BILLINGS, Mont. — Montana officials said Monday that a cow has tested positive for brucellosis — a serious livestock disease that had been declared nonexistent in U.S. cattle earlier this year.

The infection means Montana ranchers must undergo an expensive testing program before shipping cattle out of state and will have to increase vaccinations. The state will lose its federal brucellosis-free status until it can demonstrate it has the disease under control.

Brucellosis is a disease carried by wild animals including bison and elk in the greater Yellowstone area. It can cause pregnant cows to abort their calves but transmission to humans is rare.

Consuming animals with the disease is not considered dangerous, state officials said.

In February, the U.S. Department of Agriculture declared all 50 states to be brucellosis-free in their commercial herds — the first time that had happened in 74 years.

States lose their brucellosis-free status if the disease is found in more than one herd of cattle in two years. Federal agriculture officials discovered the disease in another herd in May 2007.

The recent case was discovered late last month in the Paradise Valley, north of Yellowstone National Park, during testing of a small herd of cattle. Confirmation came Monday, following a second positive test conducted at a federal lab in Iowa.

"At this point, this is the only animal we have" with the disease, said state Veterinarian Marty Zaluski. "We don't think the disease is widespread in that herd."