THERE are more than a billion chickens in Asia. In 1968, the year of the last pandemic, there were 13 million* chickens in China. Now, there are more than 13 billion in mainland China alone. And since the time from hatching to slaughter is only a matter of weeks or months, depending on whether the chicken is raised for meat or eggs, there are multiple cycles of these billions passing through the system in the course of the year.

Back then, there were 5 million pigs in China; now there are 500 million. "High concentrations of animals," concluded the International Food Policy Research Institute, "can become breeding grounds for disease."

This comment on the escalating number of livestock in the world is a quote from thebirdflubook, Michael Greger's publication which is freely available online. It makes riveting reading. It also provides the evidence that the modern poultry industry, with its rapid cycles of production, can increase the chances of changing a low pathogenic form of avian influenza into a highly pathogenic form.

"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."

-Johns Hopkins neurovirologist R H Yolken and Stanley Medical Research Institute director E.F. Torrey.

Wild birds are, of course, the natural reservoir for avian influenza viruses. They circulate as LPAI - the low pathogenic form. Wild birds do not themselves generate HPAI - the highly pathogenic form. The World Health Organisation explains in its 2005 assessment of the pandemic threat: 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.

The Flu Report details how this statement was borne out in an experimental setting. This is summary of the work: US 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. After three days they killed the chicks, ground up their lungs, and squirted the viral lung slurry down the throats of other chicks. They again allowed a few days for the virus to adapt further before repeating the cycle two dozen times.

With every passage, the virus grew more adept at overwhelming and outwitting the fledging birds' immune systems. 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. If you do this with the original swan virus nothing happens, but influenza is a fast learner. 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 single one. The researchers concluded: "These findings demonstrate that the avirulent harmless avian influenza viruses can become pathogenic during repeated passaging in chickens.

Now imagine the mad scientist scenario. Tens of thousands of chickens crammed into a filthy, football field-sized broiler shed, left to lie beak-to-beak in their own waste. The air is choked with moist fecal dust and ammonia, which irritates the birds' respiratory passages, further increasing susceptibility in chickens already compromised by the stress of confinement. Since the birds are standing in their own excrement, 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. The dose of virus transmitted from one bird to another might also play a crucial role.

Top UN coordinator David Nabarro http://www.khaleejtimes.com has also said: "The virus is likely to be with us for the five or ten years to come . . . . I do think we a have to put now plenty of energy towards a long term reform of the poultry farming techniques, in order to reduce the risks of human infection."

I wonder what sort of reforms he has in mind?

Why not read it in full?

* The figures for the chicken population in China 1968 should perhaps be higher than that stated. Note that the figures do represent the population at a given point in time, not the population accumulated over one year. Nevertheless, transmissibility of diseases increases as animal and human populations grow. It's an outcome of overpopulation. Over the past 20 years the poultry industry has undergone substantial changes resulting in shorter production cycles and higher stocking densities. Due to these changes, infectious animal diseases are significantly more difficult to control because of the greater number of susceptible animals reared per given unit in a given unit of time. As any organic farmer knows, intensification and high stocking densities lead to parasite and disease problems which need to be managed - OR the system has to be radically changed.