Japan, the world's largest customer for U.S. beef, banned imports from the United States last month after the mad cow case was discovered. Canadian beef was banned seven months earlier when a case of the illness was detected there.
Both Washington and Ottawa are pressing Japan to drop the bans, arguing that their beef products are safe, but the findings of the 11-day Japanese mission to the United States and Canada advised caution.
The cow discovered in Washington state with bovine spongiform encephalopathy, or BSE, as the disease is formally known, was imported from Canada.
"It cannot be guaranteed that there will not be a recurrence of BSE in the United States," the five-member team said in its report.
The report cited the close links between the two North American countries' beef industries. The United States imposed restrictions on Canadian cattle and beef after the case was discovered there in May.
Mad cow disease is a public health concern because scientists believe humans who eat brain or spinal matter from an infected cow can develop variant Creutzfeldt-Jakob disease. The incurable disease was blamed for 143 deaths in Britain during a mad cow disease outbreak in the 1980s.
That concern prompted several countries to close their borders to American beef last month.
Before the ban, Japan was the top importer of U.S. beef, snapping up $1 billion worth of American beef and beef products a year. It also imported about $55 million worth of Canadian beef in 2002 before halting trade in May.
The sharp drop in beef supplies has sent the price of both domestic and imported beef soaring in Japan. The Agriculture Ministry said Monday that retail prices reached a record high last week since it began monitoring such data in August.
Tokyo has scrambled to make up for the shortfalls, sending delegations to try to secure more beef from Australia and New Zealand. Neither country has reported a case of mad cow disease.
Japan tests all the 1.3 million cattle it slaughters every year for the disease and is pressing beef-exporting nations to adopt similar safeguards.
In comparison, the U.S. tested only about 20,000 cows each year the past two years. Three-quarters of those tested were so-called downer cows — sick animals that can no longer walk.
After the outbreak in Washington State, the U.S. Department of Agriculture imposed a series of stepped up regulations. It banned downer cattle and certain cow parts from the food supply and prohibited some methods of killing cattle and stripping their meat.
While acknowledging the safety measures implemented by the United States and Canada, the team concluded that the threat of further infections remained, Agriculture Ministry representative Shukichi Kugita said.
"U.S. safety measures compared to those of Japan are inadequate," he said, citing the continued use of feed containing protein or bone meal.
Mad cow disease is believed to spread by recycling meat and bones from infected animals back into cattle feed. U.S. authorities have outlawed giving it to cattle but still allow it to be fed to other livestock.
"The threat of cross-contamination remains because such feed can inadvertently get mixed up on farms," Kugita said.
The U.S. Food and Drug Administration is considering tightening restrictions on feeding animal protein to other animals. Companies already must test all feed shipments bound for the United States.
Since 1997, the United States and Canada have banned animal feeds that contain tissues of cattle, goats or sheep to keep out the illness.
The report, which was to be used as a basis for future discussions on the issue, said U.S. and Canadian officials plan to provide additional details about questions unanswered during the mission. Washington will send a negotiating team to Japan later this week.