CNN reports that Cambodia is seeing a spike in the number of deaths due to the H5N1 strain of bird flu. In a related case, Mexico recently slaughtered more than 1 million chickens infected with the H7N3 strain of bird flu. Despite the increase in bird flu in Cambodia, H5N1 is currently not very contagious among humans (most people who contracted the virus were in direct contact with sick farmed animals), and H7N3 is not known to cause harm to humans.
In spite of our current low risk, it is just a matter of time before H5N1, H7N3 or another influenza strain evolves into a dangerous form that results in a pandemic. And the events in Mexico and Cambodia beg the question: Are we ever going to be safe from bird flu?
As long as we continue to treat animals raised for food poorly, the answer is a definite "no."
We are consuming more animals than ever before. Once viewed as a luxury, meat is now becoming a dietary staple for many due to a worldwide growth in urbanized populations and affluence. Today, more than 64 billion animals are raised and killed for food worldwide annually (1).
To meet this demand, the industry has chosen to sacrifice the space and well-being of animals in the name of efficiency. Farmed animals are treated as "production units" and are denied their most basic needs. The overwhelming majority of animals raised for food are housed in extremely filthy, overcrowded conditions without access to fresh air, sunlight, or room to move about normally. This demand-driven transformation of animal agriculture is so dramatic that it has been dubbed the "Livestock Revolution" (2).
The conditions on these farms greatly contribute to the creation of deadly pathogens, including influenza viruses. Here's how it works: Wild aquatic birds are the primordial source of all influenza A viruses -- the ones that have the potential to cause pandemics. However, people rarely become infected directly from aquatic birds. Usually, an intermediate host must be involved. This intermediate host provides the right biological setting for the virus to transform into something that can easily infect a human. And that's where chickens and other farmed animals come in.
Most avian influenzas are mild, low-pathogenic (i.e., not very lethal) viruses. However, once they enter poultry factory farms (through insects or workers carrying the virus, for example), they can rapidly mutate into highly-pathogenic (very lethal) viruses, even over very short periods of time. Since 1990, outbreaks of highly-pathogenic virus subtypes have increased substantially among farmed birds compared with the years prior to 1990 (3, 4). The intensive confinement of birds has been found to facilitate both the increasing frequency and scale of these outbreaks (3, 5).
H5N1 demonstrates how a virus emerged from wildlife, adapted to domestic poultry and, after circulating in these populations, acquired the ability to infect humans (6).
In another twist, pigs are highly susceptible to both avian and human influenza viruses and could be the means by which H5N1 gains the ability to widely infect humans. Indeed, in the scientific community, pigs are commonly referred to as "mixing vessels" in whom viruses from birds, humans and other pigs co-mingle. In pigs, viruses swap genes, accelerating their own mutation.
As with birds, the crowding of pigs in confined operations increases the transmission of influenza viruses. Until relatively recently, there were only sporadic reports of pigs infected with H5N1, but a study published in 2010 not only confirmed widespread H5N1 infection in pigs, but the lack of symptoms in pigs carrying H5N1 means that the virus can easily evolve and evade detection in pig populations transported throughout the world.
Even more concerning is the fact that one particular viral isolate from pigs acquired the ability to recognize a cell receptor in the noses of both pigs and humans, a change that could allow it to spread easily among humans. The discovery of H5N1 in pigs portends that our problems with the virus may just be beginning.
Despite Mexico's attempts, slaughtering or "culling" animals isn't effective -- it provides only short-term solutions, at best. More than 100 million birds were killed throughout Asia to thwart the spread of H5N1 (7). However, the next wave of H5N1 reestablished itself in the same countries and spread to new ones. Additionally, vaccinating farmed animals may spur the evolution of the virus (8). It is suspected that the vaccination programs in China led to greater genetic diversity of the H5N1 virus and perhaps contributed to current strains (9).
The only lasting solution is to decrease our consumption of animals. By densely confining animals by the billions to feed our large appetite for animal products, we are unwittingly accelerating the mutation of the influenza virus into strains more deadly than we have yet seen.
Dr. Akhtar is the author of the book: Animals and Public Health. Why Treating Animals Better is Critical to Human Welfare. You can learn more at her website.
1. Food and Agriculture Organization of the United States. FAOSTAT, 2009.
2. Pearson J, et al. "Global risks of infectious animal diseases" Council for Agricultural Science and technology, Issue Paper No. 28, 2005.
3. Peiris JSM, et al. "Avian influenza virus (H5N1): A threat to human health" Clinical Microbiology Reviews 2007; 20: 243-267
4. Capua I, Alexander DJ. "Animal and human health implications of avian influenza infections" Bioscience Reports 2007; 27: 359-372
5. Bavinck V, et al. "The role of backyard flocks in the epidemic of highly pathogenic avian influenza virus (H7N7) in the Netherlands in 2003" Preventive Veterinary Medicine 2009; 88: 247-254
6. Leibler JH, et al. "Industrial food animal production and global health risks: Exploring the ecosystems and economics of avian influenza" EcoHealth 2009; 6: 58-70.
7. Yee KS,et al. Epidemiology of H5N1 avian influenza. Comp Immunol Microbiol Infect Dis 2009; 32: 325-340.
8. Lee, C-W.,et al. (2004) Effect of vaccine use in the evolution of Mexican lineage H5N2 avian influenza virus. Journal of Virology 78: 8372-8381.
9. Mackenzie D. Bird flu vaccination could lead to new strains. New Scientist. March 24, 2004.
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