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

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

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