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Animal Ag Drivers of Zoonoses

Animal ag heightens zoonotic disease risk through both direct and indirect routes.

Direct exposure is at the factory farm level where the farmed animals are the original hosts, amplifiers, or transmitters of pathogens. Transmission of pathogens to humans can be via direct contact with the animal, manure, or with contaminated water, soil, or air.[1,2]

Indirectly, risk stems from the vast impacts of feed crop production and grazing on land use, including deforestation, and from the loss of biodiversity, especially wildlife.[3] These changes to land use and biodiversity heighten the risk of transmission of pathogens from wildlife or farmed animals to humans.

 

  1. Klous, G., et al., (2016). Human–livestock contacts and their relationship to transmission of zoonotic pathogens, a systematic review of literature. One Health, 2, 65-76, p. 66. [“Contact with livestock animals can lead to transmission of micro-organisms by inhalation, ingestion, via conjunctiva, or during incidents such as biting or other injuries inflicted by animals… aerosols contaminated with micro-organisms from respiratory or fluid sources… the presence of animals or some type of contact with (livestock-) animals is crucial.”] 
  2. Jones, B. A., et al., (2013). Zoonosis emergence linked to agricultural intensification and environmental change. PNAS, 110(21), 8399-8404, p. 8401. [“Large quantities of waste are produced that contain a variety of pathogens capable of survival for several months if left untreated.”]
  3. Shepon, A., et al., (2023). Exploring scenarios for the food system–zoonotic risk interface. The Lancet Planetary Health, 7(4), e329-e335, pp. e330-331.  [“These drivers include (1) biodiversity loss; (2) land fragmentation; (3) pesticide use; (4) water use; (5) fertiliser application; (6) antibiotics use; (7) wildlife hunting; (8) aquaculture; (9) livestock densities; and (10) farmworker densities.”]

High-density livestock and poultry operations are optimal environments for incubating, amplifying, and transmitting zoonotic diseases.[1] The main factors:

 Large numbers confined: Where animals are crowded into sheds or on feedlots, “Thousands of animals can be infected within a few days.”[2,3]  

 Stress: Factory farmed animals are deprived of normal behaviors, lack access to natural light, are fed unnatural diets, and undergo multiple mutilations and amputations. The associated stress is known to weaken the immune system.[4]

 Genetic homogeneity: Large groups of genetically similar animals bred for growth and consistency are more vulnerable to infection than genetically diverse populations because the latter are more likely to include individuals that better resist disease.[5] Narrow genetic selection focused on productivity can compromise immune function “as energy that would otherwise be used for defence is diverted to growth and reproduction.”[6]

 Antibiotic use: The intensive use of antimicrobials can suppress the immune system of mammals, which leads to a cycle of increasing disease treated with more antibiotics.[7] In addition, persistent use of antibiotics facilitates the emergence of antibiotic-resistant pathogenic strains. The incidence of drug-resistant emerging infectious diseases is growing.[8]

 Transport and comingling: The modern factory farm system tends towards specialization, with operations transporting animals multiple times during their short lives. This increases animal stress and the exposure to pathogens, ultimately increasing exposure to farm workers.[9,10]

 Environmental pathways: Infection routes include factory farm ventilation systems and massive amounts of waste dispersed into soil and water where pathogens can be harbored for months.[11]

The most common transfers of pathogens are to workers in contact with live, ill, or dead animals via inhalation, ingestion, conjunctiva, biting, or aerosols from respiratory or fluid sources.[12]

 

  1. Wegner, G. I., et al., (2022). Averting wildlife-borne infectious disease epidemics requires a focus on socio-ecological drivers and a redesign of the global food system. eClinicalMedicine, 47, p.4. [“Instead, intensive livestock production and transportation systems are known to play a major role in increasing the likelihood of domesticated animals acting as intermediate and amplifier host for wildlife-origin diseases…”]
  2. Jones, B. A., et al., (2013). Zoonosis emergence linked to agricultural intensification and environmental change. PNAS, 110(21), 8399-8404, p. 8401. [“Intensification of livestock production, especially pigs and poultry, facilitates disease transmission by increasing population size and density.”]
  3. Espinosa, R., et al., (2020). Infectious Diseases and Meat Production. Environmental & Resource Economics, 76(4), 1019–1044, p. 1020. […intensive animal farming creates conditions for the emergence and amplification of epidemics because of the physical and genetic proximity of the billions of animals, often in frail health, that are raised indoors every year.” at p. 1023]
  4. Schuck-Paim, C. et al., (2023) Animal Welfare and Human Health, in Routledge Handbook of Animal Welfare (eds. Andrew Knight et al.) Taylor & Francis Group, New York, NY, pp. 323-327.
  5. United Nations Environment Programme and International Livestock Research Institute (2020). Preventing the Next Pandemic: Zoonotic diseases and how to break the chain of transmission. Nairobi, Kenya, p. 15, 25.
  6. Schuck-Paim, C. et al., (2023), p. 323.
  7. Yang, J. H., et al., (2017). Antibiotic-Induced Changes to the Host Metabolic Environment Inhibit Drug Efficacy and Alter Immune Function. Cell Host & Microbe, 22(6), 757-765, p. 757.
  8. Jones, K. E., et al., (2008). Global trends in emerging infectious diseases. Nature, 451(7181), 990–993, p. 991. [Emerging infectious diseases “caused by drug-resistant microbes (which represent 20.9% of the EID events in our database) have significantly increased with time … probably related to a corresponding rise in antimicrobial drug use, particularly in high-latitude developed countries.”]
  9. Emily Anthes and Linda Qiu (May 20, 2024) Farm Animals Are Hauled All Over the Country. So Are Their Pathogens. The New York Times. https://www.nytimes.com/2024/05/20/health/livestock-disease-transport.html
  10. Espinosa, R., et al., (2020), pp. 1024-1025.
  11. Jones, B. A., et al., (2013), p. 8401.
  12. Klous, G., et al., (2016). Human–livestock contacts and their relationship to transmission of zoonotic pathogens, a systematic review of literature. One Health, 2, 65-76, p. 66. [“Contact with livestock animals can lead to transmission of micro-organisms by inhalation, ingestion, via conjunctiva, or during incidents such as biting or other injuries inflicted by animals… aerosols contaminated with micro-organisms from respiratory or fluid sources… the presence of animals or some type of contact with (livestock-) animals is crucial.”]                      

The major indirect factors are animal ag’s part in land use change and biodiversity loss, primarily due to the impacts of feed crops and grazing.[1]

 

  1. See answers below for details.

It is generally understood that the conversion and fragmentation of natural ecosystems due to agricultural and urban expansion increase the spread of zoonotic disease. Land use change and habitat loss, especially deforestation and habitat fragmentation, expand opportunities for human interaction with diseased or pathogen-carrying wildlife and change the mix of species in ways that increase the chances of zoonotic disease spread.[1-5]

This is a complex subject with high levels of uncertainty; most reports end their analyses with that understanding.[6]

 

  1. Gibb, R., et al., (2020). Zoonotic host diversity increases in human-dominated ecosystems. Nature, 584(7821), 398-402. [“Our results suggest that global changes in the mode and the intensity of land use are creating expanding hazardous interfaces between people, livestock and wildlife reservoirs of zoonotic disease.” at Abstract]                   
  2. Gottdenker, N. L., et al., (2014). Anthropogenic land use change and infectious diseases: a review of the evidence. EcoHealth, 11, 619-632. [“In response to anthropogenic (land use) change, more than half of the studies (56.9%) documented increased pathogen transmission, 10.4% of studies observed decreased pathogen transmission, 30.4% had variable and complex pathogen responses…” at Abstract]
  3. Rulli, M. C., et al., (2021). Land-use change and the livestock revolution increase the risk of zoonotic coronavirus transmission from rhinolophid bats. Nature Food, 2(6), 409-416, p. 409. [“Emerging infectious diseases frequently originate from pathogen spillovers from wildlife to humans; contributing factors include forest fragmentation, habitat destruction, agricultural expansion, concentrated livestock production and human penetration into wildlife habitats.”]
  4. Barbier, E. B. (2021). Habitat loss and the risk of disease outbreak. Journal of Environmental Economics and Management, 108, 102451, p. 10. [“Increasing evidence suggests that emerging infectious diseases, such as COVID-19, originate from wildlife species, and that land-use change is an important pathway for the transmission of zoonotic diseases from wildlife to humans.”]
  5. Marcolin, L., et al., (2024). Early-stage loss of ecological integrity drives the risk of zoonotic disease emergence. Journal of the Royal Society Interface, 21(215), 20230733. [“We found EID (emerging infectious disease) risk was strongly predicted by structural integrity metrics such as human footprint and ecoregion intactness, in addition to environmental variables such as tropical rainforest density and mammal species richness.” at Abstract]
  6. Gottdenker, N. L., et al., (2014), p. 630. [“Although there are many cases in which land use change is associated with increased disease transmission, there is a great deal of uncertainty regarding the direction, magnitude, and mechanisms of anthropogenic disturbances on infectious disease transmission and persistence.”]              

Agriculture is the primary driver of land use change.[1] Animal agriculture, including the land used for feed crops and grazing, is responsible for the greatest use and expansion of agricultural lands.[2,3] It is central to deforestation in the tropics, causing habitat loss, and an increase in zoonoses.[4-6]

More than half of U.S. crop acreage is devoted to crops specifically used for animal feed. And cropland expansion continues in the U.S. intermittently, mostly for corn and soybean production which is predominantly used for animal feed. That cropland expansion “is infringing upon high-quality natural land in such a way as to disproportionately affect the wildlife that depends on it.”[7] Livestock grazing also has negative impacts on biodiversity, and grazing is the primary use of ~35% of total land in the continental U.S.[8]

Fertilizers and pesticides – a large share of which is used on feed crops – also damage habitats and exacerbate zoonotic disease rates.[9,10]

 

  1. IPBES (2019) Global assessment report of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, Brondízio, E. S. et al. (eds). IPBES secretariat, Bonn, Germany, p. 12. [“Agricultural expansion is the most widespread form of land-use change, with over one third of the terrestrial land surface being used for cropping or animal husbandry.” ]
  2. Alexander, P. et al., (2015). Drivers for global agricultural land use change: The nexus of diet, population, yield and bioenergy. Global Environmental Change, 35, 138–147, p. 1. [“The results show that the production of animal products dominates agricultural land use and land use change over the 50-year period (1961-2011), accounting for 65% of land use change.”]
  3. Machovina, B., et al., (2015). Biodiversity conservation: The key is reducing meat consumption. The Science of the Total Environment, 536, 419–431. [“Livestock production is the single largest driver of habitat loss, and both livestock and feedstock production are increasing in developing tropical countries where the majority of biological diversity resides.” at Abstract]
  4. Pendrill, F., et al., (2022). Disentangling the numbers behind agriculture-driven tropical deforestation. Science, 377(6611), eabm9267. [More than 90% of deforestation across the tropics is driven by agriculture, with about half due to cattle production and about 10% due to soybean production.]
  5. Bernstein, A. S., et al., (2022). The costs and benefits of primary prevention of zoonotic pandemics. Science Advances, 8(5), eabl4183, p. 1. [“The three main drivers of pathogen emergence: (i) wildlife trade and hunting, (ii) agricultural intensification and expansion, and (iii) destruction of tropical forests.”]
  6. Alimi, Y., et al., (2021). Report of the scientific task force on preventing pandemics. Harvard Chan C-CHANGE and Harvard Global Health Institute, Cambridge, MA, Chapter 5, p. 9. [“One of the most common types of land use change contributing to zoonotic spillover is deforestation.]
  7. Lark, T. J., et al., (2020). Cropland expansion in the United States produces marginal yields at high costs to wildlife. Nature Communications, 11(1), 4295–4295, p. 7.
  8. Winters-Michaud, C. et al., (2024) Major Uses of Land in the United States, 2017, USDA ERS Bulletin No. 275, Table 1, p. 5.
  9.  Rohr, J.R. et al., (2019) Emerging human infectious diseases and the links to global food production, Nature Sustainability, 2, 445-456, p. 450.
  10. Shepon, A., et al., (2023). Exploring scenarios for the food system–zoonotic risk interface. The Lancet Planetary Health, 7(4), e329-e335, pp. e330-331. [Includes pesticide use and fertilizer application in top ten drivers of zoonoses]           

Although this is a complex area with a lack of consensus[1,2], most researchers support the concept that pressures on biodiversity lead to heightened risks of disease.[3-7] For instance, habitat loss forces the migration of wild animal populations, facilitating the spread of any foreign diseases.[8] And diverse ecological communities inhibit the growth of parasites likely to spread disease (e.g., ticks, mosquitoes).[9]

The rare report points out that our intuitive understanding of nature and ecology along with the precautionary principle would lead us to assume that protecting wildlife and natural ecosystems is good for our health.[10]

 

  1. Keesing, F., & Ostfeld, R. S. (2021). Impacts of biodiversity and biodiversity loss on zoonotic diseases. PNAS, 118(17), e2023540118. [“The disease ecology community has struggled to come to consensus on whether biodiversity reduces or increases infectious disease risk, a question that directly affects policy decisions for biodiversity conservation and public health.”]
  2. Glidden, C. K., et al., (2021). Human-mediated impacts on biodiversity and the consequences for zoonotic disease spillover. Current Biology, 31(19), R1342-R1361. [“There has been a contentious debate about the existence and generality of the relationship between biodiversity and disease…”]
  3. Keesing, F., et al., (2010). Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature, 468(7324), 647-652. [“Overall, despite many remaining questions, current evidence indicates that preserving intact ecosystems and their endemic biodiversity should generally reduce the prevalence of infectious diseases.” at Abstract] 
  4. Johnson, C. K., et al., (2020). Global shifts in mammalian population trends reveal key predictors of virus spillover risk. Proceedings of the Royal Society B, 287(1924), 20192736. [“Among threatened wildlife species, those with population reductions owing to exploitation and loss of habitat shared more viruses with humans.” at Abstract]
  5. Bedenham, G., et al., (2022). The importance of biodiversity risks: Link to zoonotic diseases. British Actuarial Journal, 27, e10. [“The likelihood of future epidemics and pandemics is increased by the destruction of natural habitats, leading to increased interaction between humans and wildlife. Destruction of habitats also opens up the transport pathways from remote areas to population centres, and the combination of these two factors act together to increase the likelihood of zoonotic diseases emerging. The spread of zoonotic diseases shares the same underlying causes that drive biodiversity loss, including climate change and habitat loss.“]
  6. Glidden, C. K., et al., (2021), p. R1354. [“We identified mechanistic evidence in the literature that anthropogenically driven biodiversity change may increase zoonotic spillover risk.”]
  7. Jones, B. A., et al., (2013). Zoonosis emergence linked to agricultural intensification and environmental change. PNAS, 110(21), 8399-8404. [This systematic review found several examples of zoonotic disease emergence at the wildlife–livestock–human interface that were associated with varying combinations of agricultural intensification and environmental change, such as habitat fragmentation and ecotones, reduced biodiversity, agricultural changes, and increasing human density in ecosystems.]
  8. Bedenham, G., et al., (2022), p. 4.
  9. Civitello, D. J., et al., (2015). Biodiversity inhibits parasites: broad evidence for the dilution effect. PNAS, 112(28), 8667-8671, p. 8667. [“Consequently, human induced declines in biodiversity could increase human and wildlife diseases… Biodiversity conservation might then limit the abundance of many parasites of wildlife and humans.”]
  10. Shepon, A., et al., (2023). Exploring scenarios for the food system–zoonotic risk interface. The Lancet Planetary Health, 7(4), e329-e335. [“As a general rule of risk management based on the precautionary principle and our understanding of disease ecology, intact ecosystems tend to be healthier ones, posing lower threats to human and livestock health.”]  

Perhaps the key indicator of animal ag’s impacts on biodiversity is the ratio of farmed mammal biomass to wild land mammal biomass – a staggering 25 to 1.[1] The current biomass of wild land mammals is approximately one-seventh of its pre-human size.[2] We use the great majority of the world’s land and resources for animals raised for food, with ever smaller shares allocated to wild animals.

It is commonly recognized that the management and expansion of land used for crops and animal production is the primary driver of biodiversity loss.[3-5] Feed crops, factory farms, and grazing make up the bulk of agriculture’s footprint.[6]

About two-thirds of agriculture’s impacts on deforestation is attributed to animal ag.[7] Deforestation is understood to have major impacts on biodiversity as well as zoonotic disease.[8-10]

Feed crop production techniques, including pesticide and fertilizer use, adds to the loss of biodiversity.[11]

 

  1. Greenspoon, L., et al., (2023). The global biomass of wild mammals. PNAS, 120(10), e2204892120 [Similarly, the biomass of farmed birds (e.g., chickens, turkeys) is ~70% of all bird biomass]
  2. Bar-On, Y. M. et al., (2018). The biomass distribution on Earth. PNAS 115(25)650606511, p. 3. (Biomass = aggregated weight) and see, Farmed Animal Biomass and Biodiversity
  3. Campbell, B. M., et al., (2017). Agriculture production as a major driver of the Earth system exceeding planetary boundaries. Ecology and society, 22(4). [“In the absence of better information, we suggest 80% as the role of agriculture in the status of the biosphere integrity…”]
  4. Jaureguiberry, P. et al., (2022) The direct drivers of recent global anthropogenic biodiversity loss. Science Advances 8, eabm9982, p. 3. [“We have shown clearly that land/sea use change—mainly in the form of rapid expansion and intensifying management of land used for cropping or animal husbandry—and direct exploitation—mostly through fishing, logging, hunting, and wildlife trade—have been the two dominant drivers of global biodiversity loss overall over recent decades.”]  
  5. Díaz, S., et al., (2019). Pervasive human-driven decline of life on Earth points to the need for transformative change. Science, 366(6471), eaax3100, p. 4. [“Within terrestrial and freshwater ecosystems, the driver with the highest relative impact is land use change, mainly land conversion for cultivation, livestock raising, and plantations.”]
  6. Poore, J., & Nemecek, T. (2018). Reducing food’s environmental impacts through producers and consumers. Science, 360(6392), 987-992, p. 4. [“…the impacts of animal products can markedly exceed those of vegetable substitutes, to such a degree that meat, aquaculture, eggs, and dairy use ~83% of the world’s farmland…”]
  7. Poore, J., & Nemecek, T. (2018), p. 5. [“…we find that deforestation for agriculture is dominated (67%) by feed, particularly soy, maize, and pasture…”]
  8. Lewis, S. L., et al., (2015). Increasing human dominance of tropical forests. Science, 349(6250), 827-832. [“Tropical forests house over half of earth’s biodiversity…” at Abstract]
  9. Bernstein, A. S., et al., (2022). The costs and benefits of primary prevention of zoonotic pandemics. Science Advances, 8(5), eabl4183, p. 1. [“the three main drivers of pathogen emergence: (i) wildlife trade and hunting, (ii) agricultural intensification and expansion, and (iii) destruction of tropical forests.”]
  10. Dobson, A. P., et al., (2020). Ecology and economics for pandemic prevention. Science (American Association for the Advancement of Science), 369(6502), 379–381, p. 379. [“The clear link between deforestation and virus emergence suggests that a major effort to retain intact forest cover would have a large return on investment even if its only benefit was to reduce virus emergence events.”]
  11. See also, Fertilizer Harms to Biodiversity; Insecticide Harm to Biodiversity; Herbicide Harm to Biodiversity        

Human Health Impacts