Farmed animals as food – increasing the risk of zoonotic pandemics

Three aspects of human behaviour are particularly devastating and increase both the risk of pandemic occurrence and the severity of its impacts. In our previous blogs we explored the first factor – The destruction of ecosystems and the loss of biodiversity, and the second factor – The use of wild animals for food, both resulting in increased contact and virus spillover to humans and farmed animals. We’ll now take a closer look at the third factor – The use of farmed animals for food in high-density, intensified animal agriculture, resulting in ideal conditions for viral mutation, spread, and spillover to humans and wild animals.

Outbreaks of animal-borne infectious diseases such as Ebola, SARS, avian flu, and now COVID-19, caused by a novel coronavirus, are on the rise.^1 2 With COVID-19 most likely having emerged from bats and other wild animals, many people associate zoonotic diseases with exotic wild animals. However, spillover events do not occur only between wild animals and humans. The intensification of animal agriculture and aquaculture plays a key role and further escalates the risk of zoonotic pandemics. Cramming large numbers of genetically similar individuals into unsanitary, high-density settings that induce poor health and high stress levels strongly increases the chances of pathogenic spillovers between wild animals and farmed animals, and ultimately humans.

Sharing viruses – farmed animals as an interface for spillovers

There is mounting evidence that human activities facilitating contact between different animal species have likely accelerated the selection of viruses that are shared by a variety of animal hosts.³ Farmed animals frequently function as an interface which encourages virus spillover to, and subsequent spread among, humans.⁴ The key role of this transmission pathway is illustrated by the fact that it is domesticated animals such as livestock who share the highest number of viruses with humans.^5 6 Diseases such as diphtheria, measles, mumps, the rotavirus, smallpox, and influenza A all have their origin in domesticated animals.⁷

Growing demand for animal protein is driving the intensification of animal agriculture

Today, the world is seeing a rapid growth and massive intensification of animal agriculture, fuelled by a rising global demand for meat, eggs, dairy, and seafood. Accelerated population growth and increased prosperity levels have led to a growing appetite for animal-based products – with chicken and pigs at the very centre of this development.^8 9

Our hunger for animal products – staggering numbers, trending upwards

Globally, more than 75 billion land animals are slaughtered for food, every single year. This is about 10 times the number of humans living on this planet. At any point in time, there are more than 30 billion farmed animals on earth, the vast majority (82%) of them poultry such as chickens, ducks, and turkeys.¹⁰ Today, livestock accounts for 60% of all mammal biomass, and poultry for 70% of bird biomass,¹¹ with these figures continuing to grow. While these numbers are already staggeringly high, they leave fish out of the equation – with aquaculture estimated to account for up to 167 billion individual fish slaughtered each year.¹² The global production of meat, eggs, dairy, and seafood from intensive-production facilities is forecast to increase by 15% by 2028.¹³

Maximising productivity and pathogenic risks – breeding our way to zoonoses

Alongside strongly intensified husbandry conditions, the creation of new and more ‘productive’ breeds of cows, pigs, chickens, and fish have made these high livestock numbers possible. And they have helped to maximise the yield of meat, eggs, and milk per animal. Maximising productivity has put the world’s livestock species and their genetic diversity at risk, making them less resilient to environmental changes and pathogens.¹⁴ This approach has also radically increased the number of individuals confined in high-density settings. The unnatural and unhygienic conditions of large-scale animal agriculture leads to poor health and high stress levels in individual animals.¹⁵ The sum of these developments makes farmed animals more susceptible to infections,^16 17 and has thus created the perfect conditions for the emergence and spread of zoonotic diseases.

High density and high virulence – the opposite of social distancing

Moreover, the close and unsanitary proximity of individuals in intensive, high-density facilities can favour the development of high virulence – that is, the increased ability of a pathogen to infect and harm a host.^18 19 A well-studied example of the complex connection between virulence and transmission246 is the salmon louse and its host. Lice originating from farmed salmon are more harmful, i.e. they have a higher virulence (greater damage to skin tissues as a measure of virulence), than lice from wild-caught salmon.²⁰ The reasons for this are various – including high host density and limited genetic diversity, as well as reduced lifespans of the fish due to scheduled slaughtering, which may cause parasites to adapt to shorter life cycles.²¹ Under natural outdoor conditions, high virulence is costly to the virus, since killing its host too fast stops it from spreading if there is no new host nearby. This naturally limiting mechanism is bypassed, however, under the cramped and unhygienic conditions of factory farms and aquaculture. There, virus transmission, even from severely sick or dead animals to live animals, is easily possible. Literally constituting the opposite of social distancing, this makes industrialised animal agriculture a hotbed for the evolution of pathogens with a greater virulence than is naturally possible. And it strongly encourages their eventual spread.

Factory farm waste – spreading pathogens to the outside world

This alarming situation is further aggravated by the poor management of faeces, waste, and water in intensive-farming facilities, affecting not only the animals in those facilities but also those in close proximity outside. The sheer magnitude of the outputs of these facilities, including both living and dead animals, excrements, and other bodily fluids, makes it effectively impossible to contain pathogens. Existing biosecurity protocols can do little to change that (when they are in place at all). With animal agriculture continuing to rapidly expand and intrude into the natural environment, the chances of close contact between other domesticated animals (both inside and outside of farming settings) and wild animal species increase dramatically. As does the risk of zoonotic spillover events between them.^{22 23 24 25 26 27} There are several pathways to zoonotic spillover, including contaminated aerosol particles which can transmit viruses between farm facilities and humans.^28 29 For example, pig farms can be a source of infectious aerosol particles which are transported downwind.³⁰ Pathogens can also travel together with faeces, dust, debris, water, respiratory fluids, bedding, and hair particles.³¹ Smaller particles can remain suspended for long periods, facilitating the infectivity of pathogens.³²

Bigger, faster, tighter – a risky paradigm shift

While all animal agriculture intensifies the emergence and spread of zoonotic diseases, this holds especially true for large-scale, high-density operations. Aiming for ‘optimisation’ in terms of productivity and economic efficiency, small-scale farming with a few animals, kept predominantly outdoors and foraging for food in fields, is increasingly a fading memory of the past.³³ Research demonstrates that significantly higher risks of H5N1 outbreaks were found in large-scale commercial poultry operations, compared to backyard flocks. In Canada, H5N1 spread rapidly, also owing to air exchange between neighbouring poultry barns. The facilities’ industrial scale ventilation systems generate aerosolised dust which facilitates pathogen transmission. Air samples from one study revealed particle concentrations in factory farms being a million times higher than in semi-rural areas.³⁴ Given that factory farms and aquaculture are estimated to account for more than 90% of global meat and fish production,³⁵ the overall trajectory points towards a greater risk of zoonotic outbreaks in the future.^36 37

The destruction of ecosystems and loss of biodiversity

The emergence of a novel zoonotic disease is a highly complex process, involving many factors. Yet, there is compelling evidence that certain human activities strongly increase the likelihood of such developments. In this blog, we’ll have a closer look at the first factor – The destruction of ecosystems and the loss of biodiversity (driven largely by animal agriculture) – resulting in increased contact and virus spillover to humans and farmed animals.

Factory farming – an industrial-scale zoonoses incubator

Modern-day animal agriculture is much like a large-scale petri dish, providing perfect conditions for viruses to emerge, spread, and cross species barriers. The actual spillover can happen when viruses undergo genetic changes, either through antigenic shift (when different strains of a virus recombine – a process potentially accelerated by the close proximity of multiple hosts) or through antigenic drift (when small changes in the genetic information accumulate).³⁸ Both mechanisms can lead to the emergence of viruses which have the ability to infect humans. An example of an antigenic shift is the 1918 Spanish flu outbreak, which was an avian H1N1 influenza strain that mutated, probably with pigs functioning as a mixing vessel, and subsequently became transmissible between humans.^39 40 An example of antigenic drift is seasonal influenza.^41 42

Influenza – the classic among the zoonotic diseases

One of the most well-known examples of a constantly changing and mutating zoonotic disease that is connected to animal farming is the influenza A virus (IAV). While this virus occurs naturally among wild aquatic birds across the globe,^43 44 certain strains of IAV also occur in humans. This implies that the virus jumped the species barrier at some point.⁴⁵ While there is widespread awareness of the threat that IAV poses to human health, little is known by the general public about its animal origins.

Birds, pigs, and humans – growing populations and increasing influenza spillover risks

As probable intermediate hosts, pigs are thought to be a particularly good fit to host the processes mentioned above. Since they are susceptible to both avian and mammalian influenza viruses, they are seen as mixing vessels and transmitters for viruses, leading to the creation of new strains of viruses with zoonotic or even pandemic potential.^{46 47 48 49} One of the primary risk factors for spillover to humans is exposure to infected live or dead animals, for example, when raising, slaughtering, processing, or preparing them for consumption. However, humans also transmit influenza viruses and other pathogens to animals such as pigs (reverse zoonosis), potentially also making humans the catalyst for future pandemics.^50 51 Either way, the ongoing close contact with, and use of, farmed animals by humans increase the future risks of further zoonotic transmission.⁵² This holds particularly true for the transmission of influenza viruses, as the three species involved in their emergence – poultry, pigs, and humans – are all predicted to increase in number.⁵³

The H5N1 influenza outbreak in 2004 – just short of a global disaster

With H5N1, the world has already witnessed a frightening example of how serious a threat zoonotic spillovers involving factory farming can be. After two relatively mild pandemics in 1957 and 1968, the world teetered on the brink of catastrophe in 2004, when large parts of Asia experienced unprecedented outbreaks of the highly pathogenic avian influenza strain H5N1. There is evidence that H5N1 avian flu may have started to spread when migratory birds wound up in close proximity to poultry farms as the intensification of farming practices brought them closer together. The virus evolved, crossing the species barrier and infecting humans – with a devastating case-fatality rate of up to 60%, taking its heaviest toll on children and young adults.^54 55 This particular strain of the virus met all necessary prerequisites for a devastating pandemic – with only the lack of efficient human-to-human transmission preventing its extensive spread and a subsequent emergency of unforeseeable magnitude.⁵⁶

Our growing hunger for poultry – breeding the next influenza pandemic

At the time of writing, the avian influenza strain H5N8 is causing havoc in Eastern Europe. Since the end of 2019, there has been an increase in outbreaks of bird flu in poultry farms inEastern Europe, leading to the killing of millions of birds.⁵⁷ Although the likelihood seems low, human infection with H5N8 is indeed a possibility.⁵⁸ However, the Asian avian influenza strain H7N9, which has been circulating in poultry in China since 2013, is rated by the Centers for Disease Control and Prevention (CDC) as the influenza A strain with the greatest potential to cause a zoonotic pandemic and to severely impact public health if it were to achieve sustained human-to-human transmission.⁵⁹ So far, human infections have only occurred sporadically but have killed about 40% of patients, making it 400 times more dangerous and deadly than normal seasonal influenza. Thus, our growing hunger for chicken reveals itself to be one of the most critical risk factors in breeding the next influenza pandemic.⁶⁰

Wild animals as food – increasing the risk of zoonotic pandemics

There is compelling evidence that certain human activities strongly increase the likelihood of zoonotic diseases. In this blog, we’ll take a closer look at the second factor: the use of wild animals for food, resulting in additional contact and virus spillover to humans and farmed animals.

Putting zoonotic risks into perspective – one mutation away from a global disaster

The case-fatality rate of COVID-19. caused by the virus SARS-CoV-2, is currently estimated to be somewhere between 0.1% (rounded up from 0.05) and 8.5%,⁶¹ with a global average of about 2.1% (as of 25 January 2021).⁶² The virulence of a virus such as H5N1 or H7N9, paired with the infectivity of SARS-CoV-2, would have catastrophic consequences (see graphic in 1.5). And it would take just one mutation for this to occur. To put things into perspective: if a pandemic similar to the 1918 Spanish flu occured today, experts expect 100 to 400 million deaths globally.⁶³ The likelihood of this event becoming a reality increases with every single chicken and pig housed for food production – and with every single day this practice is maintained.

Infectious disease outbreaks – backfiring on animal agriculture

The development of new infectious zoonotic diseases as a result of intensified animal agriculture does not only pose a threat to human health and healthcare systems. It also backfires on the industry itself in various ways – in turn, negatively impacting humans, animals, and the food system. Animal agriculture is affected by a host of endemic and reemerging infectious diseases on an ongoing basis, including African swine fever (ASF), swine flu, and avian flu. Not only do these diseases have profound ethical implications – with animals suffering and dying from them, as well as being culled to curb their spread. They also cause enormous economic damage to meat, dairy, and poultry producers – from small-scale subsistence agriculture to large-scale commercial farming.

Swine flu is a respiratory disease that is endemic in pig populations around the world, with a morbidity rate of up to 100%.⁶⁴ Due to the constantly evolving nature of the virus and pigs acting as ‘mixing vessels’, swine flu has transformed from a seasonal disease to a disease that is prevalent all year round. And because of the constant mutation, significant efforts are needed to continuously develop new vaccines.

Animal industry workers – victims and vectors

Many of the diseases circulating among farmed animals can infect humans, thus becoming zoonotic. Being in constant contact with potentially infected animals exposes those working in the animal industry to greater risk, putting them on the frontline of possible spillover events. There is substantial evidence that farmers, veterinarians, and abattoir workers, particularly, are at an increased risk of contracting zoonotic diseases and play an important role in their spread,⁶⁵ with one particular outbreak putting this group at a 1,500-times higher risk than the general population (see also Food & Pandemics Report chapter 3.1).⁶⁶

Food & Pandemics Report

By exploring the crucial connection between the current crisis and our animal-based food system, the ProVeg Food & Pandemics Report highlights how our food choices help to create a recipe for zoonotic pandemics.

1. Carrington, D. (2020): Coronavirus: ‘Nature is sending us a message’, says UN environment chief. The Guardian. Available at https://www.theguardian.com/world/2020/mar/25/coronavirus-nature-issending-us-a-message-says-un-environment-chief. [Accessed: 30.3.2020]
2. Smith, K. F., M. Goldberg, S. Rosenthal, et al. (2014): Global rise in human infectious disease outbreaks. Journal of The Royal Society Interface 11(101), 20140950. doi:10.1098/rsif.2014.0950
3. Kreuder Johnson, C., P. L. Hitchens, T. Smiley Evans, et al. (2015): Spillover and pandemic properties of zoonotic viruses with high host plasticity. Scientific Reports 5(1), doi:10.1038/srep14830
4. Kingsley, D. H. (2016): Emerging Foodborne and Agriculture-Related Viruses. Microbiology Spectrum 4(4), doi:10.1128/microbiolspec.PFS-0007-2014
5. Johnson, C. K., P. L. Hitchens, P. S. Pandit, et al. (2020): Global shifts in mammalian population trends reveal key predictors of virus spillover risk. Proceedings of the Royal Society B: Biological Sciences 287(1924), 20192736. doi:10.1098/rspb.2019.2736
6. Wells, K., S. Morand, M. Wardeh, et al. (2020): Distinct spread of DNA and RNA viruses among mammals amid prominent role of domestic species. Global Ecology and Biogeography 29(3), 470–481. doi:10.1111/geb.13045
7. Wolfe, N. D., C. P. Dunavan & J. Diamond (2007): Origins of major human infectious diseases. Nature 447(7142), 279–283. doi:10.1038/nature05775
8. WHO & FAO (2003): Diet, Nutrition and the Prevention of Chronic Diseases. WHO Technical Report Series. World Health Organization, Geneva. Available at https://apps.who.int/iris/bitstream/handle/10665/42665/WHO_TRS_916.pdf?sequence=1 [26.05.2020]
9. Vale, P., H. Gibbs, R. Vale, et al. (2019): The Expansion of Intensive Beef Farming to the Brazilian Amazon. Global Environmental Change 57 101922. doi:10.1016/j.gloenvcha.2019.05.006
10. Food and Agriculture Organization of the United Nations (2020): Live animals. FAOSTAT Database. Rome, Italy. Available at: http://www.fao.org/faostat/en/#data/QA [26.06.2020]
11. Bar-On, Y. M., R. Phillips & R. Milo (2018): The biomass distribution on Earth. Proceedings of the National Academy of Sciences 115(25), 6506–6511. doi:10.1073/pnas.1711842115
12. Fishcount.org: Numbers of farmed fish slaughtered each year. Available at: http://fishcount.org.uk/fish-count-estimates-2/numbers-of-farmed-fish-slaughtered-each-year [Accessed 12.06.2020]
13. OECD & FAO (2019): OECD-FAO Agricultural Outlook 2019-2028. OECD Publishing, Paris/ Food and Agriculture Organization of the United Nations, Rome. doi:10.1787/agr_outlook-2019-en
14. FAO (2016): The Contributions of Livestock Species and Breeds to Ecosystem Services.Available at: http://www.fao.org/3/a-i6482e.pdf
15. Kumar, B., A. Manuja & P. Aich (2012): Stress and its impact on farm animals. Frontiers in Bioscience (Elite Edition) 4 1759–1767. doi:10.2741/496
16. Mourkas, E., A. J. Taylor, G. Méric, et al. (2020): Agricultural intensification and the evolution of host specialism in the enteric pathogen Campylobacter jejuni. Proceedings of the National Academy of Sciences 117(20), 11018–11028. doi:10.1073/pnas.1917168117
17. National Research Council (US) Committee on Achieving Sustainable Global Capacity for Surveillance and Response to Emerging Diseases of Zoonotic Origin, G. T. Keusch, M. Pappaioanou, et al. (2009): Sustaining Global Surveillance and Response to Emerging Zoonotic Diseases. Washington (DC): National Academies Press (US); 2009. 3, Drivers of Zoonotic Diseases. Sustaining Global Surveillance and Response to Emerging Zoonotic Diseases. National Academies Press (US). Available at https://www.ncbi.nlm.nih.gov/books/NBK215318/
18. Payne, S. (2017): Chapter 9 – Viral Pathogenesis. in Viruses. (ed. Payne, S.) Academic Press p.87–95 doi:10.1016/B978-0-12-803109-4.00009-X
19. Mennerat, A., F.Nilsen, D. Ebert, and A. Skorping (2010): Intensive Farming: Evolutionary Implications for Parasites and Pathogens. Evolutionary Biology 37, no. 2 (September 1, 2010): 59–67. https://doi.org/10.1007/s11692-010-9089-0
20. Ugelvik, M. S., A. Skorping, O. Moberg, et al. (2017): Evolution of virulence under intensive farming: salmon lice increase skin lesions and reduce host growth in salmon farms. Journal of Evolutionary Biology 30(6), 1136–1142. doi:10.1111/jeb.13082
21. Kennedy, D. A., G. Kurath, I. L. Brito, et al. (2016): Potential drivers of virulence evolution in aquaculture. Evolutionary Applications 9(2), 344–354. doi:10.1111/eva.12342
22. Hu, Y., H. Cheng & S. Tao (2017): Environmental and human health challenges of industrial livestock and poultry farming in China and their mitigation. Environment International 107 111–130. doi:10.1016/j.envint.2017.07.003
23. Kingsley, D. H. (2016): Emerging Foodborne and Agriculture-Related Viruses. Microbiology Spectrum 4(4), doi:10.1128/microbiolspec.PFS-0007-2014
24. Bryony, A. J., G. Delia & R. Kock (2015): Zoonosis Emergence Linked to Agricultural Intensification and Environmental Change. in Emerging Viral Diseases: The One Health Connection: Workshop Summary.
25. Staggemeier, R., M. Bortoluzzi, T. M. da Silva Heck, et al. (2015): Animal and human enteric viruses in water and sediment samples from dairy farms. Agricultural Water Management 152 135–141. doi:10.1016/j.agwat.2015.01.010
26. Chen, J., C. Zhang, Y. Liu, et al. (2017): Super-oxidized water inactivates major viruses circulating in swine farms. Journal of Virological Methods 242 27–29. doi:10.1016/j.jviromet.2017.01.002
27. Krog, J. S., A. Forslund, L. E. Larsen, et al. (2017): Leaching of viruses and other microorganisms naturally occurring in pig slurry to tile drains on a well-structured loamy field in Denmark. Hydrogeology Journal 25(4), 1045–1062. doi:10.1007/s10040-016-1530-8
28. Zhang, H., X. Li, R. Ma, et al. (2013): Airborne spread and infection of a novel swine-origin influenza A (H1N1) virus. Virology Journal 10(1), 204. doi:10.1186/1743-422X-10-204
29. Scanes, C. G. (2018): Chapter 18 – Impact of Agricultural Animals on the Environment. in Animals and Human Society. (eds. Scanes, C. G. & Toukhsati, S. R.) Academic Press p.427–449 doi:10.1016/B978-0-12-805247-1.00025-3
30. Corzo, C. A., M. Culhane, S. Dee, et al. (2013): Airborne detection and quantification of swine influenza a virus in air samples collected inside, outside and downwind from swine barns. PloS One 8(8), e71444. doi:10.1371/journal.pone.0071444
31. Sing, A. (2014): Zoonoses – Infections Affecting Humans and Animals: Focus on Public Health Aspects. Springer
32. Alonso, C., P. C. Raynor, P. R. Davies, et al. (2015): Concentration, Size Distribution, and Infectivity of Airborne Particles Carrying Swine Viruses. PLOS ONE 10(8), Public Library of Science, e0135675. doi:10.1371/journal.pone.0135675
33. Hartung, J. (2013): A short history of livestock production. in Livestock housing. (eds. Aland, A. & Banhazi, T.) Wageningen Academic Publishers, 21–34. doi:10.3920/978-90-8686-771-4_01 doi:10.3920/978-90-8686-771-4_01
34. Graham, J. P., J. H. Leibler, L. B. Price, 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. doi:10.1177/003335490812300309
35. Anthis, K. (2019): Global Farmed & Factory Farmed Animals Estimates. Sentience Institute. Sentience Institute, Available at https://sentienceinstitute.org/global-animal-farming-estimates. [Accessed: 18.5.2020]
36. Samuel, S. (2020): The meat we eat is a pandemic risk, too. Vox. Available at https://www.vox.com/future-perfect/2020/4/22/21228158/coronavirus-pandemic-risk-factory-farming-meat. [Accessed: 24.4.2020]
37. Lawrence, R. S. (2012): How Industrialized Farming Could Facilitate Pandemic Swine Flu. The Atlantic. Available at https://www.theatlantic.com/health/archive/2012/08/how-industrialized-farming-could-facilitate-pandemic-swine-flu/261356/. [Accessed: 18.3.2020]
38. CDC (2019): How Flu Viruses Can Change. CDC – Centers for Disease Control and Prevention. Available at https://www.cdc.gov/flu/about/viruses/change.htm. [Accessed: 16.4.2020]
39. Taubenberger, J. K. & D. M. Morens (2006): 1918 Influenza: the Mother of All Pandemics. Emerging Infectious Diseases 12(1), 15–22. doi:10.3201/eid1201.050979
40. Gibbs, M. J. & A. J. Gibbs (2006): Was the 1918 pandemic caused by a bird flu? Nature 440(7088), E8–E8. doi:10.1038/nature04823
41. CDC (2019): How Flu Viruses Can Change. CDC – Centers for Disease Control and Prevention. Available at https://www.cdc.gov/flu/about/viruses/change.htm. [Accessed: 16.4.2020]
42. WHO: How pandemic influenza emerges. World Health Organization Europe. World Health Organization, Available at http://www.euro.who.int/en/health-topics/communicable-diseases/influenza/pandemic-influenza/how-pandemic-influenza-emerges. [Accessed: 27.4.2020]
43. CDC (2017): Avian Influenza in Birds. CDC – Centers for Disease Control and Prevention. Available at https://www.cdc.gov/flu/avianflu/avian-in-birds.htm. [Accessed: 9.4.2020]
44. WHO (2018): Influenza (Avian and other zoonotic). World Health Organization. Available at https://www.who.int/news-room/fact-sheets/detail/influenza-(avian-and-other-zoonotic). [Accessed: 4.9.2020]
45. Nelson, M. I., D. E. Wentworth, M. R. Culhane, et al. (2014): Introductions and Evolution of Human-Origin Seasonal Influenza A Viruses in Multinational Swine Populations. Journal of Virology 88(17), 10110–10119. doi:10.1128/JVI.01080-14
46. Ito, T., J. N. S. S. Couceiro, S. Kelm, et al. (1998): Molecular Basis for the Generation in Pigs of Influenza A Viruses with Pandemic Potential. Journal of Virology 72(9), 7367–7373.
47. Ma, W., R. E. Kahn & J. A. Richt (2008): The pig as a mixing vessel for influenza viruses: Human and veterinary implications. Journal of molecular and genetic medicine : an international journal of biomedical research 3(1), 158–166.
48. Trebbien, R., L. E. Larsen & B. M. Viuff (2011): Distribution of sialic acid receptors and influenza A virus of avian and swine origin in experimentally infected pigs. Virology Journal 8(1), doi:10.1186/1743-422X-8-434
49. Kahn, R. E., W. Ma & J. A. Richt (2014): Swine and Influenza: A Challenge to One Health Research. in Influenza Pathogenesis and Control – Volume I. (eds. Compans, R. W. & Oldstone, M. B. A.) 385 Springer International Publishing, 205–218. doi:10.1007/82_2014_392
50. Nelson, M. I. & A. L. Vincent (2015): Reverse zoonosis of influenza to swine: new perspectives on the human–animal interface. Trends in Microbiology 23(3), 142–153. doi:10.1016/j.tim.2014.12.002
51. Messenger, A. M., A. N. Barnes, and G.C. Gra (2014): Reverse Zoonotic Disease Transmission (Zooanthroponosis): A Systematic Review of Seldom-Documented Human Biological Threats to Animals.” PLoS ONE 9, no. 2. https://doi.org/10.1371/journal.pone.0089055.
52. Lawrence, R. S. (2012): How Industrialized Farming Could Facilitate Pandemic Swine Flu. The Atlantic. Available at https://www.theatlantic.com/health/archive/2012/08/how-industrialized-farming-could-facilitate-pandemic-swine-flu/261356/. [Accessed: 18.3.2020]
53. OECD & FAO (2019): OECD-FAO Agricultural Outlook 2019-2028. OECD Publishing, Paris/ Food and Agriculture Organization of the United Nations, Rome. doi:10.1787/agr_outlook-2019-en doi:10.1787/agr_outlook-2019-e
54. WHO FAQs: H5N1 influenza. World Health Organization. World Health Organization, Available at https://www.who.int/influenza/human_animal_interface/avian_influenza/h5n1_research/faqs/en/. [Accessed: 10.04.2020]
55. WHO (2005): Avian influenza: assessing the pandemic threat. World Health Organization. p.5 Available at https://apps.who.int/iris/bitstream/handle/10665/68985/WHO_CDS_2005.29.pdf;sequence=1. [Accessed: 28.5.2020
56. WHO (2005): Avian influenza: assessing the pandemic threat. World Health Organization. p.11 Available at https://apps.who.int/iris/bitstream/handle/10665/68985/WHO_CDS_2005.29.pdf;sequence=1. [Accessed: 28.5.2020]
57. WHO (2020): Increase in ‘bird flu’ outbreaks – WHO/Europe advice for handling dead or sick birds. World Health Organization – Regional Office for europe. World Health Organization, Available at http://www.euro.who.int/en/health-topics/communicable-diseases/influenza/news/news/2020/01/increase-in-bird-flu-outbreaks-whoeurope-advice-for-handling-dead-or-sick-birds. [Accessed: 28.5.2020]
58. WHO Assessment of risk associated with influenza A(H5N8) virus. World Health Organization. World Health Organization, Available at http://www.who.int/influenza/human_animal_interface/avian_influenza/riskassessment_AH5N8_201611/en/. [Accessed: 28.5.2020]
59. CDC (2020): Asian Lineage Avian Influenza A(H7N9) Virus | Avian Influenza (Flu). CDC – Centers for Disease Control and Prevention. Available at https://www.cdc.gov/flu/avianflu/h7n9-virus.htm. [Accessed: 14.6.2020]
60. WHO Avian influenza. World Health Organization. World Health Organization, Available at http://www.who.int/features/qa/avian-influenza/en/. [Accessed: 28.5.2020]
61. CEBM (2021): Global Covid-19 Case Fatality Rates. Centre for Evidence-Based Medicine. Available at https://www.cebm.net/covid-19/global-covid-19-case-fatality-rates/. [Accessed: 25.05.2020]
62. Johns Hopkins University & Medicine (2021): COVID-19 Map. Johns Hopkins Coronavirus Resource Center. Available at https://coronavirus.jhu.edu/map.html. [Accessed: 25.01.2021]
63. Nuzzo, J. B., L. Mullen, M. Snyder, et al. (2019): Preparedness for a High-Impact Respiratory Pathogen Pandemic. Johns Hopkins Center for Health Security. Available at https://apps.who.int/gpmb/assets/thematic_papers/tr-6.pdf [Accessed: 28.06.2020]
64. OIE (2018): Swine influenza. World Organisation for Animal health. Available at.https://www.oie.int/en/animal-health-in-the-world/animal-diseases/Swine-influenza/ [Accessed: 26.06.2020]
65. Klous, G., A. Huss, D. J. J. Heederik, et al. (2016): Human–livestock contacts and their relationship to transmission of zoonotic pathogens, a systematic review of literature. One Health 2 65–76. doi:10.1016/j.onehlt.2016.03.001
66. Arends, J. P. & H. C. Zanen (1988): Meningitis caused by Streptococcus suis in humans. Reviews of Infectious Diseases 10(1), 131–137. doi:10.1093/clinids/10.1.131