Domesticated animals and livestock have been essential to the slow but unceasing progression of human civilization; they’ve been our companions, our work force, and our fuel. While our relationship has been overwhelming positive (at least for us– we’re not the ones being eaten, after all), it’s comforts come at a cost. Animals sustain us, but they can also carry the seeds of our destruction.
Nearly 66% of human pathogens and almost 75% of emerging or re-emerging human diseases (new diseases or diseases affecting naïve populations) are zoonotic (transmitted to humans from animals). This list includes some extremely nasty pathogens, like SARS, avian flu, and rabies. The rapid rise of zoonoses is the result of a number of anthropogenic (human-caused) factors, ranging from deforestation and urbanization to the increasing complexity and intensity of our food systems1.
Keep your friends close
There have been two revolutions in livestock management in recent history1. In the 19th century, European populations were booming, society was urbanizing at an unprecedented rate, and the demand for meat skyrocketed. To increase supply, farmers industrialized animal husbandry, packing thousands of animals into high throughput facilities and replacing forage with nutrient dense artificial feeds1,4. This trend continued throughout the 20th century, leading to the second revolution: the explosion of pig and poultry farming in the 1980s1.
Globally, pig and poultry production remain the fastest growing sectors of industrialized agriculture, growing 2.6 and 3.7%, respectively, over the last decade. In 2005 alone, 25 million pigs were traded internationally, which amounts to more than 2 million per month. Poultry production has also intensified. In the US, chickens and turkeys are raised in houses containing 15,000-70,000 animals. Industrialized agriculture depends on this kind of confined housing, as well as rapid turnover of livestock at the same site, and huge populations of closely related animals4. From a public health standpoint, this is a recipe for disaster. Aside from ethical concerns about keeping thinking, feeling beings in these conditions2, industrialized farms are ideal incubators for disease.
And your enemies closer
Most zoonoses of concern are passed to humans from animals raised for consumption. Livestock herds cultivate disease in two ways. First, by providing a bridge between wild animal populations and humans. (This happens with with Nipah virus, which comes from bats, passes through domesticated pigs, and is transmitted to humans.) Second, by being the site of pathogen evolution themselves4, creating “production diseases”3. The close quarters and high turnover in industrialized agriculture can result in rapid pathogen replication, which increases the likelihood of mutation, possibly making the disease capable of infecting humans. While there are measures in place to prevent the spread of disease, the structure and design of industrial agriculture makes containment extremely challenging. Pathogens have been shown to move readily on farms, both between animals and spilling over into their human caretakers4. Workers are often unable to decontaminate themselves, and vets, farmers, livestock handlers, and slaughterhouse employees are all at higher risk for zoonoses3,4.
For zoonotic illnesses, the cure may be even worse than the disease. Zoonotic illnesses are considered huge public health risks, although they usually result in very few human deaths. Consequently, livestock are routinely treated with antibiotics to ward off sickness. This approach is a fearsome double-edged sword; it has successfully controlled diseases like tuberculosis and brucellosis, but it also encourages antibiotic resistance. By confronting bacteria with antibiotics, we are selecting for survivors (the bacteria that can withstand an antibacterial attack), and we may be creating a suite of untreatable superbugs3.
Once an outbreak occurs, herds are rarely treated; zoonotic epidemics are usually dealt with by culling livestock populations3,4, making the potential economic toll of these diseases enormous. The Dutch avian flu epidemic of 2003 led to the demise of 31 million birds, and amounted to hundreds of millions of euros in damages. There may also be cascading effects on farmers, who suffer the psychological impact of these losses, and of carrying out the slaughter of so many animals3.
For better or worse, industrialized agriculture has become a global phenomenon4, and its expansion and intensification seem inevitable in the face of the ever-increasing human population. While intensive agriculture has clear and substantial benefits, its risks are colossal. We may not be able to eradicate these practices, but we owe it to ourselves to better understand them. Our knowledge about intervention strategies against zoonoses is minimal, and it receives very little research funding. In the UK, a mere 1.6% of the funds allocated by the Medical Research Council were dedicated to health services or systems research in 20061. This must change– understanding may be the only thing that can save us from being consumed by our own appetite.
- Coker, R, J Rushton, S Mounier-Jack, E Karimuribo, P Lutumba, D Kambarage, DU Pfeiffer, K Stärk, & M Rweyemamu. (2011). Towards a conceptual framework to support one-health research for policy on emerging zoonoses. The Lancet, 11:326-331.
- Jochemsen, H. (2013). An ethical foundation for careful animal husbandry. Wageningen Journal of Life Sciences, 66:55-63.
- Kimman, T, M Hoek, MCM de Jong. (2013). Assessing and controlling health risks from animal husbandry. Wageningen Journal of Life Sciences, 66:7-14.
- Leibler, JH, J Otte, D Roland-Holst, DU Pfeiffer, RS Magalhaes, J Rushton, JP Graham, & EK Silbergeld. (2009). Industrial food animal production and global health risks: exploring the ecosystems and economics of avian influenza. EcoHealth, 6:58-70.
Image source: Creative Commons, http://en.wikipedia.org/wiki/Subtherapeutic_antibiotic_use_in_swine