A potentially fatal sheep disease spread by gnats has triggered a quarantine in eastern Montana, preventing ranchers from moving their animals at a time of year when lambs are shipped out, often to Colorado feedlots.

State veterinarian Marty Zaluski's order this week prohibits the transportation of sheep from 16 of Montana's 56 counties. The disease, bluetongue, has been confirmed in tests from eight flocks in six counties, said Lisa Schmidt, spokeswoman for the Montana Department of Livestock.

The disease, named for the tongue discoloration it causes, is suspected of killing hundreds of Montana sheep and sickening hundreds of others, Schmidt said. Bluetongue is not a risk to human health, authorities said.

The main consequence of the quarantine is that ranchers must now feed sheep that they assumed would already be sold off, forcing producers without available pasture to buy hay, said Rodney Kott, a Montana State University sheep specialist.

Kott said he has not seen a bluetongue outbreak in the state since he joined the university nearly 30 years ago.

Because of Montana's relatively cool climate, the gnats that spread bluetongue have been uncommon in the past in the state, which is ranked fifth in the nation for sheep production.

"The unusual thing about this situation is that this is the farthest north that this disease has been reported," said Larry Cooper, spokesman for the federal Animal and Plant Health Inspection Service in Fort Collins, Colo.

The quarantine is in effect until Oct. 10. Officials expect cold weather to eliminate gnats by then. "We expect a killing frost by Oct. 9 and that should reduce the risk of spreading bluetongue significantly," Zaluski said in a statement.

Mary Jane Beadle, who is a partner in a sheep operation in Roundup north of Billings, has dozens of lambs that should have been gone by now. Pasture is available and so is water, but the supply is running low, she said.

"We have feed, but it's time to sell the lambs," said Beadle, whose Rambouillet and Targhee sheep usually are trucked to Public Auction Yards in Billings around this time of year.

Whether the quarantine will affect prices for the livestock later remains to be seen, she said.

Public Auction Yards held its weekly sheep sale Monday but will not hold another until the quarantine ends, spokesman Pat Goggins said. The number of sheep passing through the stockyard annually varies between 90,000 and 150,000, Goggins said.

The quarantine is unlikely to affect the domestic supply of lamb for dinner tables, Kott said. Once the quarantine ends, lambs likely will be shipped out of the state and the rate at which they grow can be controlled in anticipation of slaughter at the usual time this winter, he said.

Bluetongue is not always fatal, but sheep that survive it often remain impaired, Schmidt said. Symptoms of the disease in sheep include fever, sores, weakness and loss of wool.