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How worried should we be about mad cow in the US?

The U.S. confirmed a new case of mad cow disease this week, and agriculture officials insist there was no danger to human health. But even as government experts investigate how the dairy cow contracted the disease, questions remain about whether the animal was an isolated, mutant cow or part of a larger cluster. There are enormous economical implications, as well as health concerns, to consider. In addition, there is confusion over the different forms of the disease, how it's spread and whether there is a serious threat in the U.S. from mad cow disease.

Q. What is mad cow disease?

A. The disease properly called bovine spongiform encephalopathy, or BSE, causes a horrible and rapid destruction of the brain and other parts of the nervous system. It came to widespread public attention in the 1980s and 90s when more than 180,000 cattle in Britain were infected and 4.4 million were destroyed to contain the epidemic.

Q.  How is it transmitted?

A. In rare cases it can arise spontaneously. But most often it occurs when one animal eats the flesh -- most often the brain or nervous tissue -- of another animal. A unique particle called a prion transmits the disease. It is the only infectious agent that is made up solely of protein. It has no DNA or RNA, or, in other words, no genetic material like viruses and bacteria contain.

Q. What is the human health danger?

A.  People who eat the flesh of infected animals can develop a condition with similar horrible symptoms known as variant Creutzfeldt-Jakob disease, or v-CJD. The disorder is not that common. Following the massive outbreak in cows in the U.K., so far 175 people have been infected. There is no cure and none has survived.

Q.  Have there been cases of v-CJD in the United States?

A.  There have been three cases in the U.S., but health officials say that in each case the victims spent large amounts of time in other countries where they ate infected beef.  There are also very rare cases that arise spontaneously, and all are checked out. Lab tests can determine whether the disease came from infected food.

Q.  Why was there such a large outbreak in Britain and a few European countries and not the US?  Didn’t the U.S. feed its cattle the flesh of dead animals as farmers did in Britain?

A.  In the opinion of many experts, the U.S. dodged a bullet. Yes, British and American feeding practices were similar. Britain had an outbreak of a related prion disease in sheep called scrapie and those infected sheep were fed to the cattle. Many experts believe the sheep infection set off the massive outbreak in cattle. After the British disaster, countries around the world stopped feeding livestock the carcasses of dead animals that could set off BSE.

Q.  How much BSE is there in the United States?

A. Officials assumed there was none until 2003 when an infected cow born in Canada was discovered in Washington state. After that -- especially because Japan and some other counties temporarily banned the import of U.S. beef -- the U.S. set up a surveillance system. Since then, the system has detected three more cattle, including this latest California case. The surveillance was cut back in 2006 because it was finding so few infected animals.

Q. How reassuring are the official statements that there is no danger to human health?

A.  The sick dairy cow was sent to a "rendering" plant that uses dead livestock to make non-food products so it was not headed for a slaughter house. Its meat was never bound for the U.S. food supply.

Q.  What about milk from the dairy cow?

A. Prions from infected animals appear mostly in brain, nerve and gut tissues. Prions have been detected in milk, so there may be a slight theoretical danger, but government health officials said the disease is not believed to be transmitted through milk.

Q.  What happens next in the investigation?

A. Officials believe this dairy cow’s BSE was the result of a spontaneous mutation.  But they need to prove this.  So far they say there is no evidence of infected material in the animal’s feed. But they will have to test other animals in the herd to be sure this is indeed an isolated case and not an outbreak.

Q. How worried should we be about mad cow disease in the U.S.?

A. We should encourage continued surveillance, but in my opinion there are many things that present far greater health threats -- even though this one sounds so scary.

Robert Bazell is NBC's chief science and medical correspondent. Follow him on Facebook and on Twitter @RobertBazellNBC


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