快猫短视频

THE discovery of a BSE-infected cow in Canada confirms warnings from European scientists three years ago that cattle in North America could be infected. And if Canadian cattle are infected it is likely that the disease is also present in the US.

The US and Canada test so few animals that low levels of BSE infection would not be detected. Indeed, the number Canada tests would be unlikely to reveal a level of infection any lower than what the UK now has. Other countries have found many more cases after increasing testing when the first infected cattle were reported.

What鈥檚 more, neither country has taken any of the measures needed to prevent people being infected by meat from diseased cattle (see 鈥淲hat to do now鈥�). Even if people are eating infected meat, there are unlikely to be many cases of vCJD, the human disease linked to BSE. In the UK, the country worst hit by the disease, it is thought unlikely that there will be more than 200 vCJD cases in total. But BSE could still have a huge economic impact on the massive $400 billion North American beef industry.

The Canadian case was an eight-year-old Black Angus beef cow in the western province of Alberta. The province tests a sample of 鈥渉igh-risk鈥� cattle: those found dead or disabled on farms, or with neurological symptoms, or that have been rejected by the abattoir. Such animals are up to 10 times as likely as other animals to have undetected BSE, so they are ideal for monitoring purposes.

This cow was rejected by an abattoir late in January because it had pneumonia. The result was not announced until 20 May due to delays caused by a backlog in testing, the 10 days it takes to do the test that Canada uses, and the need to confirm the finding at the world BSE reference laboratory at Weybridge in England.

Unlike Canada鈥檚 only previous case, diagnosed in 1993 in a cow born in Britain in 1987, this animal was born in Canada and must have been infected there, probably by eating contaminated feed eight years ago. Canada鈥檚 Agriculture minister Lyle Vanclief insists it is an 鈥渋solated case鈥�. But cases of BSE don鈥檛 happen spontaneously, and other cattle would have eaten the same feed. 鈥淭here must be more cases,鈥� says Marcus Doherr of the University of Bern in Switzerland, a leading expert on BSE epidemiology. 鈥淔or every case we detect, we estimate there were three to five animals exposed.鈥�

Europe has been urging North Americans to do more testing, Doherr says. In 2000, he and other scientists working for the European Commission concluded that BSE could be circulating at low levels in the US and Canada, because British cattle imported before 1990 were recycled as feed (快猫短视频, 10 June 2000, p 4). The risk would have peaked between 1993 and 1997. The Alberta cow was born in 1995.

But North America has been slow to test for BSE and does not use the fast tests developed in Europe. This is partly because these tests tend to produce false positives, which could be a headache in a country that claims to be BSE-free.

In Alberta, which has 2.5 million cattle over 18 months old, the target group for testing, only 1655 animals have been tested for BSE since 1996 鈥� half of them last year. Because so few animals are being tested, infected cattle could go undetected. The 849 high-risk cattle Alberta tested last year would only have been enough to reliably detect a prevalence of infection in the high-risk group of 0.035 per cent, slightly more than the current rate in the UK. Of course, there is a big margin of error: the true prevalence could be lower or higher.

The situation is slightly better in the US, where nearly 20,000 cattle found dead on farms were tested for BSE last year. All were negative. But the sample is still far too small to rule out the existence of BSE in the US herd.

And until the US border was closed to Canadian beef last week, the two countries鈥� cattle industries were closely integrated. Last year, Alberta shipped over half a million live cattle to the US. They also trade large volumes of meat and bonemeal for feeding to animals 鈥� the main way BSE is transmitted.