JACKSON — A dart loaded with tranquilizer makes a “whap” as it connects with the haunches of an unsuspecting bighorn sheep near the base of Miller Butte.

The sheep, a 3-year-old ewe, gets wobbly within a few minutes. She fades into a sleep and biologists Doug Brimeyer and Jessica Jennings-Gaines approach.

The ewe is drooling as Brimeyer and Jennings-Gaines fit a hood over her eyes and spread her jaws. “Say ‘ahh,'” Brimeyer says as he holds the animal steady.

Jennings-Gaines picks unchewed grass off the ewe’s tongue before taking saliva samples from her tonsils and gums.

At this site on the National Elk Refuge, the Wyoming Game and Fish Department employees are testing for a strain of bacteria that’s been linked to bighorn die-offs across the West. It will be several weeks before biologists ascertain if sheep E638 — the number on her new ear tag — carries the pathogen Mycoplasma ovipneumoniae, which might leave her more susceptible to fatal pneumonia.

“We get the parasite load from the feces,” Brimeyer said. Seconds earlier he had successfully mined the sheep’s anus with a gloved finger for pellet-shaped excrement.

They injected her with antibiotics and some vaccinations, then a tranquilizer antidote. Sixteen minutes after Brimeyer stuck the sheep with the dart from 45 yards, the biologists were finished.

The ewe, still trying to find her legs, ran off.

Five out of 20 of the radio-collared sheep in the Jackson bighorn sheep herd have perished this year. At least three have carried ovipneumoniae and showed signs of pneumonia. Two uncollared sheep have also been found dead carrying the pathogen.

Brimeyer carried samples from the last dead collared sheep, mostly consumed by coyotes, out of the Gros Ventres on Monday and is still awaiting test results.

While it’s not yet determined if the 25 percent mortality is indicative of an outbreak of pneumonia, it’s out of the ordinary.

We’re still on alert with what’s going on with the sheep,” Brimeyer said. “Survival should be closer to 90 percent on these adult ewes.”

In 2002, the Jackson bighorn sheep herd was cut in half by a pneumonia outbreak. It didn’t fully recover for years.

“By the end of the spring we had sheep laying dead all over the place,” Brimeyer said of the 2002 die-off. “Think of losing 50 to 60 percent of the population. We saw latent effects of that for three years afterward.”

The Jackson herd has since bounced back, both in terms of reproductive success and, as a result, population, Brimeyer said. Nearly 500 were counted this spring, equal to the department’s objective for the herd.

Other bighorn herds that have endured pneumonia outbreaks have had it even worse, Game and Fish disease specialist Hank Edwards said.

“The Whiskey Basin herd suffered an outbreak 20 some years ago and never recovered,” Edwards said of the Dubois-area sheep herd. “It can be very devastating.”

The 14 sheep Brimeyer, Jennings-Gaines, Edwards and fellow Game and Fish biologists Gary Fralich and Aly Courtemanch tested last week from the Jackson herd will help determine the contraction rate of the pathogen. It’s part of testing taking place all around Wyoming.

Besides the Whiskey Basin herd, sheep near Cody and at several other locations across the state have tested positive for ovipneumoniae, Edwards said.

The disease specialist was hesitant to definitively tie the bacteria to fatal pneumonia in bighorns, saying there’s an active debate about the cause and effect.

“One of the things we’re trying to do is get a better handle on how pathogenic” the bacteria is, Edwards said. “There’s lots of causes of pneumonia. There’s viral causes, there’s bacterial causes.”

Test methods were different at the time, but during the Jackson herd’s 2002 outbreak, Game and Fish did not find Mycoplasma ovipneumoniae, Edwards said. Another bacterial pathogen, Mannheimia haemolytica, which also weakens sheep’s immune systems, was discovered a decade ago.

The provenance of fatal strains of pneumonia that have popped up in the Jackson sheep herd is unknown, Edwards said.

“We would love to be able to figure that out,” he said.

The Teton Range bighorn sheep herd, which numbers only about 100 and has low genetic variability, has thus far tested free of the pathogens, Edwards said.

Domestic sheep are known to carry strains of pathogens linked to pneumonia that are fatal to wild sheep. Although any link is unconfirmed, domestic sheep that graze on Bridger-Teton lands to the south near Pinedale could be a source for the Jackson herd, Brimeyer said.

Different sectors of the Jackson sheep herd could also be contracting the pathogens from one another by intermingling, Brimeyer said.

Last winter, department biologists fitted GPS collars on animals wintering in five areas: at Miller Butte, in the Hoback Canyon and in the Gray Hills, Red Hills and Slide Lake areas in the Gros Ventre drainage. Data from the collars showed that some sheep are venturing far during the summer. Sheep collared on the National Elk Refuge, for example, migrated to a high-elevation plateau on the opposite side of Sleeping Indian.

“All those sheep mixing together have the potential to give that bacteria to each other during the summer,” Brimeyer said.

Information from the collars also will help Game and Fish determine the potential scope of a pneumonia outbreak, were it to occur this winter or spring.

After one successful darting last week, Brimeyer and Fralich found themselves atop Miller Butte glassing a darted ewe. She was several hundred yards away but not showing any signs of being tranquilized.

“I don’t know if we’re going to catch up with her,” Brimeyer said to his fellow biologist. “Maybe it hit up where the bones meet.”

Had she not slipped away, the sheep would have provided the 15th sample from the Jackson herd.

Little is truly known about the bacteria-pneumonia relationship Edwards said.

“From the collared sheep with ovipneumoniae that died, yes, some of those did show signs of pneumonia,” the disease specialist said of the dead Jackson herd specimens.

“On the other side, we have sampled many healthy sheep with ovipneumoniae.”

It is “pretty obvious” Mycoplasma ovipneumoniae is a significant pathogen, he said. But the bacteria need another agent to pose a danger to an animal. The agent could be another pathogenic bacteria, adverse environmental conditions or perhaps poor general health of the animal, he said.

“Something must work in concert to make it a significant pathogen,” Edwards said. “It could very well be that ovipneumoniae is a pathogen that doesn’t cause any problems but also happens to be everywhere.”