CLAIMS that abormal prion proteins have been found in American sheep are
setting alarm bells ringing in the US. But BSE experts are unimpressed.
The US government ordered the incineration of 376 sheep from Vermont, after
suggestions that four of the flock were infected with BSE鈥攖he brain
disease in cows that causes deadly new variant CJD (vCJD) in people. If the four
cases are confirmed as BSE, they will be the first sheep known to be infected by
farm feed, and would raise the spectre of more human infection.
Larry Faillace, a former animal disease researcher, imported Belgian and
Dutch milking sheep to his Vermont farm in 1996. The US Department of
Agriculture (USDA), fearing the imported sheep could have eaten Belgian feed
contaminated with BSE, tested animals culled from Faillace鈥檚 flock for prion
disease. This month it announced test results indicating that seven of the sheep
had brain damage of the type associated with prion disease. The animals had not
shown symptoms of brain disease.
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Sheep brains can be damaged in this way by both BSE and scrapie鈥攁 prion
disease that affects sheep but is not thought to harm humans. Antibodies used to
test the sheep for scrapie did not bind to the brains of these sheep, however.
Yet four came up positive for prions in two other tests: a new assay developed
by Mary Jo Schmerr of the USDA laboratory in Ames, Iowa, and a Western blot
analysis by Richard Rubenstein of the Institute for Basic Research in
Developmental Disabilities, Staten Island.
On 14 July, this led US agriculture secretary Dan Glickman to order the
animals to be destroyed because they had prion disease 鈥渙f foreign origin鈥�,
which might be BSE.
Sheep deliberately fed BSE-contaminated feed will develop prion disease.
While no one has ever shown that this happens on farms, some scientists say it鈥檚
possible that BSE has appeared in sheep only to be mistaken for scrapie.
However, many experts doubt whether the Vermont sheep have BSE. 鈥淭he
connection to BSE is far-fetched speculation,鈥� says Tom Pringle of the Sperling
Foundation in Oregon, which monitors prion research. The two tests used gave
inconsistent results, and the imported animals were supposed to have been
strictly grass-fed.
Scrapie and BSE can only be reliably distinguished by injecting brain tissue
into mice, then waiting months for the result. Ian McConnell of Cambridge
University, an adviser to the British government on vCJD/BSE, told New
Scientist: 鈥淚 have no idea how they would make this distinction. It鈥檚 a
蝉丑补尘产濒别蝉.鈥�
Cases of vCJD have risen sharply in Britain this year
(see Table). Because
the infection can lie dormant for many years, the fear is that thousands of
cases might eventually emerge.
