August 7, 2004 New Scientist by Debora MacKenzie

Amid the rush, one cow caught Louthan's attention -- a white, 6-year-old Holstein standing in a truck at the downers' entrance. She was causing trouble, and Louthan admits he should have taken her round to the pens where cattle that could walk awaited slaughter. "But I was in a hurry," he says, "and she didn't want to step down off the trailer." Before she could trample the other cattle, "I put a hole in her forehead".

At that time the US Department of Agriculture was paying slaughterhouses for brain samples from downers, to test for BSE. The Holstein was officially among the downers, so Louthan took a sample. Two weeks later, the USDA announced that the cow was the first case of BSE detected in the US.

This was the moment that USDA officials had hoped -- perhaps even believed -- would never come. Despite years of warnings from the European Union that the US herd was probably infected, they had always maintained that the country was BSE-free -- even after Canada declared its first home-grown case in May 2003.

It was devastating news. Overnight, beef exports plummeted, at a predicted cost to the industry of $2 billion for 2004 alone. The USDA immediately implemented measures to protect consumers, most importantly by banning human consumption of brain, spinal cord and intestines from any cow over 30 months -- also known as specified risk material (SRM) because it is known to carry the highest risk of infection. It also appointed an international panel of scientists to assess its measures and advise it what to do next, based on Europe's hard-won experience of BSE.

In February the panel reported back. It praised many of the actions the USDA had already taken -- in particular for facing up to the fact that BSE had arrived in the US. The infected cow had been born in Canada, so it would have been possible to claim that the US was still BSE free, though the lack of any real separation between the two countries' herds would have made this little more than a technicality. But the report also found shortcomings, many of which have yet to be rectified. And so, as millions of North Americans barbecue beef this summer, the disease may still be spreading among cattle, people are still at risk, and no one knows how many mad cows there are.

It was back in 2000 that scientists working for the European Commission predicted that the US herd might have BSE . Their reasoning was based largely on two facts: first, prior to 1996, the US imported British cattle, some of which were certainly infected. Secondly, it also imported 44 tonnes of British meat and bonemeal (MBM) feed made from infected cattle, before the UK stopped selling it in 1996. It doesn't take much infected MBM to get an epidemic going: only 20 tonnes of British MBM sparked Switzerland's sizeable epidemic. What's more, after 1996, the US imported more than 800 cattle from other European countries that subsequently discovered they had BSE.

In 1997, under pressure from trading partners and worried scientists, the US banned cattle feed containing material from the carcasses of other cattle. But as late as 2002, the US Congress's General Accounting Office found that, just as in the UK in the early days of BSE, the ban was not being properly enforced. Easy to miss

Officials continued to point to the lack of any reported instance of BSE in the US herd, though experience from mainland Europe should have warned them how easy it is to miss the disease if you rely on finding sick animals. Calculations by epidemiologist Virginie Supervie of the University of Paris VI have shown that between 1991 and 1997 there must have been at least 1725 clinically obvious mad cows in France, and possibly thousands more, New Scientist has learned. Yet only some two dozen cases were reported over that period, and Supervie calculates that another 350 went unreported between 1997 and 2001, despite intense publicity about BSE. Mainland Europe only really discovered how much BSE it had when it started testing large numbers of cattle brains in abattoirs.

Some USDA scientists fought for years for such testing in the US, if only to confirm the country's BSE-free status. The response was grudging: the US tested 5000 cows in 2002, and 20,000 last year. One of them was the Holstein that showed up at Vern's in December.

In February, the international scientific panel warned that this level of testing was not nearly enough to assess the scale of the problem. It is widely believed that cattle are usually infected in their first year of life, so the Holstein is likely to have been infected about six years ago -- possibly from feed made in the US or Canada from native-born cattle. The mantra of all the BSE experts New Scientist has contacted is that "there is no such thing as an isolated case", so it seems likely that BSE has been circulating in North America for several years. The infected cow was the tip of an iceberg whose size can only be guessed.

In line with the international panel's recommendation, the USDA is cranking up its testing programme. The plan is that between June this year and the end of 2005, between 200,000 and 300,000 "high-risk" cattle -- downers, animals found dead, or animals with symptoms that look like BSE -- will be tested. The USDA claims this will be enough to detect an incidence of BSE as low as 1 in 10 million -- that's just 10 infected cows in the entire national herd -- though experts expect to find a higher rate. By way of comparison, France, which became infected in the same way, had a rate of 2 infected animals per 10,000 in 2003.

The testing regime will involve two kinds of test. Initial "quick" tests will be carried out at a network of 12 laboratories across the country, and any positives will then be sent for confirmation at the National Veterinary Services Laboratories in Ames, Iowa. The samples will be taken only "from establishments that have voluntarily consented to sampling", as the USDA plan puts it. These will be mainly renderers and the farms themselves. More than 11,000 brains were tested during June, the first month of the programme. As New Scientist went to press none had been confirmed positive.

But there are a number of questions hanging over the scheme. The plan will only work if the USDA can find enough "high-risk" cattle to test. Since December, downers have not been allowed into the human food supply, which means they are no longer being brought to abattoirs where they might have been tested. The USDA is now asking farmers to volunteer their downers for testing, but there is a flaw in this approach. If BSE is found in a downer, the farmer's entire herd will be destroyed. So if any cow looks suspicious, there is bound to be a temptation to "shoot, shovel and shut up", US veterinary experts have privately told New Scientist. "Several big [beef] producers have told me they won't let the USDA anywhere near their downers," one said.

In Europe, farmers did exactly the same. Switzerland tried to get round the problem by randomly testing cattle in abattoirs, to deter farmers from trying to pass off suspect animals as healthy. The USDA is doing that too. But with only 20,000 of the 35 million animals slaughtered each year scheduled for a random test, and only 40 abattoirs enlisted, the deterrence value is uncertain -- especially, as those abattoirs have been named, making it easy for farmers to avoid them. Testing times

Mo Salman, an epidemiologist at Colorado State University at Fort Collins, says he is impressed with the number of high-risk cows the agencies has managed to test so far. But he adds: "I am sceptical about their ability to obtain all the needed cattle within 18 months." Salman says the USDA has "given the green light" to his request to monitor the quality of the surveillance programme as it develops.

Some other aspects of the testing programme are causing concern, however. For the initial "quick" test, two main types of test are available: one is based on a technique called ELISA and the other on the so-called "western blot". ELISA-based BSE tests are known to be prone to "false positives". Western blots tend not to give false positives.

Last year, in a submission to the Paris-based World Organisation for Animal Health (OIE) on international testing standards, USDA officials recommended that countries with low levels of BSE should use the western blot, so as to avoid a string of false positives that might alarm consumers. And yet it has gone against its own advice. Five fast BSE tests were quickly licensed in the US after the infected cow was found in December, including a western blot made by the Swiss firm Prionics. But all the USDA's surveillance labs are using an ELISA test made by California's BioRad. Predictably, by early July two false positives had been announced, sending beef futures markets reeling.

The USDA has justified its choice by saying that the other tests need field trials. Yet the Prionics western blot, to take one example, has been used 20 million times in Europe. Some observers say BioRad's test was chosen simply because its salesmen were better. Another theory is that the USDA's choice of test, and its willingness to announce unconfirmed positive results that later prove false, is aimed at desensitising beef markets and consumers to such announcements before real positives start emerging.

Compounding the uncertainty is the fact that all definitive results will be decided by one lab, Ames. There are no plans to send any samples to independent reference labs to double-check Ames's decisions, even though the test it uses is notorious for being a somewhat subjective.

Despite its flaws, surveillance is the aspect of the US response that has won the highest marks from observers with experience of the European outbreak. They are less satisfied with the other two components of the US response: stopping the spread of the infection to cattle, and protecting people.

Beyond the 1997 ban on feeding ruminant remains to other ruminants, the US has taken no further measures to stop the spread of BSE among cattle. Europe has learned the hard way that simple ruminant-to-ruminant bans don't work, and in February the international panel insisted that more was needed in the US too. Unless you close off every possible avenue to infection, BSE continues to circulate.

One such avenue is putting cattle MBM into pig and poultry feed. Cross-contamination in feed mills or accidental feeding to cattle will guarantee that some cattle eat MBM. What's more, the remains of pigs and chickens that eat cattle -- including their gut contents -- can legally go into cattle feed, as can poultry litter, which contains poultry feed.

The problem is aggravated by the fact that, since December, downers and SRM have been banned from human food. That may be a good move from the point of view of the direct risks to human health, but it also means that all this potentially infected material is going into pig feed, poultry feed and pet food. And while it is illegal to feed reject batches of pet food to cattle, it happens, US veterinary experts have told New Scientist.

The Food and Drug Administration was due to impose new rules last month banning cattle blood from cattle feed, which is still legal, and banning cattle SRM being put in pig and chicken feed, thereby removing most infection from the feed system. It also planned to ban downers in all feed, and poultry litter in cattle feed. But the rules were postponed pending "wider consultation".

Even the existing ban will the slow spread of the infection considerably, says Stuart MacDiarmid of New Zealand's agriculture ministry, a member of the international panel. But more should still be done, he says. "They can't really claim that they have an absolute barrier as long as SRM are still permitted to go into rations for non-ruminants."

As for measures to protect people, banning human consumption of SRM should prevent virtually all transmission of BSE to humans, says Danny Matthews of the UK's Veterinary Services Laboratories in Weybridge, Surrey, another panel member. But some scope for infection remains. Meat removed by advanced meat recovery systems, in which carcasses are ground and forced through a filter, is commonly contaminated with spinal cord. Such meat can still be put in beef stock or flavouring, says Peter Lurie of the Washington-based consumer advocacy group Public Citizen. These ingredients then find their way into all sorts of product, from soups to snacks.

Lurie also points to the potential risk from brains and spinal cords from cattle younger than 30 months, which can still be eaten even though there is evidence that infection can surface in these tissues earlier. Such material is often found in burgers, hot dogs and the like.

Possibly a worse risk is that, as in the UK, the seriousness of the situation has not yet been brought home to the slaughterers who remove the SRM. Even after years of restrictions in Europe, with inspections and penalties -- and the knowledge that people have died -- spinal cord still turns up in carcasses. In the US there is as yet no independent inspection to confirm that SRM is being removed.

Despite these potential hazards, American consumers seem blithely unconcerned, and sales of beef have been virtually unaffected.

Final comfort, of course, can be had from the fact that it now seems very hard for a human to catch the disease. In the UK, where millions of people were exposed to BSE through infected meat, there have so far only been 142 confirmed cases of variant CJD, the human form of the disease.

But the US still needs to be on its toes. As Europe learned to its cost, BSE punishes complacency.