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Q. What is "mad cow" disease?

A. Clinically known as bovine spongiform encephalopathy, or BSE, it is a chronic, degenerative disease of the brain and spinal cord in cattle. The animal's brain literally becomes spongy. It was first diagnosed in 1986 in Great Britain and has greatly affected the livestock industry there. There is no treatment or vaccine for BSE.

Q. How are cattle affected?

A. They can become nervous, aggressive and uncoordinated, lose weight and decrease their milk production. The animal typically does not show symptoms until two to eight years after infection; then it dies within two weeks to six months. The disease is confirmed by examining the brain tissue after death or by detection of an abnormal protein in the brain.

Q. How do cattle get the disease?

A. Through contaminated meat and bone meal fed to the animal as a protein source. The use of such feed has been banned in the United States since 1997. So has the importation of beef from the United Kingdom, Europe, Japan and Israel, where BSE has been found. Through 2002, more than 183,000 cases of BSE were confirmed in the United Kingdom alone.

It is believed that cattle cannot contract the disease from another live animal.

Q. How is the disease transmitted to humans?

A. Scientists believe it can be transmitted through the consumption of contaminated beef, especially parts of the central nervous system such as the brain. There is no evidence that it can be transmitted by milk. Called variant Crueutzfeldt-Jakob Disease, or vCJD, in humans, it is believed to be caused by an abnormal prion, a cellular protein, that folds incorrectly in the cells.

Q. How many human cases have been reported?

A. The World Health Organization says that through 2002, 129 cases have been reported in the United Kingdom, six in France and one each in Canada, Ireland, Italy and the United States. The U.S. case was in a 22-year-old Florida resident who was born and raised in the United Kingdom.

Q. What is its effect on humans?

A. Just as in cattle, it slowly deteriorates the brain and is invariably fatal. Scientists think it takes many years, even decades, to cause symptoms: early psychiatric or sensory problems, then lack of coordination and later, muscle jerking and severe dementia. Death may take several years. There is no vaccine or treatment.

Q. Isn't there a naturally occurring form of CJD?

A. Yes. Experts say more than 85 percent of the 200 cases a year in the United States occur sporadically, with no known cause or risk factor. About 5 to 10 percent are hereditary, and less than 1 percent are acquired by exposure to infected nervous-system tissue through certain medical procedures.

Q. Is there monitoring of apparently naturally occurring cases of CJD to make sure they were not caused by beef consumption?

A. Yes. The Centers for Disease Control and Prevention analyzes death-certificate information and investigates cases of CJD.

Q. How do naturally occurring CJD and vCJD, or mad cow disease, differ in their effects?

A. A person with naturally occurring CJD usually starts having symptoms in his or her 60s. Unlike the early psychiatric or sensory problems of mad cow disease, classic CJD symptoms begin with poor muscle coordination, personality changes and impaired memory, thinking and vision. Mental impairment soon becomes severe, and blindness can occur. The patient dies in a year.

Symptoms of mad cow disease typically begin at a much younger age and may last for years, until death. Of the more than 120 deaths in the United Kingdom, more than half have been in people under 30.

Q. Do I need to be worried about beef I buy at my grocery?

A. Federal agriculture officials said the infected cow's brain and spinal cord, which contain the disease-causing prion, were not sent to the meat-processing plant.

Q. Can every package of beef in the supermarket be traced to its precise origin?

A. No. But experts say all can be traced at least to a particular packing plant, and in some cases to a certain feedlot or farm. A batch of ground beef, which may mix meat from many cows, may be traceable only to a group of farms rather than a single farm.