A zoonosis is an infectious disease that is transmitted from vertebrate non-human animals (wild or domesticated) to humans. A zoonotic pathogen can be bacterial, viral, or parasitic, and can be transmitted by direct contact with an animal, through contamination of food, water, soil, or air, or indirectly via non-vertebrate “vectors” such as mosquitoes.[1-3]
Most zoonoses are viral and most originate in mammals, often rodents, bats and non-human primates.[4,5] Most zoonoses have limited and regional impact; some turn into pandemics (i.e., occurring over a wide geographic area and affecting a significant portion of the population).[6]
A zoonosis begins with a “spillover transmission” during which a pathogen moves from an animal to a human. The risk of a spillover is determined by “the prevalence of infection in the animal reservoir, the rate at which humans come into contact with these animals, and the probability that humans become infected when contact occurs.”[7]
World Health Organization – Zoonoses (2020) https://www.who.int/news-room/fact-sheets/detail/zoonoses
Rahman, M. T., et al., (2020). Zoonotic diseases: etiology, impact, and control. Microorganisms, 8(9), 1405.
Mills, J., & Driscoll, M. (2022). The hidden health impacts of industrial livestock systems: transforming livestock systems for better human, animal and planetary health. World Animal Protection, p. 4.
Morse, S. S., et al., (2012). Prediction and prevention of the next pandemic zoonosis. The Lancet, 380(9857), 1956-1965. [“Most pandemics… are caused by viruses…” at Abstract. “Most of the identified reservoirs are mammalian (roughly 80%) or, to a lesser extent, avian, although people share some pathogens with invertebrates, which act as vectors or intermediate hosts.” at p. 1957]
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, p. 4. [“We found that three mammalian orders (rodents, bats and primates) have together been implicated as hosts for the majority (75.8%) of zoonotic viruses described to date, and these orders represent 72.7% of all terrestrial mammal species.”]
Bernstein, A. S., et al., (2022). The costs and benefits of primary prevention of zoonotic pandemics. Science Advances, 8(5), eabl4183, Table 1. [Listing of “Mortality from zoonotic viral emergence since 1918”]
Lloyd-Smith, J. O., et al., (2009). Epidemic dynamics at the human-animal interface. Science, 326(5958), 1362-1367, p. 2.
Covid-19 showed why zoonoses can transform the arc of human societies. The virus is widely considered to be a zoonosis although no animal source has been definitively confirmed.[1,2] Regardless of origin, the pandemic has shown the potential health and societal impacts of zoonotic diseases.
Zoonoses can have limited regional impacts or can turn into pandemics causing major loss in many areas around the world.
The three largest zoonoses since 1900 are the “Spanish flu” with ~50 million deaths, HIV virus with ~40 million, and Covid-19 with ~7 million.[3-5] The Black Death (bubonic plague) of the 1300’s was a zoonotic disease, estimated to have killed a quarter to a half of the European populace.[6] The rate of emerging zoonotic diseases is understood to be growing.[7-9]
Gostin, L. O., & Gronvall, G. K. (2023). The origins of Covid-19—why it matters (and why it doesn’t). New England Journal of Medicine, 388(25), 2305-2308, p. 2307. [“Of the three possibilities — natural, accidental, or deliberate — the most scientific evidence yet identified supports natural (i.e., zoonotic) emergence.”]
Haider, N., et al., (2020). COVID-19-Zoonosis or Emerging Infectious Disease? Frontiers in Public Health, 8, 596944–596944. [“The pandemic of Coronavirus disease (COVID-19) caused by SARS-CoV-2 has been classified as a zoonotic disease, however, no animal reservoir has yet been found…” at Abstract]
World Health Organization (2024) Number of COVID-19 deaths reported to WHO. https://data.who.int/dashboards/covid19/deaths?n=o [Cumulative total as of September 2024 was 7.1 million]
World Health Organization (2024) HIV data and statistics. https://www.who.int/teams/global-hiv-hepatitis-and-stis-programmes/hiv/strategic-information/hiv-data-and-statistics [“HIV continues to be a major global public health issue, claiming 42.3 million (35.7–51.1 million) lives so far.”]
Bernstein, A. S., et al., (2022). The costs and benefits of primary prevention of zoonotic pandemics. Science Advances, 8(5), eabl4183, Table 1. [Listing of “Mortality from zoonotic viral emergence since 1918”]
Morens, D. M., et al., (2008). Emerging infections: a perpetual challenge. The Lancet infectious diseases, 8(11), 710-719, p. 712.
Jones, K. E., et al., (2008). Global trends in emerging infectious diseases. Nature, 451(7181), 990–993. [Emerging infectious diseases, predominantly zoonoses, “are increasing significantly over time.”]
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, Lancet, 47, 101386, p. 3. [“…numerous proponents have argued that zoonoses, and especially those from wildlife, may have been emerging and re-emerging at an unprecedented and increasing rate in recent decades.”]
Smith, K. F., et al., (2014). Global rise in human infectious disease outbreaks. Journal of the Royal Society Interface, 11(101), 20140950.
Major zoonotic diseases (i.e., pandemics) are global and their impacts enormous. And yet the understanding of the range of sources and types of spillovers is still limited.[1] While the body of literature has grown substantially (accelerated by Covid-19), simmering controversies have also grown.[2,3] Add to this the inherent challenge of evaluating future risk and there’s a lot to unpack.[4]
Normally on this site we try to document what is happening and we focus on the U.S. However, when the risk, heightened by animal ag, potentially includes the elimination of large parts of the human population, it warrants extra attention. We try to present the middle ground among differing opinions, while also pointing out the general acceptance of heightened risk from an industrial animal ag system that is “feeding the world.”
Jones, B. A., et al., (2013). Zoonosis emergence linked to agricultural intensification and environmental change. PNAS, 110(21), 8399-8404. [“However, available research inadequately addresses the complexity and interrelatedness of environmental, biological, economic, and social dimensions of zoonotic pathogen emergence, which significantly limits our ability to predict, prevent, and respond to zoonotic disease emergence.” At Abstract. Note: Like this report, most end with an analysis of what they don’t know, and the list is usually long.]
Bartlett, H., et al., (2022). Understanding the relative risks of zoonosis emergence under contrasting approaches to meeting livestock product demand. Royal Society Open Science, 9(6), 211573–211573, p. 3. [“…a Google Scholar search on ‘emerging infectious disease’ and ‘agriculture’ returned 271,000 highly diverse papers in February 2022.”]
Rohr, J. R., et al., (2020). Towards common ground in the biodiversity–disease debate. Nature ecology & evolution, 4(1), 24-33.
Morse, S. S., et al., (2012). Prediction and prevention of the next pandemic zoonosis. The Lancet, 380(9857), 1956-1965.
More than 60% of known infectious diseases are zoonotic, i.e., those with a non-human animal source. About 75% of emerging or reemerging infectious diseases are zoonotic.[1-5]
Emerging or reemerging infectious diseases are a sub-category of infectious diseases “that have newly appeared in a population or have existed previously but are rapidly increasing in incidence or geographic range.”[6]
U.S. CDC, About Zoonotic Diseases. https://www.cdc.gov/one-health/about/about-zoonotic-diseases.html [“Scientists estimate that more than 6 out of every 10 known infectious diseases in people can be spread from animals, and 3 out of every 4 new or emerging infectious diseases in people come from animals.”]
Taylor, L. H., et al., (2001). Risk factors for human disease emergence. Philosophical Transactions of the Royal Society B. Biological Sciences, 356(1411), 983–989. [Zoonotic = 61% of all infectious disease, 75% of emerging]
Karesh, W. B., et al., (2012). Ecology of zoonoses: natural and unnatural histories. The Lancet (British Edition), 380(9857), 1936–1945, p. 1936. [“More than 60% of human infectious diseases are caused by pathogens shared with wild or domestic animals.”]
Espinosa, R., et al., (2020). Infectious Diseases and Meat Production. Environmental & Resource Economics, 76(4), 1019–1044, p. 1020. [75% of emerging infectious diseases are zoonotic]
Woolhouse, M. E. J., & Gowtage-Sequeria, S. (2005). Host range and emerging and reemerging pathogens. Emerging Infectious Diseases, 11(12), 1842–1847, p. 1844. [In a comprehensive literature survey of 1400 pathogens, 58% were zoonotic – 73% of emerging or reemerging pathogens were zoonotic.]
Morens, D. M., et al., (2004). The challenge of emerging and re-emerging infectious diseases. Nature, 430(6996), 242-249, p. 242.
Although researchers have differing opinions about primary causes, there is agreement that the key drivers are:[1-6]
(Bolded are factors connected to animal agriculture.)
Human contact with wildlife – through hunting, trading, and exotic pets. Land use changes – spurred by agriculture, including grazing and feed crops for animal ag. Deforestation – as a key sub-category of land use changes, especially in the tropics. Livestock transmission and amplification of disease – both in extensive pasture-raised systems and intensive factory farm systems.
Other drivers: Loss of biodiversity – including habitat loss and changes to species compositions. Increasing antibiotic resistance – partially due to livestock usage. Globalization – including human travel and product transportation. Animal transport – of wild and farmed animals. Climate change – affecting wildlife and all ecosystems. Human population growth – including urbanization and industrial growth. Agricultural practices – including water use, pesticides, and fertilizers.
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.”]
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. 7 [“Seven human-mediated factors are most likely driving the emergence of zoonotic diseases: 1) increasing human demand for animal protein; 2) unsustainable agricultural intensification; 3) increased use and exploitation of wildlife; 4) unsustainable utilization of natural resources accelerated by urbanization, land use change and extractive industries; 5) increased travel and transportation; 6) changes in food supply; and 7) climate change.”]
Alimi, Y., et al., (2021). Report of the scientific task force on preventing pandemics. Cambridge, MA: Harvard Chan C-CHANGE and Harvard Global Health Institute. Chapter 5, pp. 8-15 [Drivers: Land use change especially deforestation, expansion of agricultural lands, livestock intensification, urbanization, wild animal hunting and consumption, wildlife trade, climate change.]
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, 101386, p. 3. [“…being linked to faster human and livestock population growth, heightened urbanization and human crowding, accelerated encroachment into natural habitats and agricultural intensification, increased antibiotic use resulting in multi-drug resistance and the internationalisation of travel and trade.”]
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.”]
Rohr, J.R. et al., (2019) Emerging human infectious diseases and the links to global food production, Nature Sustainability, 2, 445-456. [Report covers agricultural drivers including increasing irrigation, pesticide usage, nutrient pollution from excess nitrogen and phosphorus, and the impacts of poor nutrition on the spread of disease.]
It is estimated that ~70-75% of zoonotic emerging infectious diseases are caused by pathogens with a wildlife origin.[1,2] Presumably, the remaining portions originate in farmed animals, companion animals, other domesticated animals, and exotic pets, though we are not aware of studies quantifying these figures by category. Since farmed animals are by far the larger biomass and they are known to harbor high levels of viral infections, our assumption is that they make up a large portion of the non-wildlife sources.[3,4]
Farmed animals can be the original source of zoonotic diseases or more commonly act as an intermediary host.[5] Even where the originating source (or natural “reservoir”) of disease is a wild animal, domestic animals are often the “bridging host to human infection.”[6] The virus known as “bird flu” is an example.[7]
Jones, K. E., et al., (2008). Global trends in emerging infectious diseases. Nature, 451(7181), 990–993, p. 990. [71.8% of wildlife origin]
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, p. 4. [“We found that three mammalian orders (rodents, bats and primates) have together been implicated as hosts for the majority (75.8%) of zoonotic viruses described to date, and these orders represent 72.7% of all terrestrial mammal species.”]
Johnson, C. K., et al., (2020), p. 4. [“As a group, domesticated mammals host 50% of the zoonotic virus richness but represent only 12 species.” “…mammalian orders with more species are the source of more zoonotic viruses.”]
Rahman, M. T., et al., (2020). Zoonotic diseases: etiology, impact, and control. Microorganisms, 8(9), 1405. [See, Table 1 for “Major Zoonotic Diseases, their etiological agents, hosts, and the major symptoms in humans.”]
Leibler, J. H., et al., (2009). Industrial food animal production and global health risks: exploring the ecosystems and economics of avian influenza. EcoHealth, 6, 58-70. [“Attention has often focused on wild animal reservoirs, but most zoonotic pathogens of recent concern to human health either originate in, or are transferred to, human populations from domesticated animals raised for human consumption.” at Abstract]
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. 11.
Lycett, S. J., et al., (2019) A brief history of bird flu. Phil. Trans. R. Soc. B 374: 20180247, p. 4. [“Phylogeographic analyses have revealed the role of migratory wild birds in the intracontinental circulation of LPAI in North America and have implicated wild birds following North American flyways in the introduction of H7N3 strains into Mexico in 2012–2013.”]
Improbably, some researchers posit that the intensification of animal production may reduce the emergence of zoonotic diseases, while others have not come to a conclusion about whether intensification (i.e., more concentrated factory farms) is preferable to extensive (pasture-raised) systems with regards to zoonotic disease proliferation.[1-3]
Additionally, there is controversy about the role of biodiversity loss in zoonotic emergence.[4] Therefore, the major role that industrial animal ag plays in biodiversity loss (through land use, air and water pollution, pesticides, water usage, etc.) is not commonly scrutinized in the context of zoonoses.
Still, many reports view intensive animal farming (industrial animal ag) as a major contributor to the emergence and amplification of epidemics.[5-8] Some researchers acknowledge that all forms of meat production, both intensive and extensive, raise zoonotic risks, and that paths to reducing risk should include efforts to shrink demand.[9,10]
Bartlett, H., et al., (2022). Understanding the relative risks of zoonosis emergence under contrasting approaches to meeting livestock product demand. Royal Society Open Science, 9(6), 211573–211573, p. 9. [“Our overview of the limited evidence available suggests that calls to reduce ‘intensive’ livestock farming practices to mitigate EID risk are premature…”]
Leibler, J. H., et al., (2009). Industrial food animal production and global health risks: exploring the ecosystems and economics of avian influenza. EcoHealth, 6, 58-70. [“It is often assumed that confined food animal production reduces risks of emerging zoonotic diseases.” at Abstract]
Jones, B. A., et al., (2013). Zoonosis emergence linked to agricultural intensification and environmental change. PNAS, 110(21), 8399-8404, p. 8399. [“However, the evidence is not sufficient to judge whether the net effect of intensified agricultural production is more or less propitious to disease emergence and amplification than if it was not used.”]
Keesing, F., & Ostfeld, R. S. (2021). Impacts of biodiversity and biodiversity loss on zoonotic diseases. PNAS, 118(17), e2023540118, pp. 1-2. [“Taken together, these conflicting findings appeared to mean that the loss of natural biodiversity could simultaneously increase human exposure to existing pathogens, and decrease the probability of the emergence of new ones. Such a potential contradiction has complicated the ability of scientists to provide useful information about diversity–disease relationships for public policy and management.”]
Espinosa, R., et al., (2020). Infectious Diseases and Meat Production. Environmental & Resource Economics, 76(4), 1019–1044, p. 1020. [“…intensive farming amplifies the impact of the disease due to the high density, genetic proximity, increased immunodeficiency, and live transport of farmed animals.”]
Leibler, J. H., et al., (2009). [“Attention has often focused on wild animal reservoirs, but most zoonotic pathogens of recent concern to human health either originate in, or are transferred to, human populations from domesticated animals raised for human consumption… This article provides evidence suggesting that these industrial systems may increase animal and public health risks unless there is recognition of the specific biosecurity and biocontainment challenges of the industrial model.” at Abstract]
Shepon, A., et al., (2023). Exploring scenarios for the food system–zoonotic risk interface. The Lancet Planetary Health, 7(4), e329-e335, p. e330. [Of the 10 food-related drivers, almost all are linked to industrial animal ag. “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.”]
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. [Of 7 factors driving zoonotic disease emergence #1 is “Increasing demand for animal protein” and #2 is “Unsustainable agricultural intensification,” including the use of crop lands for animal feed and the impacts on deforestation.]
Espinosa, R., et al., (2020), p. 1037. [“All types of meat production increase zoonotic risks.…it appears more and more urgent to find ways to curb the demand for meat.”]
Hayek, M. N. (2022). The infectious disease trap of animal agriculture. Science Advances, 8(44), eadd6681, Figure 2. [“A three-pillar approach for preventing zoonotic disease emergence and reducing environmental impacts from animal agriculture” includes “semi-intensification,” “shifts to a plant-based diet,” and “forest conservation.”]
First, there is the logical focus on wildlife as the origin of most zoonotic diseases, which in many reports then translates into a surprising exclusion of other factors.[1]
Secondly, many researchers assume that the steady increase in world meat consumption via factory farming is a given: both necessary and/or inevitable.[2] They treat the industrial ag system as one that must be managed and monitored rather than reformed. They recommend efforts to more efficiently manage the “interfaces” between factory farmed animals and wildlife and between farmed animals and workers. Therefore, heightened “biosecurity” is the path to warding off disease transmission, rather than questioning the place of the factory farming model in human and planetary health.[3,4]
Third, many health researchers have little understanding of the impacts of grazing and feed crops on earth ecosystems and biodiversity. The transformation of huge portions of the earth into farmland supporting animal food production is not well understood, and so the impacts on biodiversity and wildlife health are downplayed.
U.S. GAO (May 2023) Zoonotic Diseases: Federal Actions Needed to Improve Surveillance and Better Assess Human Health Risks Posed by Wildlife, GAO-23-105238.
Gilbert, W., et al., (2021). Mitigating the risks posed by intensification in livestock production: the examples of antimicrobial resistance and zoonoses. animal, 15(2), 100123, p. 2. [“An increase in the supply of LSFs (livestock source foods) will therefore be critical to global food security as the world’s population expands.” Note: Almost all analyses of the connections between industrial animal ag and zoonoses begin their reports with a paean to meat production and the feeding the world storyline.]
Graham, J. P., et al., (2008). The animal-human interface and infectious disease in industrial food animal production: rethinking biosecurity and biocontainment. Public health reports, 123(3), 282-299, p. 284. [“Biosecurity is defined as any practice or system that prevents the spread of infectious agents from infected to susceptible animals, or prevents the introduction of infected animals into a herd, region, or country in which the infection has not yet occurred.”]
Leibler, J. H., et al., (2009) Industrial food animal production and global health risks: exploring the ecosystems and economics of avian influenza. EcoHealth, 6, 58-70. [“This article provides evidence suggesting that these industrial systems (confined food animal production) may increase animal and public health risks unless there is recognition of the specific biosecurity and biocontainment challenges of the industrial model.” at Abstract]
We’re not aware of a specific quantification of risk attributable to factory farming. It is one of many possible animal-to-human transmission routes that could result in a major pandemic.[1] And yet, factory farming is acknowledged as a major factor, undoubtedly one of the largest, by many sources. This is based on the inclusion of all direct and indirect factors, including the impacts of grazing and the industrialized methods of raising feed crops, i.e., the overuse of chemical fertilizers, pesticides, water, and the overall effects on land use, deforestation, and biodiversity.[2,3]
The relative global biomass (aggregated weight) of farmed mammals versus wild mammals is about 25 to 1.[4] This starkly demonstrates the earth’s role as an animal farm, pushing wildlife into ever smaller corners and likely harming their health in ways not fully understood.
Some sources are unsure whether intensive or extensive meat farming systems are the greater risk, since extensive systems use more land and can cause more fragmentation and interactions with farmers and workers for similar amounts of production.[5,6]
Given the continuing growth of meat production and the steady transition to industrial systems, the “precautionary principle” of risk management would lead us to a more cautious approach, especially in the face of uncertainty about an existential risk.[7-9]
Linder, A., et al., (2023) Animal Markets and Zoonotic Disease in the United States. Animal Law & Policy Program, Harvard Law School, p. 31.
Shepon, A., et al., (2023). Exploring scenarios for the food system–zoonotic risk interface. The Lancet Planetary Health, 7(4), e329-e335.
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. 11.
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. See also, Yinon Bar-On et al., (2018). The biomass distribution on Earth. PNAS 115(25)650606511, p. 3] For more details, see, Farmed Animal Biomass and Biodiversity
Bartlett, H., et al., (2022). Understanding the relative risks of zoonosis emergence under contrasting approaches to meeting livestock product demand. Royal Society Open Science, 9(6), 211573–211573, p. 9. [“Our overview of the limited evidence available suggests that calls to reduce ‘intensive’ livestock farming practices to mitigate EID risk are premature…”]
Jones, B. A., et al., (2013). Zoonosis emergence linked to agricultural intensification and environmental change. PNAS, 110(21), 8399-8404. [“However, the evidence is not sufficient to judge whether the net effect of intensified agricultural production is more or less propitious to disease emergence and amplification than if it was not used.”]
Food and Agriculture Organization of the United Nations OECD (2024), OECD-FAO Agricultural Outlook 2024-2033, Paris and Rome, p. 167. [“Global meat consumption is anticipated to rise 12% by 2033 relative to the Outlook 2021-23 base period.”]
Shepon, A., et al., (2023), p. e330. [“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.”]
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, 101386, p. 8. [“These trends imply that, in the absence of targeted mitigation measures, there is a strong risk that increasing global livestock production will greatly destabilise the planetary ecosystem processes on which the web of life and human wellbeing depend.”]