The livestock industry is a vast network of expectations. A farmer expects meat, dairy, and eggs from his animals, and a consumer expects to obtain these products from grocery stores. Industry expects profitable revenue from the sales of these products. Given the intensiveness of modern agriculture, this chain of action has been massively amplified. Meat production has doubled since the 1950s, and currently almost 10 billion animals—not including additional goods such as dairy and eggs—are consumed every year in the United States alone.1 Due to the magnitude of this industry, even small changes can bring about large scale effects. Infections exemplify this chain of events.
Though animal infections might initially seem to be a lesser concern, their effects on human health are rapidly becoming more pronounced and pervasive. During the past few years, an increased number of food-borne disease outbreaks have been traced to products such as beef, pork, poultry, and milk.2 These outbreaks are especially concerning because the pathogens involved are new strains previously harmless to humans. Rather, these pathogens have become infectious to humans due to mutations that occur in animal hosts; such diseases that jump from animals to humans are termed zoonotic. Within the food industry, zoonotic illnesses can be transmitted by consumption or through contact with animals. Crucially, zoonotic cases are much harder to treat because there is no precedent for their treatment.
How often does this transmission occur? Since 1980, 87 new human pathogens have been identified, out of which a staggering 80% are zoonotic.3 Furthermore, many of these have been found in domestic animals, which serve as reservoirs for a variety of infectious agents. The large number of zoonoses raises several key questions. Are these outbreaks the product of our management of livestock or simply a natural phenomenon? How far could zoonotic illnesses escalate in terms of human cases and mortality? What practices or perspectives should we modify to prevent further damage?
Prominent virologist and Nobel laureate in medicine Sir Frank MacFarlane Burnet provided a timeless perspective to this issue in the mid-20th century. He conceptualized infectious disease as equally fundamental to other interactions between organisms such as predation, decomposition, and competition.4 Taking into account how we have harnessed nature, particularly with the aim of producing more food, we can see how farming animals has also inadvertently farmed pathogens.
Treating animals as living environments that can promote pathogenic evolution and diffusion is crucial to creating proper regulations in the livestock industry that protect the safety of consumers in the long run. Current practices risk the emergence of zoonotic diseases by facilitating transmission under heavily industrialized environments and by fostering antibiotic resistance in bacteria. Cooperative action between government, producers, and educated consumers is necessary to improve current practices and preserve good health for everyone.
Influenza: Old Threats, New Fears
The flu is not exactly a stranger to human health, but we must realize that the influenza virus not only affects humans but also other species such as pigs and birds. In fact, what is known as “the flu” is not a single virus but rather a whole family of viruses. The largest family of influenza viruses, influenza A, has different strains of viruses classified with a shorthand notation for their main surface glycoproteins –H for hemagglutinin and N for neuraminidase (Figure 1). These surface glycoproteins are important because their structure and shape determines if the virus will attach to the cellular receptors of its host and infect it. For example, the influenza H7N7 virus has a structure that allows it to specifically infect horses but not humans. Trouble arises when these surface glycoproteins undergo structural changes and the virus gains the capacity to infect humans, as was the case during the 2003 avian flu and the 2009 swine flu pandemics, when the influenza virus jumped from poultry and swine to humans.
Since 2003 when it was first documented in humans, avian influenza H5N1 has been responsible for over 600 human infections and associated with a 60% mortality rate due to severe respiratory failure.5 The majority of these cases occurred in Asia and Africa, particularly in countries such as Indonesia, Vietnam, and Egypt, which accounted for over 75% of all cases.5-6 Though no H5N1 cases have been reported in the U.S., there have been 17 low-pathogenicity outbreaks of avian flu in American poultry since 1997, and one highly pathogenic outbreak of H5N2 in 2004 with 7,000 chickens infected in Texas.5
Poultry is not the only area of livestock industry where flu viruses are a human health concern. The 2009 outbreak of influenza H1N1—popularly termed “swine flu” from its origin in pigs—was officially declared a pandemic by the WHO and the CDC. With an estimated 61 million cases and over 12,000 deaths attributed to the swine flu since 2009, H1N1 is an example of a zoonotic disease that became pandemic due to an interspecies jump that turned it from a regular pig virus to a multi-species contagion.7
The theory of how influenza viruses mutate to infect humans includes the role of birds and pigs as “mixing vessels” for mutant viruses to arise.8 In pigs, the genetic material from pig, bird, and human viruses (in any combination) reassorts within the cells to produce a virus that can be transmitted among several species. This process also occurs in birds with the mixing of human viruses and domestic and wild avian viral strains. If this theory is accurate, one can infer that a high density of pigs in an enclosed area could easily be a springboard for the emergence of new, infectious influenza strains. Thus, the “new” farms of America where pigs and poultry are stocked to minimize space and maximize production provide just the right environment for one infected pig to transfer the disease to the rest. Human handlers then face the risk of exposure to a new disease that can be as fatal as it is infectious, as the 2009 swine flu pandemic and the 2003 avian flu cases demonstrated. As consumers, adequate care of our food sources should not only be priority in avoiding disease but also in national and global health.
Feeding our Food: Antibiotic Resistance in the Food Industry
Interspecies transmission is not the only way through which new diseases can become pathogenic to humans. In the case of bacteria, new pathogenic strains can arise in animals from the action of another mechanism: antibiotic resistance. Antibiotic resistance is the result of the fundamental concept of evolutionary biology—individuals with advantageous traits that allow survival and reproduction will perpetuate these traits to their offspring. Even within the same population, antibiotic resistance varies among individual bacteria—some have a natural resistance to certain antibiotics while others simply die off when exposed. Thus, antibiotic use effectively selects bacteria with such resistance or, in some cases, total immunity. In this way, the livestock industry provides a selective environment.
The rise of these resistant strains—commonly termed “superbugs” for their extensive resistance to a variety of common antibiotics—has been a serious threat in hospitals; there, antibiotic use is widespread, and drug resistance causes almost 100,000 deaths each year from pathogens such as Methicillin-resistant Streptococcus aureus, Candida albicans, Acenitobacter baumanni, and dozens of other species.9 Our attention should not be exclusively focused to hospitals as sources of superbug infections, however. The widespread use of antibiotics in the livestock industry to avoid common bacterial diseases in food animals also poses the risk of emerging superbug strains, and it has not been without its share of outbreaks and casualties.
The Center for Science in the Public Interest –a non-profit organization that focuses on advocating for increased food safety in the US—has reported that antibiotic-resistant pathogens have been the cause of 55 major outbreaks since 1973, and that the majority of cases have come from dairy products, beef, and poultry. Furthermore, the same study reported that most of these pathogens exhibit resistance to over 7 different antibiotics.10 One of the main culprits identified in these outbreaks is the bacterium Salmonella typhimurium along with other Salmonella species, which account for over half of these cases. Salmonella is especially dangerous because it is so pervasive; it is able to lay dormant in a variety of livestock products such as uncooked eggs, milk, cheese, poultry, and beef until incubating in a live host for infection. Escherichia coli 0157:H7 (commonly known as E. coli), a bacterium that usually resides in the intestines of mammals, has also been implicated in a number of outbreaks related primarily to beef products. Overall, antibiotic-resistant pathogens have been the cause of over 20,500 illnesses, with over 31,000 hospitalizations and 27 deaths.10
These cases demonstrate how the widespread use of antibiotics in the food industry is perpetuating the risk of infections and damage to human health with antibiotic-resistant bacteria. Currently, the Food and Drug Administration (FDA) in the U.S. still approves of the use of antibiotics as a treatment for sick animals; furthermore, the organization allows antibiotic use in healthy animals as prevention and even as growth enhancers.11 In fact, over 74% of all antibiotics produced in the United States are used in livestock animals for these reasons.9,11 Using antibiotics in non-infected animals in this way generates a greater environmental pressure for superbugs to emerge; this type of use in particular should be restricted. Managing a proper use of antibiotics to reduce the risk of emerging strains of superbugs should be prioritized in the food industry just as it is in health care.
Hungry for a Solution
Still open to debate is the question of how many resources should be allocated to the problem of widespread antibiotic use. Currently, diseases are transmitted from animals to humans faster than they are evolving within humans. Not only that, many of these zoonotic diseases have high potential to become a pandemic due to their high infectivity, as in the case of H5N1 avian influenza. Measures to prevent the transmission of viruses among livestock animals and to reduce the rate of emergent antibiotic-resistant strains need to take into account the environmental and evolutionary nature of a zoonosis.
A more thorough surveillance of livestock animals and monitoring signs of new emerging strains are important in preventing the spread of such deadly pathogens. This strategy requires intensive molecular analysis, a larger number of professionals working in the field, and a nationwide initiative. Keeping an accurate record of where new strains arise and the number of animal and human cases would significantly improve epidemiological surveillance of infectious disease. This process requires cooperation at multiple levels to ensure that the logistics and public support for these initiatives is ongoing and effective. Additionally, educating people about the nature of zoonotic pathogens is crucial to fostering the dialogue and action necessary to secure the good health of animals, producers, and consumers.
References
- John’s Hopkins Center for a Livable Future: Industrial Food Animal Production in America. Fall 2013. http://www.jhsph.edu/research/centers-and-institutes/johns-hopkins-center-for-a-livable-future/_pdf/research/clf_reports/CLF-PEW-for%20Web.pdf (accessed Oct 24, 2013).
- Cleaveland, S. et al. Phil. Trans. R. Soc. B. 2001, 356, 991.
- Watanabe, M. E. BioScience 2008, 58, 680.
- Burnet, F. M. Biological Aspects of Infectious Disease. Macmillan: New York, 1940.
- Centers for Disease Control and Prevention: Avian Flu and Humans. http://www.cdc.gov/flu/avianflu/h5n1-people.html. (accessed Oct 12, 2013)
- Cumulative number of confirmed human cases of avian influenza A(H5N1) reported to WHO. http://www.who.int/influenza/human_animal_interface/H5N1_cumulative_table_archives/en/ (accessed March 14, 2013)
- Chan, M. World Now at the Start of the 2009 Influenza Pandemic. http://www.who.int/mediacentre/news/statements/2009/h1n1_pandemic_phase6_20090611/en/ (accessed March 14, 2013).
- Ma, W. et al. J. Mol. Genet. Med. [Online] 2009, 3, 158-164.
- Mathew, A. G. et al. Foodborne Pathog. Dis. 2007, 4, 115-133.
- DeWaal, C. S.; Grooters, S. V. Antibiotic Resistance in Foodborne Pathogens. http://cspinet.org/new/pdf/outbreaks_antibiotic_resistance_in_foodborne_pathogens_2013.pdf (accessed March 14, 2014).
- Shames, L. Agencies Have Made Limited Progress Addressing Antibiotic Use in Animals http://louise.house.gov/images/user_images/gt/stories/GAO_Report_on_Antibioic_Resistance.pdf. (accessed Jan 20, 2014).