An instruction on the content of livestock feed had been expected since January, when the FDA recommended that no cows, sheep or goats should eat feed that included parts of cow, sheep, goat, deer or mink. The new rule, however, bans the use of all animal protein except pure pork or pure horse (pigs and horses are not known to be affected by BSE).
The US government says there have been no cases of BSE in American cattle. Announcing the ban, the FDA said that if a case of BSE were ever discovered in the US, the ban would prevent it from spreading through feed.
The US livestock industry instituted a voluntary ban on certain animal proteins in cattle feed last year, and their associations yesterday welcomed the FDA ban, saying that it would put a "protective blanket around the cattle industry".
Some specialists say the fact that the ban does not affect pet food or chicken- or pig-feed, however, leaves open the possibility of this cheaper feed being used for cattle - as happened in some European countries.
Others, including a spokeswoman for the US Consumers' Union, objected that the pork exemption made the ban "totally inadequate". She said that British research showed that pigs could be susceptible to BSE.
US consumer groups and veterinarians pressed earlier this year for more research into the possibility of a mad-pig disease, after unearthing a dormant 1979 study that found pigs at one New York slaughterhouse with symptoms suggesting a disorder of the central nervous system.
The study said that because the pigs came from many different farms, there was a possibility that there was a widespread problem, but the findings were never followed up.
While there have been consistent denials that there have ever been any BSE cases in the US, there are documented instances of what is described as a "spontaneously occurring form of spongiform encephalopathy" in mink and farmed deer. Some scientists believe that the risk of a BSE epidemic exists in the US, where there are 100 million head of cattle, because around the same proportion of animal protein was added to livestock feed, before the new ban, as in Britain.
When a food safety specialist ventured to suggest on a popular television show two years ago, however, that the US might not be immune from BSE, his remarks precipitated a sharp fall in beef prices and left some farmers with huge losses. A Texas farmer who says he lost almost $7m is suing the Oprah Winfrey Show, and the specialist, under a US law that makes "disparaging the quality of food products" an offence.