11.2 How do livestock affect the risk of zoonotic disease emergence and spread?

11.2.1 The major point of interconnection between animals and humans

Livestock are an important point of interconnection for many infection pathways linking humans to sources of zoonotic pathogens (Figure 11).

Figure 11: Illustrative flows of pathogens at the human-animal-environment interface between species. Peri-domestic wildlife signifies animals such as rats, commonly found in urban areas. Reproduced from Jones, et al. 2013.

By sharing the landscape with wildlife and disease vectors, livestock are routinely exposed to the pathogens they carry, and may become infected. At the same time, human exposure to livestock can be considerable – especially in low-income countries where many households keep animals.

Ever since the introduction of agriculture and the domestication of animals, farms have provided a venue for human and animal pathogens to intermingle and share genetic information; and the activities of farming, butchery, preparation, and consumption of livestock products all provide pathways for zoonotic pathogens to be transmitted to humans.

The sourcing and consumption of bushmeat is another important pathway for zoonotic pathogens to infect humans directly from wildlife.

11.2.2 Growth in livestock numbers

Globally, livestock production has increased substantially. Estimates of livestock numbers are uncertain but are thought to total around 23 billion poultry and 1 billion pigs living on farms today. Some individual farms contain tens to hundreds of thousands of animals.

With a larger global livestock population comes an overall increase in the:

  • Frequency of interactions between wildlife, livestock, and humans;
  • Likelihood of disease emergence and transmission events occurring;
  • Overall diversity of pathogens found in separate animal populations; and
  • Potential for new diseases to spread among livestock and human populations.

All things being equal – i.e. without stricter measures to mitigate the risk – this trend increases the likelihood that diseases will spill over from wild animals and livestock, to affect humans (Figure 12).

Figure 12: Transmission and amplification of zoonotic diseases. Transmission of a pathogen to people can occur directly from a wild animal or following an outbreak in livestock that amplifies the likelihood of transmissions to humans. Redrawn from Karesh, et al. 2012.

11.2.3 Livestock-driven land use change

Global land use for agriculture has grown dramatically over the last two hundred years, with about three-quarters of the world’s agricultural land used to rear livestock (Figure 13).

Figure 13: Growth in global land use for agriculture over the last 2000 years. Redrawn from Goldewijk, et al. 2011.

Expansion of agricultural land into natural ecosystems increases the likelihood of contact between wild animals, disease vectors, and livestock, and so also increases the likelihood of subsequent disease transmission to humans.

11.2.4 Growing trade in livestock and livestock products

Geographically separate populations of livestock and their pathogens, and those of the wild animals to which livestock are exposed, can be connected through trade – especially the trade in live animals that may join herds/flocks in new regions, or in the case of wild animals, that may escape and infect local species.

Modern trade and transport networks also mean that it is possible for zoonotic diseases to spread quickly, both within countries and internationally. And from countries with higher burdens of disease and poorer animal health services to those with much stronger controls.

Alongside growing animal numbers, the trade in live animals, animal feed, and animal-based products has increased substantially (Figure 14).

Figure 14: Live animal exports of pigs and chickens, 1961 to 2013. Data from FAOSTAT. 2017.

Without stricter measures to mitigate the risk, this trend increases the likelihood that zoonotic diseases will spread to new locations through trade. And while some preventative national and international regulations already exist, zoonotic disease outbreaks still occur regularly as a result of both legal and illegal trade. This is true both of trade in farm animals and in wild animals and their products.

11.2.5 Changes in livestock production systems

Farms are artificial ecosystems. By shaping the ecological context for pathogens, their evolution, and their opportunities for transmission, the management of livestock farming affects the risk of new diseases emerging and being transmitted to humans.

Many factors affect this risk, and different forms of livestock production come associated with different costs, benefits and trade-offs for public health that need to be actively managed (Figure 15).

Figure 15: A conceptual framework for zoonotic disease risk in livestock industries. Reproduced from Liverani, et al. 2013.

A shift away from domestic or ‘backyard’ forms of livestock production in lower-income countries is likely to reduce levels of sickness from those zoonotic diseases that are almost exclusively transmitted via ongoing direct contact between people and infected animals (i.e. endemic diseases).

This type of shift has already been observed in countries such as China. However, it has also been associated with an increase in more commercial and intensive models of livestock production that bring a whole new set of health risks.

Intensive livestock systems

In simplistic terms, ‘intensive’ livestock production systems are characterised by their greatly increased scale and speed of production, alongside significant investments in the use of technology. The goal of this being to optimise levels of output (i.e. meat, milk, eggs) in relation to inputs such as human labour, land, animal feed, and other resources, and ultimately, to reduce the overall costs of producing a unit of food.

By making animal products more affordable for more people, intensive livestock production systems have enabled significantly increased overall consumption of animal products. Data on numbers of livestock in different systems is sparse and uncertain. However, it has been estimated that, globally, this type of production system is responsible for about:

  • 76% of all pork;
  • 79% of all poultry; and
  • 61% of eggs.

On the one hand, greater availability and ability to access even small amounts of nutrient-dense animal products can help to alleviate problems associated with malnutrition among those in extreme poverty. However, a by-product of more intensive production methods is an increased risk of infectious disease resulting from:

  • Large and densely populated livestock populations that enable pathogens to be quickly transmitted and spread to large numbers of animals;
  • Greater susceptibility to infection due to the animals’ genetic similarity and optimisation for faster growth, as well as weakened animal immune systems as a response to stress;
  • More frequent movements of people, vehicles, and livestock between farms that may spread disease to new locations;
  • Increased levels of airborne particles and large concentrations of animal waste, each containing pathogens that may lead to environmental contamination;
  • The exchange of genetic information between both animal and human pathogens, co-existing within livestock populations, to create entirely new forms of infectious disease;
  • Increased use of antibiotics and the development of antibiotic resistance (Section 11.4.1).
Figure 16: A schematic representation of potential pathways for exposure to and transfer of pathogens within the environs of an intensive chicken farm. Reproduced from Graham, et al, 2008.

Due to these elevated risks, it is necessary to monitor and maintain a high degree of control over livestock’s physical environment and biological functioning (i.e. ‘biosecurity’; Figure 17 below), in order to:

  • Prevent new pathogens from being introduced to and infecting a livestock herd/flock;
  • Identify and contain the spread of new pathogens within a farm; and
  • Stop dangerous pathogens from being released and spreading off-farm.
Figure 17: A hierarchy of risk mitigation measures to maintain the biosecurity of livestock systems using vaccination measures. Redrawn from Layton, et al. 2017.

Unfortunately, strong biosecurity practices and food safety measures are not universally adopted, especially in low-income countries where most of the world’s livestock production actually takes place. Neither are they always effective at preventing or containing outbreaks of disease, even where implemented.

Intensive livestock production systems also present significant health risks because of the routine use of high levels of antibiotics (Section 11.4.2).