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 nonhuman 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 conversion to human-dominated systems drive the loss, turnover, and homogenization of biodiversity and expose adjacent human communities 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 environmental 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 efficiency, 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 confinement 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 intensification that remains unaddressed in the scientific literature: Intensified 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 agriculture could therefore undermine the “sustainable” nature of sustainable 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 interactions 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 reduce the likelihood of zoonotic disease emergence, including (i) selectively intensifying the least productive regions, namely, LMICs, without resorting to confinement and other common high-risk intensive 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 implemented 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 implicated in zoonotic disease emergence in the literature (
Table 1). 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-resistant infectious bacteria including methicillin-resistant
Staphylococcus aureus and
Escherichia coli (
19,
20).
Intensified animal agriculture is often, but not always, characterized by a shift toward “landless” or “industrialized” systems (as defined 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 animals 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 fattened to reach their final market weights on enriched feeds in feedlots, with stocking densities for cattle on outdoor feedlots of less than 4 m
2 per steer/heifer (
24). These environments entail physiological and mental stress, close proximity to each other and wastes, and the routine administration of subtherapeutic (infection-preventing) and growth-promoting antibiotics (
Table 1). 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 livestock (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 pastoralist 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 (
Table 1), 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 production or lead to net agricultural expansion. For instance, eliminating feedlot beef cattle systems in the United States by shifting to intensive 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 efficient than semi-intensive methods. Shifting away from industrialized 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.
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 potential sources of the next zoonotic disease outbreak. Well-trodden prevention strategies include suppressing disease in vulnerable animals, monitoring transmission and spillover events of pathogens with pandemic potential, and stopping detected outbreaks in domesticated animals through culling (
81). These decade-long pursuits have only tackled pathogens of concern after some initial emergence or spillover. They do not address root causes of transmission, mutation, spillover, and proliferation of emerging infectious zoonotic 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 agriculture than any one pillar in isolation (
Fig 2). 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 antibiotic uses (
82), forming multilateral commitments among countries importing and exporting tropical commodities linked to deforestation (
73), ambitiously scaling community-based approaches to popularizing plant-rich diets (
68), supporting open and public alternative protein research (
77), and facilitating sustainable and just transitions for producers. Commitments should also set quantifiable science-based goals and fund ongoing research to monitor and accelerate progress. Together, the three pillars of primary prevention can guide and empower decision-makers to escape the zoonotic disease trap of business-as-usual animal agriculture.
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.