Eight cows in Sheridan County, Mont., died from anthrax last week, and state livestock officials said more cases could follow.

The dead cows were part of a single herd on a ranch near Raymond, which is between Plentywood and the Canadian border, Montana Department of Livestock spokeswoman Lisa Schmidt said. They were buried 10 feet underground in an effort to keep the disease from spreading further, she said.

Field tests indicated it was naturally occurring anthrax, a disease caused by a spore-forming bacterium found in some soil types. Spores can live in soil for decades, and livestock–such as cattle, horses, sheep, and goats–can get infected after inhaling or ingesting spores during grazing. The spores often become active during changes in climate, especially in very wet or very dry conditions.

The Raymond-area cases were confirmed Friday, Schmidt said.

Anthrax cases also have been reported this year in North Dakota and South Dakota, indicating conditions are ripe for the spores to become active, livestock officials said.

“When an outbreak is confirmed in a location, it is reasonable to expect additional cases within the same area,” Schmidt said in a news release.

Jeanne Rankin, DVM, Montana’s acting state veterinarian, urged livestock owners to contact their veterinarians immediately if they notice livestock dying sudden, unexplained deaths.

A vaccine for horses is available. Vaccination is recommended for livestock in or near an outbreak area, state officials said.

The last cases of naturally occurring anthrax in Montana were in 2005 in Roosevelt County, when more than 35 cows died. Separate outbreaks were reported in Yellowstone County and McCone County in 1999.