For the United States Department of Agriculture (USDA), it was about worst the Christmas present imaginable: the country's first case of mad cow disease. Nations around the world responded by banning the import of US beef.

The diagnosis of bovine spongiform encephalopathy (BSE) — in a 'downer' cow, unable to walk yet still sent for slaughter and human consumption — was confirmed on 25 December. Two days later, the USDA tentatively traced the animal to a herd imported from Alberta, Canada, which announced North America's first 'home-grown' case of BSE last May (see Nature 423, 467; 2003 10.1038/423467b ). A previous Canadian case, in 1993, occurred in a cow imported from Britain.

The new US case is from a farm in Mabton, Washington, about 200 kilometres southeast of Seattle. Efforts are being made to recall potentially contaminated meat products. Even though the cow may not have been infected in the United States, the case has raised questions about the adequacy of measures designed to keep BSE out of the US cattle herd — and to protect people from infection with the brain-wasting disease. In particular, scientists are calling for an expansion of the BSE-testing programme for slaughtered cattle.

The animal was probably infected by eating feed contaminated with the 'prion' proteins that are thought to cause the disease. Both the United States and Canada have banned the feeding to cattle of meat and bone meal from sheep and cows, although the meal can still be fed to other livestock. Experience in Britain, which introduced a similar ban in 1988, suggests that such bans are difficult to enforce unless meat and bone meal from ruminants is completely excluded from animal feed.

Experts also say that too few cattle are being tested for BSE in the United States to be sure that infected animals are not entering the human food supply. There is no routine testing for the 30-million-plus cattle slaughtered each year — and only about 10% of the 200,000 or so downer cattle slaughtered in 2003 were tested, even though an inability to walk is a known symptom of BSE. “We should be testing more animals,” argues Frederick Murphy, a veterinary scientist at the University of California, Davis.

The case is also likely to lead to a revival of attempts to exclude all downer cows from the human food supply. Such efforts have previously been defeated after lobbying from agricultural interests.