A federal appeals court has refused to block grazing on a half-million acres of public land in Oregon, dismissing environmentalist claims that cattle cause irreparable harm to sage grouse.

The 9th U.S. Circuit Court of Appeals has held that an injunction against grazing was not warranted in the Louse Canyon area of southeast Oregon, which is overseen by the U.S. Bureau of Land Management.

Although cattle can pose some threats to the sage grouse, "the danger is site-specific and depends on a number of factors," so environmentalists failed to prove an injunction was necessary, the 9th Circuit said.

"One at least one rangeland, exclusion of livestock had no effect on the sage grouse population and some evidence demonstrated potential positive effects from grazing," the ruling said.

The 9th Circuit specifically rejected as the claim that puddles from cattle hoofs will serve as breeding grounds for mosquitoes that can transmit West Nile virus.

The Oregon Natural Desert Association argued that sage grouse face "clear and inescapable risk of exposure" to the disease due to grazing, but the 9th Circuit found the connection between cattle and the virus "too remote."

"An outbreak requires the presence of a complex set of conditions, but the record did not indicate that those conditions were likely to be present," the ruling said.

Last year, ONDA convinced a federal judge that BLM grazing plans for the area, which called for new fencing and water pipelines, violated environmental law.

While the judge overturned 2010 grazing permits for the area, he refused to stop grazing in Louse Canyon, ruling that the practice could proceed under previous permits issued in 2006.

ONDA appealed that decision, claiming that BLM could not revert back to the previous plans and allow grazing to resume this year. According to the group, the old permits were no longer valid.