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Synthesis/Regeneration 50   (Fall 2009)

Why Should Factory Farming of Swine and Poultry Create a Pandemic Influenza Threat?

by Stan Cox

We know that indiscriminate feeding of antibiotics to livestock has led to higher rates of human infection by antibiotic-resistant bacteria. But how might factory-farming practices increase the threat from the influenza virus?

Michael Greger, M.D., director of public health and animal agriculture for The Humane Society of the United States, listed on the Society's website some of the reasons that "Factory farms can be considered viral breeding grounds." Quoting Dr. Greger, they are: [1]

The World Health Organization explains why influenza A viruses (including the one causing swine flu) evolve and spread so rapidly [2]: Influenza viruses are grouped into three types, designated A, B, and C. Of greatest concern are the influenza A viruses. They have characteristics that make influenza A one of the most worrisome of all the well-established infectious diseases. These viruses mutate much more rapidly that type B viruses, and this gives them great flexibility. In addition to humans, they infect pigs, horses, sea mammals and birds. They have a large number of subtypes, all of which are maintained in aquatic birds, providing a perpetual source of viruses and a huge pool of genetic diversity.

When pigs are intensively confined, the large viral loads necessary for the emergence of rare flu mutants can rapidly transfer from animal to animal.

As a result of their unique features, influenza A viruses regularly cause seasonal epidemics in humans that take a heavy toll in morbidity and excess mortality, especially when pneumonia is a complication. At recurring yet unpredictable intervals, influenza A viruses cause pandemics. Scientists describe these viruses as sloppy, capricious, and promiscuous. Their labile and unpredictable nature is notorious. As they lack an RNA proof-reading mechanism, the small errors that occur when the virus copies itself are left undetected and uncorrected.

As a result, influenza A viruses undergo constant stepwise changes in their genetic make-up. This strategy, known as antigenic drift, works well as a short-term survival tactic for the virus: the speed with which slight variations develop keeps populations susceptible to infection.

Though small, the changes are sufficient to evade the defenses of the immune system. Populations protected, whether because of previous infection or vaccination, against one virus strain will not be protected when the next slightly different virus arrives.

Add yet another feature: the genetic content of these viruses is neatly segmented into eight genes. This facilitates the most greatly feared event: the swapping of gene segments during co-infection with human and avian influenza viruses, creating a new virus subtype that will be entirely or largely unfamiliar to the human immune system.

If this new "hybrid" virus contains the right mix of genes, causing severe disease and allowing easy and sustainable human-to-human transmission, it will ignite a pandemic. This strategy, known as antigenic shift, works well as a long-term survival tactic: immunologically, a new virus subtype starts from scratch and is guaranteed a very large population of susceptible hosts.

Stan Cox is lead scientist for the Land Institute in Salina, KS, and author of Sick Planet: Corporate Food and Medicine. Write to him at t.stan@cox.net.

[26 oct 09]

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