The Department of Agriculture in March announced a tenfold increase in the number of cattle tested for bovine spongiform encephalopathy. The plan targets an estimated 446,000 cows in the higher risk group of BSE infection—and 20,000 older, apparently healthy cows at slaughter.
The plan is based on recommendations from a panel of international BSE experts and the Harvard Center for Risk Analysis. Agriculture Secretary Ann M. Veneman appointed the panel after the nation's first case of the degenerative neurologic disease in cattle was discovered in Washington state last December (see JAVMA, March 15, 2004).
"This enhanced plan will augment our aggressive measures taken over the past decade by strengthening BSE surveillance in the high-risk cattle population and establishing random surveillance in the general population," explained Veneman on March 15.
"This intensive effort," added Dr. Ron DeHaven, USDA chief veterinary officer, "will allow the department to more definitively determine if BSE exists in our national herd and, if so, accurately estimate the prevalence of the disease in the U.S. cattle population."
The expanded plan will be fully in place by June 1 of this year and last 12 to 18 months. Testing will be conducted at USDA National Veterinary Services Laboratories in Ames, Iowa, and a network of laboratories around the country. The NVSL made the initial diagnosis of BSE infection in the Washington state dairy cow.
Veneman announced the department was appropriating $70 million to offset the costs of additional testing. The USDA is also working to approve tests for rapid detection of the disease to use as part of the program. At press time in March, two rapid diagnostic tests had been approved.
For the past 14 years, the USDA's BSE surveillance program has targeted cattle populations in which the disease is most likely to be found. Each year, approximately 446,000 cows are considered at higher risk of infection for several reasons. They might be unable to walk, show signs of central nervous system disorders or signs consistent with a case of BSE, or die of unknown causes.
The primary focus of the enhanced surveillance effort will continue to be those high risk cattle, but the number tested will greatly increase. A random sampling of at least 20,000 apparently healthy, older cows will also be tested.
Older cattle will be tested at slaughter because BSE has a typical incubation period between 3 and 8 years. It's likely that those animals were born prior to the 1997 ban on feeding animal protein to ruminants.
The goal of the enhanced surveillance plan is to test as many animals as possible during the allotted period.
In the past fiscal year, 20,543 cows had been tested—a sample size that could detect the disease if it occurred in one animal per million adult cattle, with a 95 percent confidence level, which is 47 times the international standard for low-risk countries.
If 268,000 animals were tested, this would allow a BSE detection rate of one positive case in 10 million adult cattle, with a 99 percent confidence level. This means the enhanced surveillance program could detect BSE, even if only five infected cows existed in the United States, Dr. DeHaven said.
Samples from at-risk cattle will be collected from various locations, including farms, sale barns, veterinary diagnostic laboratories, veterinary clinics, and rendering facilities. Additionally, sampling will be geographically disbursed and proportional to the adult cattle populations in each state.
Samples from older cows will come from 40 U.S. slaughter plants that handle 86 percent of the aged cattle processed for human consumption in the United States. The animal carcasses will not be allowed to enter the human food chain until test results are received and show the samples are negative for BSE.
Dr. DeHaven emphasized the USDA's belief, based on more than a decade of sampling, in an "extremely low" prevalence of BSE in the nation's cattle, if the disease exists at all.
"Nevertheless," he said, "there is, and I think we need to recognize that there is, a chance that we could find more positive cattle. I think it's critically important that we keep it in perspective.
"The steps that we have already taken assume that there is the potential for infected cows in the U.S., and these measures provide the necessary safeguards for the protection of our public."
Additional information on the surveillance plan is available at www.usda.gov.