Ten of 14 bighorn sheep darted earlier this month tested positive for bacteria that may signal an impending rash of fatal pneumonia.

The pathogen, Mycoplasma ovipneumoniae, has been linked to bighorn die-offs across the West. Wyoming Game and Fish Department biologists took samples from the Jackson bighorn herd this winter in part because of higher-than expected mortality. Six of 20 radio-collared sheep in the Jackson bighorn herd have died since summer, Game and Fish wildlife biologist Doug Brimeyer said.

A 10 percent rate of mortality is more typical, he said.

“It doesn’t look very good,” Brimeyer said. “We’re just going to have to keep monitoring the population over the course of the winter.”

GPS data showed the most recent dead sheep, found Thursday, died in inaccessible terrain near the Sleeping Indian. While the location will make testing the specimens impossible, the majority of the dead collared sheep this year have tested positive for ovipneumoniae.

More encouragingly, the 14 bighorn Game and Fish tested all were healthy specimens with good fat reserves. All the bighorn sheep also tested negative for Mannheimia haemolytica, a more dangerous pathogen, Brimeyer said.

Wildlife biologists are still trying to sort out the relationship between ovipneumoniae and fatal pneumonia.

“We don’t have an estimate for the average contraction rate, because there was no sampling between 2002 and 2010,” Brimeyer said of ovipneumoniae contraction in Jackson herd bighorns. “It’s possible that 70 percent is what they normally have, but there’s not enough known about it to say.”

In 2002, the Jackson bighorn sheep herd was cut in half by a pneumonia outbreak. The Jackson Hole bighorn population took years to recover.

If an epidemic strikes the herd again, there’s little Game and Fish biologists will be able to do, Brimeyer said.

“We believe the population is at risk of developing a pneumonia outbreak,” he said. “The main thing for us to do is to keep documenting what the cause-specific agents are.”