Mad Cow

Dean Cliver: Causes of the disease and risks for humans

Dean Cliver

Dean Cliver is a professor of food safety in the UC Davis School of Veterinary Medicine and a national expert on mad cow disease or bovine spongiform encephalopathy. He recently served on a BSE advisory committee for the U.S. Food and Drug Administration. Cliver, who helps the national and local media understand the science behind BSE, discusses how the disease is caused, the security of the U.S. beef supply and regulatory measures that should ensure human and animal health. He expects U.S. beef will be embargoed for some time, with potentially huge losses for farmers.

(Cliver also participated on the Feb. 5 mad-cow threat panel sponsored by the Institute of Governmental Affairs that can be viewed as an online video.)

Q. What is a prion and how does it cause BSE?

A. Prions are small proteins that occur naturally in all of us (humans, cattle, etc.); their normal function is unknown. When the amino acid strand of this small protein becomes folded in a specific, abnormal way, it becomes resistant to heat and to enzymes that digest proteins. These abnormal prions can also transfer their abnormal folding to normal prions on contact. This transformation occurs fairly readily within a single body, and abnormal prions can sometimes be transferred to others of the same species. Very few instances are known where abnormal prions can transfer their abnormality to prions of another species. Accumulation of aggregated prions, especially within the brain, leads to spongy degeneration and loss of function that is eventually fatal. There is, as yet, no known treatment. It should be noted that most species -- for example, poultry, dogs, and pigs -- are not known to be subject to prion diseases, also called transmissible spongiform encephalopathies.

Q. Why are prions found primarily in tissue of the nervous system, such as the brain and spinal chord?

A. Early in infection, abnormal prions are apparently most common at the entry site. Ingested prions probably colonize lymphatic tissue in the lining of the intestine. Over years, the abnormal prions transmit their abnormality and migrate via both the lymphoid organs and nerve trunks. Those that arrive in the brain and spinal cord accumulate to detectable levels in a number of species, including humans, sheep, cattle, and North American deer and elk. Although no one knows precisely why prions accumulate in the central nervous system, it may be that the absence of transmissible spongiform encephalopathies in other species is because abnormal prions accumulate elsewhere in these species.

Q. How concerned are you about the potential of BSE to affect human health in the United States?

A. I believe that other food-borne diseases, already known to occur in the United States, constitute a much greater threat. I am concerned that most of the extreme BSE control measures that are being proposed will divert limited resources from needed food-safety activities, leading to a net decrease in the safety of our food. I am also concerned that expensive measures are being proposed without consulting the 9 million people in our country who are seriously undernourished.

Q. How great is the risk to an average consumer in the United States of contracting mad cow disease?

A. Essentially zero. Before it was realized that bovine spongiform encephalopathy, also called "mad cow disease," was transmissible to humans, causing variant Creutzfeldt-Jakob disease, there were fewer restrictions on what parts of cattle could be eaten in the United Kingdom. With an incubation period of 10–20 years, there have now been about 145 Creutzfeldt-Jakob disease cases reported in the United Kingdom, a ratio of about 1,300 BSE cattle per human case. Annual numbers of both diseases are going down in the UK. I believe that our precautions are adequate, and I do not expect to see variant Creutzfeldt-Jakob disease of North American origin here (imported cases have already occurred in the United States and Canada). I have no intention of changing what I eat, nor will I recommend that any of my family do so.

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