Mad cow disease, which officials said on Tuesday had been found for the first time in the US, still mystifies scientists because it is not caused by a virus, bacteria or other microbes, does not alert the immune system and can jump from species to species.
US Agriculture Secretary Ann Veneman said the disease, formally known as bovine spongiform encephalopathy (BSE), had been found in a single Holstein cow in Washington state.
"Even though the risk to human health is minimal, based on evidence, we will take all appropriate actions out of an abundance of caution," Veneman told reporters.
People can contract a form of Creutzfeldt-Jakob disease (CJD) known as vCJD from eating beef products infected by BSE. At least 137 people have died of vCJD, all linked to eating beef or to having received blood or tissue transplants from vCJD patients.
"In contrast to the traditional forms of CJD, vCJD has affected younger patients [average age 29 years, as opposed to 65 years for CJD], has a relatively longer duration of illness [median of 14 months, as against 4.5 months] and is strongly linked to exposure ... to BSE," the World Health Organization said in a statement.
BSE and CJD are members of a class of diseases called transmissible spongiform encephalopathies (TSEs). They are believed to be caused by misfolded proteins called prions.
TSEs affect cats, mink, elk, deer, cattle, sheep and a range of other animals. Rabbits, horses and dogs seem to be resistant but it is not known why.
TSEs can be passed on in food. Cattle are believed to have become infected with BSE when they were fed the rendered remains of sheep, which have their own TSE called scrapie.
Rendering or processing food can destroy the prions, but only with difficulty. Cooking alone will not do the trick.
Experts were not available to comment on Tuesday evening on the case of the infected US cow, but there is disagreement on just which beef products may carry the infectious prions that cause the disease.
British officials say milk cannot transmit the agent, but there is strong evidence that nerve tissue and other cattle parts ground up for some products may carry it.
A recent Swiss study suggested that, in theory, muscle tissue -- which would include steaks -- can carry the agent.
No one has determined how much a person must eat to become infected.
Because the misfolded proteins, or prions, believed to cause the TSEs occur naturally in the body, they do not alert the immune system as a virus or bacterium would to prompt a reaction against the disease.
Recent research suggests the "bad" prions can cause others to go bad simply by touching them.
Proteins are made by cells following instructions laid out in the genes.
But like a cardboard box, a protein must be folded to function and they sometimes get folded into the wrong shape.
Usually a cell will recognize this and eventually cause it to be broken up.
But Susan Lindquist and her colleagues from the Whitehead Institute at the Massachusetts Institute of Technology found late last year that this does not always happen.
They discovered that if the misfolded prions are not broken up quickly enough, they accumulate and alter the cell's metabolism, killing it.
In the brain this kills neurons. When the neurons die they break apart, releasing more prions into the system.
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