Zoonotic diseases account for 60% of human infectious disease cases and millions of deaths every year. The public health and economic costs of zoonoses to global health have also been well demonstrated in recent events such as the 2013-2016 Ebola virus epidemic, the 20th and 21st-century influenza pandemics, and the previous SARS outbreaks.
Livestock species currently constitute more biomass than all wild mammals combined, representing a massive zoonotic reservoir from which new emerging pathogens are likely to arise. However, of even greater concern, the propitious conditions for the emergence and transmission of highly pathogenic strains are present in intensive animal farming systems. Specifically, many of the conditions that translate into poor animal welfare are also a threat to public health, increasing the likelihood that new pathogens evolve, cross the species barrier and eventually achieve sustained transmission in the human population.
When genetic selection is focused predominantly on growth and productivity, critical organ systems and biological functions are concomitantly compromised, including cardiorespiratory function and immune resistance. For example, among broiler chickens, more productive and faster-growing breeds have shown the greatest decline in immune capacity. Genetic homogeneity, immunosuppression from chronic stress, and environmental risk factors such as poor air quality and high stocking densities aggravate these problems, increasing the likelihood of infectious disease emergence. High levels of aerial pollutants often found in closed facilities, such as ammonia, further compromise respiratory function and increase the likelihood of infection by respiratory pathogens. Accordingly, in intensively raised pigs, post-mortem findings of lesions in the respiratory tract as a result of pneumonia, pleuropneumonia, pleurisy, and other diseases are pervasive nowadays.
The difficulties of independently auditing animal welfare and biosecurity conditions, the sheer scale of the many outputs of animal farming systems, and the transport of live animals nationally and abroad represent additional challenges. Where investigated, the industry has demonstrated endemic failures of compliance with biosecurity protocols. Enforcing higher animal welfare standards in industry practices, genetic selection and stockmanship, as well as transparency and independent auditing (e.g. through the implementation of CCTV systems open to real-time and independent monitoring) will be critical to reducing the risks of emergence and spread of new pathogens, including those with pandemic potential.
Given our many years of research experience on global health, specifically the risks and burden of infectious diseases and pandemics, we have developed content for different audiences (a booklet, scientific opinions, papers, book chapters, talks) aimed at the public understanding of these risks. A short e-book is available to download below. A chapter where we explain the association between Human Health and Animal Welfare is also available at the recently published Routledge Handbook of Animal Welfare. A talks is also available at the end of this page.
In this chapter, we discuss how industrial systems of animal farming created novel routes for pathogens that gained, or regained, access to human populations, along with the conditions now eroding many health advances achieved so far. We first describe how impoverishment of animal welfare drives infectious disease emergence. We then focus on the effects of industrial farming on water and air pollution, the health of workers and local communities. We also review the impacts of wildlife hunting, trafficking, and trade. We close with a section on overconsumption of animal-sourced food and non-communicable disease risk.
