Researchers target disease-carrying bighorn sheep in Idaho
LEWISTON, Idaho — Faint beeps picked up by a transceiver alert Frances Cassirer to the nearby presence of bighorn sheep, but the animals are momentarily invisible.
Their tawny bodies and white rumps are a perfect match for the conditions in the Asotin Creek canyon. The mix of patchy snow, dry grass, sage brush and basalt outcroppings make the animals melt into the landscape.
“It’s the Bev Doolittle effect,” she said, referencing the artist known for landscape paintings sprinkled with animals that are, at first glance, hidden by their markings.
The Idaho Department of Fish and Game wildlife biologist and bighorn sheep researcher lifts a set of binoculars to her eyes, scans a hillside and quickly finds the camouflaged animals wandering up the canyon. The 11 sheep — two rams, three lambs and six radio-collared ewes — are part of the Asotin band that Cassirer is closely monitoring.
State wildlife agencies from Idaho, Washington and Oregon work cooperatively to manage sheep herds in and around Hells Canyon. That includes putting collars on some of the animals. The collars transmit tracking signals, allowing biologists with receivers to find them.
The Asotin herd was once an anomaly. For years, its members remained healthy and free of the pneumonia that has stricken so many bighorn sheep herds in the region and elsewhere across the Western United States. But that changed in 2012. The pathogen was introduced to the herd and as many as a third of its members died.
Cassirer, who has spent her career at the forefront of research attempting to understand the disease that is passed to wild sheep from their domestic cousins, is leading a study to see if intervention from wildlife managers can shorten the time it takes for bighorn populations to recover from initial infections.
The study revolves around identifying and removing the bighorn equivalents of Mary Mallon. The cook, better known as Typhoid Mary, was quarantined by New York health officials in the early 1900s after she was identified as a carrier of typhoid fever.
Cassirer said when pneumonia first hits a bighorn herd, it is passed from sheep to sheep, often killing many of them. Those that survive become immune, but the disease doesn’t go away. Years following an initial outbreak, it often continues to hammer away at herds by infecting lambs.
“We think once it gets into a population it’s the females that keep it there. We think the ewes get some immunity and the lambs don’t,” she said. “The (ewes) that don’t die, they not only get immunity, they get rid of the pathogen. But some are like Typhoid Mary; they get immunity but they carry the pathogen.”
A carrier can infect her lamb, which then has the ability through play to pass the sickness to other lambs.
“It’s like a sick kid in day care,” she said.
She said the herds don’t completely recover until the carriers die off.
Cassirer is trying to determine if wildlife managers can shorten the recovery time by taking the same tack that New York health officials did with Mallon 100 years ago — removing the carriers from the rest of the population.
They repeatedly tested the ewes in the Asotin herd to determine which ones, known as super shedders or chronic carriers, still had the pathogen. Two that tested positive in 2013 were removed.
So far, the strategy is showing promise. The herd appears to be healthy at the moment.
“We’ve had good lamb survival,” she said. “We haven’t had any evidence the pathogen is here (since 2014).”
The study includes control herds and there is a companion captive study at South Dakota State University. There, healthy sheep are exposed to super shedders or chronic carriers to see if they get sick. They do.
The results from control herds where there has been no intervention are a bit confounding, though. The Lostine herd in northeastern Oregon continues to suffer from the disease. But another control group, the Black Butte Herd that lives along the Grande Ronde River near Boggan’s Oasis and Troy, Oregon, “is doing really well even though we didn’t do anything to them,” she said.
Even if intervention proves to be effective, Cassirer said it’s not something that can be done everywhere. The process requires intensive management and ready access to wild sheep. It works at Asotin Creek because the sheep are easily reached. It would be much more difficult and expensive to implement in more remote areas like the Hells Canyon or Frank Church River of No Return wilderness areas.