglects zoonotic disease risks. Preventing zoonotic diseases
requires international coordination to reduce the high demand for animal-sourced foods, improve forest conser-
vation governance, and selectively intensify the lowest-producing ruminant animal systems without confinement.
INTRODUCTION: FOOD PRODUCTION DRIVES
ZOONOSIS EMERGENCE
Despite global advances in prosperity, nutrition, and medical care,
infectious diseases are rising in prevalence (1,2). In the past four
decades, emerging infectious diseases have increased at more than
four times the rate of prior decades (3), most of which have nonhu-
man animal (zoonotic) origins.
Since 1940, an estimated 50% of zoonotic disease emergence has
been associated with agriculture (1–3). This estimate, however, is
necessarily conservative because only direct agricultural drivers are
considered in the epidemiological literature, i.e., within the farm gate.
Food systems have environmental impacts before and after the farm
gate (4), such as land clearing, food processing, and waste disposal.
Food systems therefore affect zoonotic disease emergence indirectly.
The true contributions of food systems to recently emerged zoonotic
diseases remain poorly characterized.
The increase in zoonosis emergence has been partially attributed
to ongoing deforestation, particularly in the tropics (2,5,6). The
largest driver of deforestation is pasture expansion for ruminants
(e.g., cattle) with another substantial fraction of forest and savanna
clearing for producing feed crops like soy, predominantly fed to
monogastrics (e.g., pigs and chickens) for domestic and export
markets (7), with ongoing debate as to the precise proportions (8).
Land clearing is expected to continue through 2050 due to further
increased meat and dairy demand (9–12). Deforestation and con-
version to human-dominated systems drive the loss, turnover, and
homogenization of biodiversity and expose adjacent human com-
munities to wildlife harboring microbes that can become zoonotic
pathogens with pandemic potential (5).
To meet the rising global demand for animal-sourced foods, the
most commonly recommended development strategy in the envi-
ronmental literature is “sustainable intensification,” which refers to
increasing production while managing inputs more judiciously
(13,14). Experts recommend this strategy for virtually all low- and
middle-income countries (LMICs). By improving resource use effi-
ciency, sustainable intensification strategies for animal agriculture
can reduce greenhouse gas (GHG) emissions and deforestation
(15–17), thereby also reducing zoonotic disease risks.
However, the intensification of animal agricultural production,
in its most common forms, entails the concentration and confine-
ment of animal bodies and their wastes, trading off deforestation for
other multiple well-documented and potentially cascading risks for
zoonotic disease emergence. This creates a paradox for intensifica-
tion that remains unaddressed in the scientific literature: Intensi-
fied animal production, while decreasing marginal land use change
and GHG emissions, can often increase other zoonotic disease risks.
The risks of zoonotic disease emergence from intensive animal ag-
riculture could therefore undermine the “sustainable” nature of sus-
tainable intensification.
This review examines the zoonotic disease paradox inherent to the
sustainable intensification of animal agriculture, exploring whether
food systems can circumvent a “trap” of zoonotic disease risks as
they further develop. The review first aims to characterize interac-
tions between intensification and deforestation while examining
ways that they both contribute to zoonotic disease risk. On the basis
of these interactions, this review provides recommendations to re-
duce the likelihood of zoonotic disease emergence, including (i) se-
lectively intensifying the least productive regions, namely, LMICs,
without resorting to confinement and other common high-risk in-
tensive management techniques; (ii) strengthening and improving
conservation regulations with effective community governance; and
(iii) curbing the high and rising demand for animal-sourced food
products. These three strategies are most likely to succeed if imple-
mented in tandem and via regional and international coordination
to avoid leakage and rebound effects.
INTENSIFICATION—RISKS, OPPORTUNITIES, AND LIMITS
FOR STEMMING ZOONOTIC DISEASE
A number of intensive animal production methods have been im-
plicated in zoonotic disease emergence in the literature (Table1). The
intensification of animal agriculture through confinement and
industrialization has directly led to the emergence of viruses including
Nipah and H5N1 influenza (“swine flu”) (18) and antibiotic- re si st ant
infectious bacteria including methicillin-resistant Staphylococcus
aureus and Escherichia coli (19,20).
Department of Environmental Studies, New York University, 285 Mercer St., New York,
NY 10012, USA. Email: matthew.hayek@nyu.edu
Copyright © 2022
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
License 4.0 (CC BY).
Hayek, Sci. Adv. 8, eadd6681 (2022) 2 November 2022
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Intensified animal agriculture is often, but not always, character-
ized by a shift toward “landless” or “industrialized” systems (as de-
fined by the United Nations Food and Agriculture Organization).
These systems typically restrict animal movement and are oriented
toward rapid weight gain and productivity (21). Monogastric ani-
mals like pigs and chickens are raised indoors in sheds, each animal
with less than twice the space that their bodies occupy, with little or
no room to express natural behaviors (22,23). Many beef cattle
spend the latter part of their lives being “finished” or rapidly fat-
tened to reach their final market weights on enriched feeds in feed-
lots, with stocking densities for cattle on outdoor feedlots of less
than 4m2 per steer/heifer (24). These environments entail physio-
logical and mental stress, close proximity to each other and wastes,
and the routine administration of subtherapeutic (infection-
preventing) and growth-promoting antibiotics (Table1). Zoonotic
diseases from aquatic animals are relatively less common and are
predominantly caused by bacteria rather than viruses (25). However,
aquatic animal bacteria are expected to become more prominent
and potentially infectious among humans as finfish aquaculture
continues to grow to produce a larger share of aquatic foods globally,
and with it are confinement, stress, and antibiotic use, potentially
leading to spillover into humans (26). These intensive systems are
predominant in developed, industrialized countries but are rapidly
proliferating in developing regions (27), with encouragement and
financing from international development organizations including
the World Bank (28).
Relatively more extensive systems include pastoralism, extensive
grazing, and mixed crop-livestock grazing. Extensive systems are
used almost exclusively in developing regions, namely, through the
tropics and semitropics, and among predominantly ruminant live-
stock (e.g., cows, buffalo, sheep, and goats).
Intensification methods sit on a spectrum, with poles of landless,
industrialized production on the high end and highly extensive pas-
toralist grazing on the lowest. The most extensive and inefficient
systems have the potential to be improved using “win-win” forms of
intensification that do not entail a fully industrialized or landless
kind of confined intensification (Table1), but rather a kind of
“meeting in the middle” for the lowest, least productive systems to
improve their performance (15). Thus, intensifying low-production
ruminant systems in a selective manner could confer a neutral or
decreased risk of zoonosis emergence while improving meat and
dairy productivity in the most marginal contexts.
However, there are limitations to this form of intensification.
First, the number of animals raised in extensive systems is already
decreasing while being supplanted by highly industrialized/landless
systems throughout developing regions (11,21). Therefore, there
are regional and global limitations to how much additional food
“semi-intensive” systems can provide. Second, shifts downward
from more highly intensive forms would compromise food produc-
tion or lead to net agricultural expansion. For instance, eliminating
feedlot beef cattle systems in the United States by shifting to inten-
sive grazing would require 64 to 270% greater land use (29), while
eliminating confined indoor broiler chicken systems by shifting to
minimal pasture would require 43.8 to 60.1% greater land use (30).
Industrialized systems are often more productive and resource effi-
cient than semi-intensive methods. Shifting away from industrial-
ized systems therefore entails a GHG and land use penalty or
“sustainability gap” (30). Last, production systems for monogastric
animals, which produce two-thirds of meat globally, lack common
semi-intensive commercial methods (21). Global production and
consumption of beef, pork, and chicken are expected to rise by 39,
55, and 58%, respectively, by 2050, with the majority of additional
production expected to be achieved through intensification systems
(industrial, in the case of monogastrics) (11). Therefore, additional
food system strategies beyond intensification are needed to safely
feed a rising and more affluent global population.
INTERACTIONS BETWEEN INTENSIFICATION
AND DEFORESTATION
Intensification tends to reduce deforestation directly
Intensification, which aims to make agricultural production more
efficient, is commonly understood to decrease the pressure for de-
forestation within the environmental literature (13,31,32). How-
ever, in many developing tropical regions, both intensification and
deforestation are occurring simultaneously because they share under-
lying drivers (i.e., confounding causes): rising populations, incomes,
and demand for animal-sourced foods (Fig.1). Because the two are
visibly correlated, the epidemiological literature on zoonotic disease
often erroneously links intensification directly to deforestation. A
Table 1. Intensive animal management strategies, by qualitative risk
categories and farmed animal types.
Elevated risks Evidence of zoonotic disease
emergence
All farmed animal species
Indoor production and confinement (83–85)
Genetic homogenization (86, 87)
Subtherapeutic and growth-
promoting antibiotic use (19, 20, 25, 74, 88–90)
Long-distance transportation (91, 92)
Physiological stress from crowding,
confinement, and conflicts (e.g.,
gestation crates, veal crates, and
battery cages)
(22, 23, 26, 93)
Temporary/seasonal and transient
human labor (83, 94)
Concentrated animal wastes (88, 95)
Neutral or reduced risks Evidence of reduced land and
resource needs
All farmed animal species
Improving veterinary care and
reducing mortality (15)
Improving animal husbandry
management (e.g., lower
reproductive age)
(15, 96)
Integrating crop and livestock
production (97–99)
Ruminant species only
Optimizing grazing densities (100, 101)
Improving forage quality (15, 102)
Amending and restoring
degraded pastures (15, 102–104)
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number of recent high-profile synthesis reports on zoonoses discuss
intensification and deforestation synonymously and interchange-
ably (6,33,34), sometimes directly implicating intensification as
causing the ongoing deforestation, although the environmental lit-
erature predominantly concludes the opposite. Intensification can
lead directly to reduced deforestation in agriculture-forest fron-
tiers (35,36).
Intensification can indirectly trigger more deforestation
Intensification reduces the marginal resource requirements of animal-
sourced food production; it thus can potentially reduce pressures
for deforestation, a finding that is widely accepted and uncontro-
versial. However, after achieving higher efficiency, intensification can
lower the costs of production and sale prices of final goods, induc-
ing higher demand and production (Fig.1). This greater demand
can then incentivize additional deforestation (37), negating some or
all of the original efficiency improvements. This trade-off is known as
Jevons’ paradox (36,38,39) or “rebound effects,” more commonly.
The occurrence and magnitude of rebound effects in animal-
sourced food production are difficult and controversial to identify
because of confounding factors (40–42), leading to ongoing debates
(similarly reflected in the “land sparing versus land sharing” debate
regarding agricultural efficiency). However, some trends and investi-
gations are illuminating. In Sweden and the United States, an in-
creased consumption of chicken over the past two decades, due to
lower prices, resulted in greater aggregate GHG emissions despite
marginal efficiency gains over the same period (43,44). In South
America, beef intensification has triggered further deforestation
due to lower production costs (35,37,45). Sustainable intensifica-
tion can thus spur greater environmental impacts, undermining its
sustainable aims (46,47). Intensification is necessary but insuffi-
cient to reduce pressures for agriculture expansion and land clear-
ing. Escaping this “damned if we do, damned if we don’t” trap of
intensification (Fig.1) requires a more multipronged approach.
Effective forest conservation occurs in tandem with
other strategies
Intensification alone is an insufficient strategy for reducing zoonotic
disease risk (see the “Intensification—Risks, opportunities, and limits
for stemming zoonotic disease” section) and for mitigating and
reversing deforestation (see the previous section). Direct forest
conservation policies and incentives are widely recommended in
environmental and epidemiological literature, e.g., (6,18,33). How-
ever, known trade-offs and pratfalls exist. First, forest and wildlife
habitat conservation policies that are not appropriately designed
and enforced with the involvement of local cultures have backfired
(36,48–50). Second, conservation may lead to “leakage” effects: Global-
ization allows production to relocate, along with its deforestation,
to countries where conservation policies are insufficiently adopted
or enforced (51,52). Last, effective forest conservation policies in
the short term can boost intensification but lead to further defor-
estation in the long term and across wider regions (Fig.1) (39). These
effects can vary over space and time, changing with local livelihoods
and culture, price elasticities for agricultural goods, and how con-
nected production regions are to global markets (37).
Conservation policies should be culturally sensitive, rigorously
enforced, and have long-term community buy-in. However, a well-
crafted conservation policy is still insufficient to spare land from
agricultural pressures; additional land for rising populations and diets
richer in animal-sourced foods must come at the expense of clear-
ing native habitats somewhere (11,53).
MEAT DEMAND AT THE NEXUS OF ENVIRONMENTAL CHANGES
The largest increases in meat demand and production are occurring
in developing, tropical regions (16). Meat consumption exceeds the
dietary requirements in high-income countries and among increas-
ingly urban and middle-class populations of most middle-income
countries (54–56). As demand rises along with affluence in the
coming decades in LMICs and high-income countries continue to
sustain high levels of consumption and exports, additional land
clearing and GHG emissions will occur even with ambitious levels
of intensification (9,12).
Shifting to plant-rich diets mitigates environmental
and zoonotic disease risks
Decreasing meat consumption has cobenefits for environmental pro-
tection and zoonotic disease risks. Global dietary changes are theoret-
ically sufficient to reverse ongoing deforestation trends, providing 5
to 11 GtCO2 per year of natural carbon removal across 5 to 12 million
km2, sequestering approximately a decade worth of anthropogenic
emissions by 2050in natural vegetation (9,57–59), which would also
conserve and restore a substantial fraction of lost biodiversity (53,60).
Shifts to plant-rich diets in high-income countries alone would remove
approximately 3 million km2 from agricultural production, including
1 million km2 of natively forested areas (9,56).
To address the emerging zoonotic disease risks of animal agricul-
ture, a multipillared approach is required (Fig.2). This approach in-
cludes reducing demand for animal-sourced foods, semi-intensification
(see the “Intensification—Risks, opportunities, and limits for stem-
ming zoonotic disease” section and Table1), and direct forest con-
servation (see the “Effective forest conservation occurs in tandem
with other strategies” section). Under business-as-usual condi-
tions of rising demand for animal-sourced food, increased land
clearing is inevitable (57,61). Reducing demand can therefore avoid
leakage and rebound effects from focusing exclusively on supply-
side protections like semi-intensification and forest conservation
(Figs.1 and 2).
Deforestation
Forest
conservation
−
−
++
+
The zoonosis trap
Meat
demand
Intensification
Fig. 1. Higher incomes are associated with high meat demand that must be
met through intensification or deforestation (or both). Intensification can trigger
higher meat demand through lower prices, because meat demand is elastic with
respect to its cost. Intensification and deforestation are highlighted in orange, as both
have caused recent zoonotic disease emergence and are predicted to continue
doing so. Intensification is colored by a gradient to indicate that intensification strat-
egies lie on a gradient of helpful/neutral to harmful with respect to zoonosis risks.
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The zoonotic disease risks of rising animal-sourced food pro-
duction and consumption have been underscored by a number of re-
cent major environmental epidemiology synthesis reports (6,33,62).
These reports imply or outright state that high future demand
for animal-sourced foods is an immutable consequence of rising in-
comes, treating this trend as fait accompli rather than a decision
point for policy interventions. This fatalism contradicts behav-
ioral science research on reducing the consumption of meat and
other products with harmful public health impacts (e.g., tobacco
and sugar).
To meaningfully flatten the rising curve of animal-sourced foods,
demand-side interventions should be implemented, tested, and
scaled ambitiously (63). Even gentle changes to dining options and
presentation can create large effects (64). Effective interventions
range from these subtle “nudges” to more blatant rewards and in-
centives, as well as stringent regulations and restrictions (16,55).
This spectrum has been described using the Nuffield intervention
ladder, with lower rungs of “soft” methods or “carrots” (e.g., guid-
ance, suggestions, education, and nudging) to higher rungs of in-
creasingly forceful “hard” interventions or “sticks” at the top (e.g.,
taxes and bans) (65).
Countries lack healthy and sustainable food consumption policies
that are comprehensive and synergistic; most countries only have
education policies (e.g., dietary recommendations), with higher rungs
on the Nuffield ladder—including guiding choices through chang-
ing incentives and defaults or disincentivizing options—completely
missing (66). Promising local policies and corporate initiatives, mean-
while, are aiming to guide consumers toward more sustainable op-
tions using methods of monitoring, goal setting, and verification in
combination with multiple soft behavioral interventions to motivate
change (67).
More targeted dietary change interventions are needed; recom-
mendations for dietary change policies across most scientific litera-
ture are general and vague (16,55). Policies can leverage social,
behavioral, and organizational sciences to change the underlying
motivations and choice environments that drive consumer deci-
sions (64,67). Small successes should also be better communicated to
decision-makers and ambitiously scaled to large populations with
help from community-based advocacy and organizing (68).
Differentiating risks across food animals
Shifting production and consumption from beef to poultry is a com-
mon recommendation in the literature. Such shifts would accomplish
most of the GHG emission mitigation as reducing or eliminating all
meat (69–71). These recommendations have shaped national cli-
mate policies: Ethiopia stated plans to shift 30% of their beef pro-
duction to poultry in their 2021 Nationally Determined Contribution
to the United Nations Framework Convention on Climate Change
(72). However, such shifts could maintain or even increase zoonotic
disease risks.
Beef has higher land use and is associated with more tropical
deforestation than any other commodity (73). However, monogas-
tric animals, including pigs and chickens, require higher antibiotic
use and higher animal populations to produce the same quantity of
meat as ruminants such as cattle (Fig.3). Pigs and chickens are fed
more than three times the antibiotics than cattle in intensive sys-
tems (74) due to close confinement of animals and their wastes. It
takes three pigs or 170 chickens to produce the meat of one steer.
Intensive methods of monogastric animal production entail more
marked confinement, including hen laying and pig gestation sys-
tems wherein animals are confined without enough space to spread
their wings or turn around. Now, there are more than 33 billion
chickens on Earth, representing more than 70% of global avian bio-
mass (75). Shifts from beef to even greater chicken consumption
would entail greater confinement and subtherapeutic antibiotic use
for a larger number of animals, elevating multiple risks for zoonotic
disease emergence.
The precise zoonotic disease risks of individual foods and whole
dietary patterns have not previously been quantified. Statistical
analyses are challenging because any predictive metrics would en-
tail creating robust models from only a few (but highly costly) zoo-
notic disease spillover events and outbreaks that have emerged from
agricultural production, often from diverse pathogens and with
sometimes ambiguous origins. The lack of quantitative disease anal-
yses remains a hurdle to assessing the full costs, benefits, and trade-
offs of food system transitions. Despite this, plant-rich diets entail
cost-saving cobenefits (76,77), including environmental outcomes,
human nutrition, and animal welfare, which have been quantified
robustly in previous work (78–80).
INTERNATIONAL COORDINATION FOR PRIMARY PREVENTION
OF PANDEMICS
The coronavirus disease 2019 pandemic has increased the vigilance
of the global community in identifying and monitoring the poten-
tial sources of the next zoonotic disease outbreak. Well-trodden
prevention strategies include suppressing disease in vulnerable ani-
mals, monitoring transmission and spillover events of pathogens
with pandemic potential, and stopping detected outbreaks in do-
mesticated animals through culling (81). These decade-long pursuits
have only tackled pathogens of concern after some initial emer-
gence or spillover. They do not address root causes of transmission,
mutation, spillover, and proliferation of emerging infectious zoonotic
Shifts to
plant-rich
diets
Semi-
intensification
Forest
conservation
Leakage—
Production is
displaced
elsewhere
Remaining
demand must be
met with
confined/industrial
production
Rebound effects/
Jevons’ paradox—
Lower costs beget more
consumption &
production
Fewer cobenefits for
emerging economies
(low- & lower-middle
income)
Ecosystem
preservation/
restoration,
zoonotic disease
prevention
Inexpensive land
clearing—Low
incentives to
improve marginal
production
Lack of supportfor low-
income producers and
animals’ health
Fig. 2. A three-pillar approach for preventing zoonotic disease emergence and
reducing environmental impacts from animal agriculture (center). Within indi-
vidual circles and the intersections between the two, limitations of adopting only
one or two strategies are described.
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pathogens. The high and increasing demand for animal- sourced foods
is one such root cause.
Strategies that prevent infectious diseases at their root sources
are called primary prevention (6,18,33). This work outlines three
pillars for primary prevention that, when combined, constitute
stronger protection against zoonotic diseases from animal agricul-
ture than any one pillar in isolation (Fig2). National governments
should coordinate their support for a wide range of policies and
activities that support these pillars, including expanding veterinary
and extension services for improved animal care in LMICs (18),
phasing out and banning subtherapeutic and growth-promoting anti-
biotic uses (82), forming multilateral commitments among countries
importing and exporting tropical commodities linked to defor-
estation (73), ambitiously scaling community-based approaches to
popularizing plant-rich diets (68), supporting open and public al-
ternative protein research (77), and facilitating sustainable and just
transitions for producers. Commitments should also set quantifi-
able science-based goals and fund ongoing research to monitor and
accelerate progress. Together, the three pillars of primary preven-
tion can guide and empower decision-makers to escape the zoonotic
disease trap of business-as-usual animal agriculture.
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Cattle Pigs Chickens
Hectares (10,000 m
2
)
3.93 ha
0.78 ha
0.44 ha
A
Grams
Cattle Pigs Chickens
45 g
172 g
148 g
B
= 1 animal
3.5 cattle
11.3 pigs
Number of animals
C
Requirements for 1 metric ton of meat production in OECD countries
Native forest area cleared
Antibiotics
592 chickens
Fig. 3. Requirements to produce 1 metric ton of meat (dressed carcass weight), averaged among all OECD countries and weighted by production quantity, base
year 2010. (A) Hectares required for the production of animal feed (crops, pastures, and forages) in natively forested areas, calculated by the author from geospatial
potential vegetation data and agricultural production data in (9) and sources therein. (B) Grams of antibiotics used, derived from (74). (C) Number of animals required for
slaughter, from United Nations FAOSTAT (105). OECD, Organization for Economic Co-operation and Development.
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Acknowledgments: I thank B. Franks, J. Sebo, D. Jamieson, W. Alonso, and N. Mueller as well
as the anonymous reviewers for helpful input regarding the contents and direction of this
article. Funding: The authors acknowledge that they received no funding in support of this
research. Author contributions: M.N.H. authored this report, including all drafts and
revisions, performed the data analysis, and created and designed all figures contained
therein. Competing interests: The author declares that he has no competing interests.
Data and materials availability: All data needed to evaluate the conclusions in the paper are
present in the paper, with the exception of data in Fig. 3, the online sources of which are cited
in the caption.
Submitted 26 June 2022
Accepted 15 September 2022
Published 2 November 2022
10.1126/sciadv.add6681
