Don't Miss Out

Subscribe to OCA's News & Alerts.

Avian Influenza: Flu Factories

   In our efforts to streamline farming practices to produce more meat for more people, we have inadvertently created conditions by which a harmless parasite of wild ducks can be converted into a lethal killer of humans. University researcher E.F. Torrey.(1)

 By Dr. Michael Greger

How does naturally occurring, innocuous waterfowl flu mutate into virulent, highly pathogenic avian influenza (HPAI)?

According to the World Health Organization's 2005 assessment of the pandemic threat, HPAI viruses aren't born, they're made. "Highly pathogenic viruses have no natural reservoir. Instead, they emerge by mutation when a virus, carried in its mild form by a wild bird, is introduced to poultry. Once in poultry, the previously stable virus begins to evolve rapidly, and can mutate, over an unpredictable period of time, into a highly lethal version of the same initially mild strain."(2) Scientists have demonstrated this transformation in a laboratory setting.

Duck, Duck...Chicken

A collaboration of U.S. and Japanese researchers started with a harmless virus isolated from waterfowl and proceeded to do serial passages through baby chickens. First, the researchers took day-old baby chicks and squirted a million infectious doses into their lungs. The virus was left there for a few days to allow it to start to adapt to the chicks' respiratory tracts. The viral mutant naturally selected to predominate would presumably be the one that learned best through trial and error to undermine the hatchlings' defenses. After three days, they killed the chicks, ground up their lungs, and squirted the viral lung slurry down the throats of other chicks. They allowed a few days for the virus to adapt further before repeating the cycle two dozen times.

The researchers ground up the final baby chickens' brains and infected five additional rounds of healthy chicks with infected brain pulp. With every passage, the virus grew more adept at overwhelming and outwitting the fledging birds' immune systems to best survive and thrive in its new environment. The final infected brain sample, after two dozen cycles though lungs and five cycles through brain, was squirted into the nostrils of healthy adult chickens. By the 18th lung passage, the virus was able to kill half of the chickens exposed. After the final five brain passages, the virus was capable of rapidly killing every chicken. The researchers concluded, "These findings demonstrate that the avirulent [harmless] avian influenza viruses can become pathogenic during repeated passaging in chickens."(3)

If mad scientists wanted to create a bird flu virus of unprecedented ferocity, they could try to continually keep cycling the virus through chickens. Imagine if the serial passaging was done not two dozen times, but 20,000 times. What kind of virus would come out the other end?

Industrialized Animal Agriculture and Disease

Each year in the United States, nearly 9 billion chickens are raised and killed for meat(4) and approximately 300 million hens(5) lay eggs. The overwhelming majority of these birds are confined in industrialized factory farms. Broiler chickens are housed by the tens of thousands in football field-sized sheds,(6,7) living in their own excrement, and egg-laying hens are intensively confined in small, wire "battery cages," metal enclosures so small, the birds can't even spread their wings,(8,9) suspended over massive pits of their manure.

Of factory farming, the director of Toronto General Hospital's Centre for Travel and Tropical Medicine said, "The primary driver has been economics crush them into these abnormal environments, poke them full of whatever and make them fatter for sale. Any microbe that enters that population is going to be disseminated to thousands of animals...."(10) Indeed, in the filthy and intensive conditions on factory farms, mass disease outbreaks may be inevitable.(11)

Factory Farms and Avian Influenza

Inside overcrowded broiler chicken factory farm sheds, the birds live in their own excrement, so the virus need not even develop true airborne transmission via nasal or respiratory secretions. Rather, the virus has an opportunity to be excreted in the feces and then inhaled or swallowed by the thousands of other birds confined in the shed, allowing the virus to rapidly and repeatedly circulate. With so many birds in which to readily mutate, low virulence strains can sometimes turn into deadly ones. Highly pathogenic bird flu viruses seem predominantly to be products of factory farming.(12) Indeed, said University of Ottawa virologist Dr. Earl Brown, a specialist in influenza virus evolution, "You have to say that high intensity chicken rearing is a perfect environment for generating virulent avian flu virus."(13)

Many of the world's scientific authorities seem to agree. The World Health Organization blames the increasing trend of emerging infectious diseases in part on the "industrialization of the animal production sector"(14) in general, and the emergence of H5N1 on "intensive poultry production" in particular.(15) The OIE blames in part the shorter production cycles and greater animal densities of modern poultry production which result in "greater number of susceptible animals reared per given unit of time."(16) According to the Royal Geographical Society, "Massive demand for chicken has led to factory (battery) farming which provides ideal conditions for viruses to spread orally and via excreta which inevitably contaminates food in the cramped conditions that most birds are kept in."(17) "High concentrations of animals," concluded the International Food Policy Research Institute, "can become breeding grounds for disease."(18)

Other experts around the world similarly lay blame on "so-called factory farming,"(19) "intensive poultry production,"(20) "large industry poultry flocks,"(21) "intensive agricultural production systems,"(22) or "intensive confinement."(23) "We are wasting valuable time pointing fingers at wild birds," the U.N.'s FAO has stated, "when we should be focusing on dealing with the root causes of this epidemic spread which...[include] farming methods which crowd huge numbers of animals into small spaces."(24)

A Global Problem

With intensive confinement operations spreading around the globe, southern China may lose its distinction as purported pandemic epicenter of the world.(25) In Chile in 2002(26) and in Italy and Mexico in the 1990s, the same scenario played out: A low-grade waterfowl virus found itself locked inside a building with thousands of chickens, leading to the "now predictable mutation to a highly pathogenic virus."(27)

In Mexico, a low-grade H5N2 virus, causing no more than mild respiratory symptoms in chickens, found its way into industrial poultry facilities outside Mexico City and turned deadly,(28) eventually affecting nearly a billion birds throughout the country.(29) The "informative, but frightening"(30) lesson to be learned from the Mexico outbreak is that once a harmless waterfowl virus is introduced into millions of farmed chickens,(31) it can "accumulate multiple mutations and become a highly pathogenic strain that causes high mortality."(32)

The Italian outbreak in 1999-2000 among "intensively reared poultry" caused the deaths of over 13 million birds in 3 months, evolving into a virus with 100% morbidity and 100% mortality,(33) which means once the virus got into a flock, every bird caught it and every bird died. Over the preceding 20 years, the Italian poultry industry had grown and industrialized dramatically, particularly in the Veneto and Lombardia regions, where the 1999 epidemic broke out.(34)

The epidemic wiped out both broiler chicken and egg operations. OIE veterinary officials in Italy at the time wrote, "To date, HPAI has affected virtually all intensively reared avian species regardless of age or housing system."(35) The virus spread slower within battery-cage egg facilities than broiler chicken sheds. Although all the caged laying hens would eventually succumb, the virus spread from cage to neighboring cage, instead of exploding throughout the building at once as was seen in the broiler chickens living directly on their own waste. The investigating scientists suspect then that the behavior of viral spread "was probably related to the amount of infected feces in direct contact with the birds."(36) This suggests that outdoor flocks may be the least risk since droppings may quickly dry in the sun and open air, rapidly killing any virus contained within.(37)

Animals and Humans at Risk

The intensive chicken and egg industries are not only playing with fire with no way to put it out, they are fanning the flames. And firewalls to contain the virus don't exist. A pandemic of H5N1, or a comparable future bird flu, has the capacity to spark a devastating human catastrophe. It may be wiser to move away from intensive poultry production altogether or, at the very least, stop encouraging its movement into the developing world.

Drawing on his 37 years of experience within the poultry industry, Ken Rudd concluded his trade publication article, "Poultry reality check needed," with these prophetic words:

"Now is the time to decide. We can go on with business as usual, hoping for the best as we charge headlong toward lower costs. Or we can begin making the prudent moves needed to restore a balance between economics and long-range avian health. We can pay now or we can pay later. But it should be known and it must be said, one way or another we will pay."(38)

Michael Greger, M.D., is director of public health and Animal Agriculture in the Farm Animal Welfare division of The Humane Society of the United States.


1. Torrey EF and Yolken RH. 2005. Beasts of the Earth: Animals, Humans, and Disease (New Jersey: Rutgers University Press).

2. World Health Organization. 2005. Avian influenza: assessing the pandemic threat, January 1.

3. Ito T, Goto H, Yamamoto E, et al. Generation of a highly pathogenic avian influenza A virus from an avirulent field isolate by passaging in chickens. Journal of Virology 75(9):4439-43.

4. U.S. Department of Agriculture National Agricultural Statistics Service. 2005. Poultry slaughter: 2004 annual summary.

5. U.S. Department of Agriculture National Agricultural Statistics Service. 2005. Chickens and eggs. Published January 24, 2005.

6. Ernst RA. 1995. University of California Cooperative Extension, Poultry Fact Sheet No. 20, June 1995.

7. Voris JC. 1997. University of California Cooperative Extension, Poultry Fact Sheet No. 16c, September 1997.

8. Stamp Dawkins MS and Hardie S. 1989. Space needs of laying hens. British Poultry Science 30:413-6.

9. Mench JA and Swanson J. 2000. Developing science-based animal welfare guidelines. A speech delivered at the 2000 Poultry Symposium and Egg Processing Workshop,

10. Brady M. 2004. The Comeback Kids: Just when you thought it was safe to go back outside, along comes another pestilence from the past. Ottawa Citizen, May 23, 2004.

11. Girard D. 2004. Coping with the flu virus. Toronto Star, April 10, p. F1.

12. Horimoto T and Kawaoka Y. 2001. Pandemic threat posed by Avian Influenza A viruses. Clinical Microbiology Reviews 14:129-49.

13. Bueckert D. 2004. Avian flu outbreak raises concerns about factory farms. Daily Herald-Tribune (Grande Prairie, Alberta), April 8, p. 6.

14. Stohr K and Meslin FX. 1997. The role of veterinary public health in the prevention of zoonoses. Archives Virology 13:S207-18.

15. Pheasant B. A virus of our hatching. Australian Financial Review, January 31.

16. Pheasant B. A virus of our hatching. Australian Financial Review, January 31.

17. Royal Geographical Society. 2004. Bird flu across Asia. February 23. Geography in the News.

18. Delgado C, Rosegrant M, Steinfeld H, Ehui S, and Courbois C. 1999. Livestock to 2020: the next food revolution. Food, Agriculture, and the Environment Discussion Paper 28. For the International Food Policy Research Institute, the Food and Agriculture Organization of the United Nations and the International Livestock Research Institute.

19. Zhihong H, Shuyi Z, and Zhu C. 2005. Zoonoses: the deadly diseases that animals pass on to us. Global Agenda: Magazine of World Economic Forum Annual Meeting.

20. Pheasant B. A virus of our hatching. Australian Financial Review, January 31.

21. Chastel C. 2004. Emergence of new viruses in Asia: is climate change involved? Médecine et maladies infectieuses 34:499-505.

22. Prowse S. 2005. Biosecurity and Emerging Infectious Diseases.  Australia Academy of Technological Sciences and Engineering Focus No 136, March/April.

23. Boyd W. 2001. Making meat: science, technology, and American poultry production. Technology and Culture 42:631-64.

24. United Nations. 2005. UN task forces battle misconceptions of avian flu, mount Indonesian campaign. UN News Centre, October 24.

25. Shortridge KF, Peiris JSM, and Guan Y. 2003. The next influenza pandemic: lessons from Hong Kong. Journal of Applied Microbiology 94:70-9.

26. Suarez DL, Senne DA, Banks J, et al. 2004. Recombination resulting in virulence shift in avian influenza outbreak, Chile. Emerging Infectious Diseases 10:693-99.

27. Capua I, Mutinelli F, Marangon S, and Alexander DJ. 2000. H7N1 avian influenza in Italy (1999 to 2000) in intensively reared chickens and turkeys. Avian Pathology 29:537-43.

28. Webster RG, Shortridge KF, and Kawaoka Y. 1997. Influenza: interspecies transmission and emergence of new pandemics. Federation of European Microbiological Societies Immunology and Medical Microbiology 18:275-9.

29. Infectious Disease Society of America. 2006. Avian Influenza (Bird Flu): Agricultural and Wildlife Considerations. April 4.

30. Webster RG, Shortridge KF, and Kawaoka Y. 1997. Influenza: interspecies transmission and emergence of new pandemics. Federation of European Microbiological Societies Immunology and Medical Microbiology 18:275-9.

31. Webster RG. 1997. Predictions for future human influenza pandemics. The Journal of Infectious Diseases 176:S14-9.

32. Webster RG. Influenza: An emerging microbial pathogen. In Emerging Infections. Academic Press, 1998:275-300.

33. Capua I, Marangon S, Selli L, et al. 1999. Outbreaks of highly pathogenic avian influenza (H5N2) in Italy during October 1997 to January 1998. Avian Pathology 28:455-60.

34. Capua I and Marangon S. 2000. The avian influenza epidemic in Italy, 1999-2000: a review. Avian Pathology 29:289-94.

35. Capua I and Marangon S. 2000. The avian influenza epidemic in Italy, 1999-2000: a review. Avian Pathology 29:289-94.

36. Capua I, Mutinelli F, Marangon S, and Alexander DJ. 2000. H7N1 avian influenza in Italy (1999 to 2000) in intensively reared chickens and turkeys. Avian Pathology 29:537-43.

37. Shortridge KF, Zhou NN, Guan Y, et al. 1998. Characterization of avian H5N1 influenza viruses from poultry in Hong Kong. Virology 252:331-42.

38. Rudd K. 1995. Poultry reality check needed. Poultry Digest, December 1995, pp. 12-20. Reprinted in

Related Links

Avian Influenza: Unjustly Blaming Outdoor Flocks

An HSUS Report: The Welfare of Animals in the Broiler Chicken Industry

Copyright © 2006 The Humane Society of the United States. All rights reserved.