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Fearing BSE, USDA Seizes, Tests, and Destroys Vermont Sheep 

After winning a court battle against two Vermont sheep farmers last March, U.S. Department of Agriculture (USDA) officials seized more than 300 sheep, shipped them to Iowa for testing, and soon after taking samples for analysis destroyed the animals over concerns that some of them are carrying a form of transmissible spongiform encephalopathy (TSE), either bovine spongiform encephalopathy (BSE, also known as mad cow disease) or scrapie. Although officials remained uncertain about the extent and exact nature of TSE that was affecting the now-destroyed sheep, “we opted to err on the side of safety,” says Arthur Davis, chief of the Pathobiology Laboratory at the USDA National Veterinary Services Laboratory in Ames, Iowa.

BSE Concerns Propel Herd Kills in Europe, U.S.

The founding members—65 animals—of the Vermont sheep flocks were imported from Europe, where they were bred for high-capacity milk production for use in making specialty cheeses. Even though most of the other sheep in the destroyed herds were bred and raised in Vermont, these sheep were all kept under close watch--in part, because that is customary when sheep, cows, or other such animals are imported and also, in this case, because of suspicions that the founder animals were fed BSE-contaminated materials while still in Europe. The incident involving the Vermont sheep gathered momentum last July when one of the Vermont farmers voluntarily relinquished his small herd of 21 sheep to USDA officials, who declared their intention to remove additional sheep from two other farms in that state totaling more than 350 animals (ASM News, February 2001, p. 64). 

Although the Vermont sheep showed none of the behavioral signs associated with cases of TSE, samples of brain tissues were collected for analysis from animals that died during the several years since the founder sheep were imported and the numbers of animals in the several flocks grew, according to Davis. Some of those test samples “suggested some features” consistent with TSE, he says. However, he adds, “We use four criteria for ‘full-blown’ scrapie, and we never found that all four criteria were met in any one sample—only one or maybe two.”

Subsequently, responding to a request from one of the Vermont sheep owners, USDA investigators ran an experimental test on blood samples from some of the animals and, “lo and behold, it came up positive for about four animals,” Davis continues. “But because the [blood] test method is not validated, we used Western blots on brain samples for those sheep and some [of the latter tests] were positive . When we put the evidence together, even though we did not find typical histopathology, USDA was convinced to take precautionary action—taking the position that it is better to be safe than sorry.”

The results from testing samples that were taken from the larger flock of sheep, which was moved from Vermont to Iowa and destroyed in March, were still not available in mid-April, when Davis said several additional months would be needed before all the follow-up analysis could be completed. “We are not determining whether it's BSE or scrapie at USDA, but only whether it's a prion,” he says. “Because these animals are of non-U.S. origin, that is enough justification to eradicate them  since they were imported from Europe and they could be incubating the BSE prion.” Even before BSE concerns arose, USDA began restricting sheep imports as part of an effort to avoid importing new strains of scrapie, adds Linda Detwiler of the USDA Animal and Plant Health Inspection Service facility in Robbinsville, N.J.

“The most important thing is that it's sheep  and the real worry is that BSE from sheep in lab tests looks just like scrapie,” Detwiler says. Follow-up typing can distinguish BSE from scrapie, but definitive testing depends on a bioassay in mice that can take several years to complete. Another important concern over BSE in sheep is that, unlike with scrapie, it apparently distributes throughout a wider assortment of anatomic sites in affected animals and is more readily transmitted, including by blood, among them, she points out. “This poses an unbelievable risk . . . a zoonotic disease that is masked by scrapie.

Yet another component of the risk revolving around the Vermont sheep was the disposition of the young rams that the flock was producing, according to Detwiler. Although the herds were being raised for their milk-producing capacity, roughly half the animals born each breeding season were male, could produce no milk, but were deemed too risky to be sold for their meat. “USDA was buying them to keep them out of the food chain,” she says, Now that worry is past because all the animals from the several flocks from Vermont are “gone.”

USDA officials decided not to conduct longer-term studies on the Vermont sheep in part because of shortages in funds and for lack of appropriate facilities, according to Davis. “If we had the space and time to hold the sheep to do experiments, that would be reasonable,” he says. “But we're a regulatory agency, not a research agency.”

  Jeffrey L. Fox