Living on Earth: Farming the Flu

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Ellen Silbergeld is professor of environmental health at Johns Hopkins Bloomberg School of Public Health.


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YOUNG: Why do you suspect these large-scale farms? What is it about this particular method of farming that you think makes it easier for viruses to change and to jump from species to species?

SILBERGELD: Well, this, of course, is not strictly limited to influenza viruses, but is based on observations by us and others in studying bacteria as well as viruses.

These operations contain elements that really are conducive to raising concerns for public health. You have thousands of animals, in the case of pigs usually between two and six thousand animals, held in close confinement under conditions that are not sanitary, that is frankly that they're housed with their wastes. And therefore you've got a lot of hosts available to exchange a pathogen, which is one of the processes by which viruses and bacteria evolve and acquire mutations. And then the way in which these operations are run – and I want to stress that this is a worldwide issue, it is not peculiar or restricted to Mexico.

You have situations that are not bio-secure and also large amounts of waste that are not well regulated in terms of their management or disposal. In terms of bio-security, it's generally not recognized that these operations have to be high ventilated. When you put two to six thousand animals in side a building, you have to have very high rates of ventilation or that animals will die of heat stress. So several researchers have in fact reported that in the environs of these large operations, you can detect pathogens in the outflow air from these exhaust fans. People have also isolated influenza virus from the legs and feet of flies in the vicinity of these operations. This was noted in some outbreaks of avian influenzas, for example, in Japan. So, the lack of bio-security, the very dense populations, the conditions that are conducive to viral evolution and mutation and the lack of control over disposal of waste from these animals which also contain pathogens are really the elements that have given rise to concern for us and many others as to the public health implications of this manner of food animal production.

YOUNG: So despite their efforts to keep it secure, you can't stop the flow of air, you can't stop insects.

SILBERGELD: Well, the issue is most of the bio-security practices that have been developed by the industry – most of them have been to protect the animals from a person bringing in a disease that could be dangerous to the health of the animals. In terms of what we call bio-containment – that is preventing from something that might be in the house from coming out – there's very, very imperfect and limited protection.

YOUNG: Is there evidence that the people who work at these type farms are more susceptible to catching diseases from the animals and able to spread disease to other people.

SILBERGELD: There's considerable evidence that workers in these operations, farmers and farm workers are in fact at greater risk of exposure. Studies conducted in Iowa and other places have demonstrated that workers in swine CAFOs have a high rate of infection by swine influenzas. In the same way that we and others have reported that workers in poultry houses have a high rate of being infected by poultry born pathogens. Now – and here's one of the tricky things – they may not be sick. I'm sure you've heard of Typhoid Mary – and the really worrying thing in public health is what we call the asymptomatic carrier, that is a person who's become infected by a pathogen but does not get sick.

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http://www.loe.org/shows/segments.htm?programID=09-P13-00018&segmentID=3

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