WASHINGTON (AP) – The Agriculture Department is ending its search for additional cases of mad cow disease even though officials have not found several animals suspected of having eaten the potentially infectious feed believed to have caused the only known U.S. case.

“Our investigation is now complete,” Dr. Ron DeHaven, the department’s chief veterinarian, said Monday. “We feel very confident the remaining animals, the ones we have not been able to positively identify, represent little risk.”

The closure leaves officials not knowing what happened to 11 head of cattle among 25 that authorities say were most likely to have eaten the same feed as that given to a Holstein diagnosed in Washington state with mad cow after it was slaughtered on Dec. 9.

All 25 were among 81 born on a farm in Alberta, Canada, and shipped into the United States in 2001. Officials have found 29 of the 81, including 14 considered most at risk.

The search for the 81 cattle led authorities to 189 farms and ranches and the testing of 255 animals, none of which had bovine spongiform encephalopathy, or BSE, the technical name for mad cow disease, DeHaven said. Some may have gone to slaughter, but BSE tests would have spotted any slaughtered animal that had had mad cow, he said.

The likelihood of finding more cases “is pretty slim at this point,” DeHaven told reporters during a telephone conference.

An international review panel created by Agriculture Secretary Ann Veneman said last week that U.S. officials had done a thorough job of searching for the animals but added that more focus should be put on preventing future cases.

The government also has found 2,000 tons of rendered protein into which tissue from the Holstein could have been mixed, said Dr. Stephen Sundlof, director of the Food and Drug Administration’s Center for Veterinary Medicine, which regulates animal feed.

The material is being destroyed so none can make its way into animal feed, Sundlof said.

Experts say meat and bone meal from infected animals can transmit mad cow’s infectious protein to other animals.

The decision to end the search for potentially infected cattle does not close the books on the case.

Officials do not know the source of the feed thought to have sickened the Holstein, and they cannot be certain that all America’s 95 million cattle are free of mad cow disease.

The international review panel warned of the probability of other cases.

Also unknown is what happened to all 10,410 pounds of meat that was recalled from the Washington state slaughterhouse that processed the Holstein and mixed its meat with that of 19 other animals. They expect some may have been eaten. However, the meat supply is safe, DeHaven said. USDA had said the parts of the Holstein that could have contained infectious material, such as the brain and spinal cord, were removed before processing.

The recall was initiated out of “an abundance of caution,” DeHaven said.

Eating meat from animals with mad cow has been linked to a rare but fatal condition in people, variant Creutzfeldt-Jakob disease, although no cases have been traced to U.S. beef.

Beyond the domestic impact is the loss of crucial international markets, which had absorbed about 10 percent of U.S. beef production. About 50 countries have banned U.S. beef, its byproducts or live cattle. Among them are Japan and Mexico, America’s best customers. Despite a procession of foreign visits by high-level U.S. trade officials, almost all the bans remain.

The government plans to follow up on initiatives to assure U.S. and foreign buyers that U.S. cattle will be safe from mad cow disease. The main bulwark is a 1997 ban on the feeding of cattle with protein or bone meal of cattle or other ruminants, such as sheep.

Since the first U.S. case became known Dec. 23, the government has banned meat from nonambulatory animals such as the Holstein from being used for food. It also is considering a ban on blood and blood products from feed for ruminants.

The five-member international review panel proposed even stricter measures. They would include banning the use of any mammal and poultry parts in cattle feed.

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On the Net:

Agriculture Department BSE information: http://www.usda.gov/BSE/

Food and Drug Administration: http://www.fda.gov

FDA BSE information: http://www.fda.gov/oc/opacom/hottopics/bse.html

AP-ES-02-09-04 1548EST