In May, Idaho Fish and Game reported a rare outbreak of the plague among southern Idaho ground squirrels southwest of Boise, after Boise State researchers and the Idaho Army National Guard noticed and reported striking changes in populations they were monitoring. Now researchers are hoping that continued monitoring of the ground squirrels will yield valuable insights about how the disease spreads to predators and even the role raptors — who are considered non-susceptible to the disease — may play in its transmission.
“Our collaborators with the Idaho Army National Guard have been monitoring this population for years, so we’ll be able to document changes in both raptors and plague species,” explained associate professor of biological sciences Jennifer Forbey.
“This is an opportunity for research on something that is really novel,” added biological sciences professor Jim Belthoff. “This is the perfect opportunity to examine what a plague epizootic is like on a variety of prey raptor species.”
Plague outbreaks among wildlife are rare but not unheard of in Idaho. The last plague outbreak was in the early 90s, explains Zoe Tinkle, a Boise State graduate student who’s been trapping and monitoring ground squirrels in the National Guard’s Orchard Combat Training area since 2013.
Plague is caused by a bacterium called yersinia pestis, which is passed via flea bites. The disease also can be transmitted via contact with infected animals, like Idaho’s ground squirrels, which is why Forbey’s students are trained to only handle wild animals with protective equipment when out in the field.
“We essentially assume that every animal we touch, work with, in any habitat, contains diseases. Plague or not, we operate as if it’s out there,” she said.
Forbey’s team of graduate and undergraduate students, lead by Tinkle, monitor the ground squirrels, which are a major prey species for raptors, in conjunction with the National Guard. Adult ground squirrels emerge from hibernation in late January or early February and remain above ground for four to five months, feeding and reproducing. In late June or early July they return to their dens. Forbey says her students began noticing changes in the population in April of this year. “We were seeing animals moving slowly, things were just a bit weird.”
“It wasn’t until very early May that we were beginning to see adult carcasses on the ground, which is very strange because they actually eat their own,” Tinkle added. Her team’s video research has shown that even roadkill carcasses are consumed within a day. “To actually see a carcass on the ground is very rare.”
Tinkle’s team coordinated with the National Guard to collect carcasses, which were then turned over to Fish and Game for testing. She says that it came back positive for plague.
“We know when the plague started, now we can understand its consequences by comparing this year’s data to our pre-plague data,” Forbey said. The effect of the ground squirrel die-off on raptors will probably be indirect — e.g. birds searching for alternative forms of prey rather than birds getting sick off dead and dying squirrels, as they’re considered non-susceptible to the disease.
Tinkle’s research has been focused on the behavior of ground squirrels and how their personalities — timid versus bold, for example — may affect everything from their overall lifespan to how they forage for food. Next year, she says, her research will shift slightly in light of this year’s outbreak. “I’m interested in exploring whether behavioral tactics make them more or less susceptible to contracting the disease,” she said.
And there’s still much to learn about the role raptors play in an outbreak. Belthoff said that scientists have long speculated on the role of birds of prey have in transmitting plague across the landscape — for instance by capturing an infected prairie dog and transferring it somewhere else before consuming it.
“The fleas from those infected animals could potentially jump on one of those raptors,” he explained. “Now if one of those fleas were to bite a raptor, nobody really knows what the response of those birds would be; the inoculation studies haven’t been done on that response.”
Belthoff’s main area of study is the burrowing owl. The raptor is unique because it normally has several species of fleas living on it whereas most birds of prey do not. With collaborators like the Idaho Bureau of Labs, in 2012 Belthoff began studying the fleas on burrowing owls to try and understand what role, if any, burrowing owls play in spreading the plague. Belthoff captured burrowing owls in five different states where plague was endemic — South Dakota, Washington, Idaho, Oregon, and Colorado — and examined the owls for infected fleas and plague antibodies. However, there were no active plague outbreaks in any of the five states when Belthoff was conducting his study.
“All our test results were negative to plague — in antibodies, in the fleas and in the blood of the owls,” he explained.
The current outbreak represents an incredible research opportunity for scientists like Forbey and Belthoff.
Belthoff theorizes that there’s an advantage to having burrowing owls in an active plague system because any fleas that bite burrowing owls will not bite other susceptible hosts. In other words, burrowing owls could take infected fleas out of circulation, thereby slowing the force of infection because they’re feeding on hosts that are not susceptible to contracting the plague (versus hosts that are highly susceptible, like rodents).
“Our study needs to be replicated during an active die off,” Belthoff said. “It’s not common for that to happen here in large numbers. I want to look again, while there’s a die-off, and see if ground squirrels infected with fleas transfer those fleas to the burrowing owls.”
BY: CIENNA MADRID PUBLISHED 12:52 PM / JUNE 9, 2015