In our industrialized and technologically sophisticated world, the relationship between human beings and the far greater number of nonhuman animals on the earth may seem of limited importance to public health. Yet evidence suggests the opposite: The parallel increases in global human and food animal populations and their effects on wildlife and ecosystem health are creating such unprecedented linkages between global human health and animal health that new models are needed to study this area further.
DEMOGRAPHIC CHANGES IN HUMAN AND ANIMAL POPULATIONS
The earth's increasing human population impacts the health not only of our own species, but that of food and companion animals and of our environment. Increased meat consumption, globalization of the food supply, and migration of humans and animals all play a pivotal role in how we address current and future challenges.
The Parallel Growth of Human and Food Animal Populations
The projected increase in the world's human population (from 6.1 billion in 2000 to 10.1 billion by 2100) has been the subject of numerous public health studies and predictions. Some of the attendant phenomena include the growth of mega cities in response to both population growth and urban movement, strains on water and food resources, and management of sewage and solid waste. A seldom discussed consequence of human population growth is the increase in food animal populations needed to feed the growing human demand (see Figure 1). In 2010, an estimated 9 billion food animals were raised in the United States alone. This growing demand for meat and dairy production is due not only to the sheer increase in human numbers, but also to an increase in per capita consumption of meat, dairy, and other livestock products in relation to rising per capita incomes in populous countries such as China and India. Investigators therefore estimate that roughly an 80% increase in animal production will be needed to meet the population demands of 2050.
The increase in food animal numbers has influenced changes in livestock management, including a rapid transition toward concentrated animal-feeding operations (CAFOs) and other intensive forms of livestock agriculture, which may involve housing thousands of animals for most of their life spans in indoor enclosed facilities. Thus, increasing the concentrations of animals and humans in limited land areas may have numerous consequences. These include the risk of infectious disease emergence: CAFOs may amplify mechanisms for the rapid evolution and spread of novel viruses, including zoonotic (transmitted between animals and humans) influenza and antibiotic-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA).
Gibbs et al. report that air plumes impacting communities downwind of CAFOs contain S. aureus and other antibiotic resistant bacteria, whereas Heederik et al. report that the hydrogen sulfide in such plumes has resulted in odor complaints. Community water supply contamination with nutrient pollution, fecal coliforms, and veterinary drugs has also been linked to CAFOs. Although research to date on the environmental health effects of animal agriculture has focused on large facilities, evidence indicates that in developing countries, small-scale animal husbandry practices could be affecting water supplies and increasing risk of zoonotic diarrheal and other waterborne diseases. This situation can negatively impact HIV-infected and other immunocompromised populations (such as patients receiving cancer treatments) in particular.
Animal Workers
The more than one billion people worldwide who work on a daily basis with food animals or their products are at particularly high risk. These workers include farmers, abattoir workers, and market workers. Risks to such workers include injury, allergic exposures, exposures to organic dusts, and chemicals from confinement operations as well as infection such as zoonotic influenza, MRSA, brucellosis, Q fever, Rift Valley Fever, and Escherichia coli. In addition to their personal risks, animal workers can be the bridge between zoonotic pathogens and the general human population.
Companion Animals
In addition to more food animals, the rise in human populations is swelling the world’s numbers of companion animals. In the United States, more than 50% of households have either a dog or a cat. While, studies have suggested positive health effects from such attachments, including improvements in physical and mental well-being, we also need to consider the risks that are being shared with humans from the health issues of animals like Rabies.EMERGING ZOONOSES
In recent decades, many infectious diseases have emerged with often devastating consequences for global health and impacts on trade and economies. These include SARS, Nipah virus, avian influenza, pandemic H1N1, and MRSA. Systematic reviews have determined that ∼75% of emerging infections in the past two decades are zoonotic in origin. Therefore, human-animal linkages play a key role in infectious-disease emergence.
STRATEGIES FOR LINKING HUMAN, ANIMAL, AND ENVIRONMENTAL HEALTH
There are a number of One Health challenges that demand development of specific, evidence-based protocols that can serve as guidelines for coexistence between increasing populations of humans and animals. To develop such guidelines, collaboration and cooperation among human, animal, and environmental professionals will be not only advisable, but critical.
Optimal Ways to Produce Food Animals
Addressing this challenge involves recognition that animal agriculture has developed to the point that it shares much in common with the manufacturing industry sector and that techniques developed to deal with environmental health and safety in industrialized settings could be applied to food animal production. A unique aspect of animal agricultural production and processing as an industry is that it involves the health not only of the workers and the environment, but of the animals as well. This fact argues for a One Health approach to animal agriculture with involvement of animal health, human health, and environmental health experts.
Healthy Models for Companion Animals and Coordination of Clinical Care
Because companion animals are playing an increasingly important role in households, and their clinical care increasingly resembles that provided to humans, it is logical to create better guidelines for how human and animal health professionals can interact regarding companion animal issues. This process involves developing and testing pilot mechanisms of collaboration between veterinarians and human health care providers and defining the role of the veterinarian in the human health care team.
Cross-Sectoral Prevention Between Human, Animal, and Environmental Health
Evidence of the benefit of linking human and animal disease prevention efforts includes the vaccination campaigns in Africa with nomadic pastoralists; investigators found that when combined teams of human and animal health workers linked animal vaccination campaigns for brucellosis to efforts to vaccinate humans for other diseases, the human vaccination rates improved substantially.
Many aspects of the human-animal-ecosystem approach have yet to be fully implemented, though. For example, few zoonose control intervention efforts track outcomes in humans, animals, and the environment, and few have attempted such an integrated approach. However, a recent study in China to reduce rates of Schistosoma japonicum (a zoonotic form of schistosomiasis) found that treating cattle as well as humans decreased the load of parasite in snails in the environment, decreased the rate of infection in humans, and was superior to the use of antiparasitic treatment in humans alone