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Brucellosis monitoring in Montana’s wild elk

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By Jessianne Wright EBS Contributor

BOZEMAN – The Greater Yellowstone is home to tens of thousands of elk, and their range extends from the far reaches of Wyoming and Idaho, to the core of Yellowstone National Park. While known for its rich wildlife ecosystem, the Greater Yellowstone is also the only place in the U.S. where the highly contagious brucellosis infection continues to thrive.

This bacterial disease is localized in the reproductive track of female cattle, elk and bison and can induce abortion in pregnant animals. Though no longer common in the U.S. due to pasteurization, brucellosis can also be transmitted to humans, causing fever, joint pain and fatigue.

Wildlife officials continue to monitor the impact of brucellosis on the region’s elk population and earlier this spring, Montana Fish, Wildlife and Park biologists completed a series of annual elk captures in partnership with the Montana Department of Livestock in order to better understand the disease.

“There’s just a lot of unknowns with how the disease is circulating,” said Kelly Proffitt, FWP wildlife biologist and one of the lead researchers on the brucellosis study.

Up until about the year 2000, she said, scientists attributed brucellosis transmission to feeding grounds in Wyoming and exchange with bison. “But in the last 10 years, we’ve seen that it’s circulating in free-roaming elk. Current management of bison has really taken them out of the equation.

“The primary mode of transmission between elk and cattle is an abortion event,” she said, adding that an infected cow elk will expel a big load of viable Brucella abortus—the bacteria that causes brucellosis—at the birth site. Domestic livestock could then sniff and lick the area and eat the grass, introducing the bacteria into their system.

Following exposure, a pregnant female will develop an infection and likely abort her pregnancy that year. Because her body has mounted an immune response to the disease and antibodies are present in her blood, she will likely test positive for the presence of the disease well into the future.

“For the most part, most of the seropositive animals—[those who test positive for the presence of brucellosis]—go on to have healthy births [following the first abortion],” Proffitt said. However, it’s possible seropositive animals could continue to shed Brucella even during normal births, and some infected elk have gone on to never calve again.

“Working with wildlife is really difficult. Like everything else, elk are unpredictable … the common pattern is not always followed,” she said, adding that “pregnancy rates aren’t what drives elk populations, so it’s a pretty low consequence for elk, but of really high consequence for livestock.”

Within the Montana portion of the Greater Yellowstone is an identified area where brucellosis is common and cattle producers are required to vaccinate their livestock and test them prior to transport.

Positive test results in this designated surveillance area could have implications for mandatory testing and shipping holds statewide. Delayed or canceled shipments could see producers lose millions of dollars, said Eric Liska, brucellosis program veterinarian for the Department of Livestock. In 2008, he added, producers lost $12.3 million because two cattle herds tested positive.

Beginning in 2011, FWP and the Department of Livestock have worked to identify high-risk populations of elk, which are then monitored for five years. Initially, 100 females are captured using nets shot from a helicopter. They are tested in the field for brucellosis and pregnancy and if positive, the cows receive a GPS collar.

In the years following, these elk are recaptured and tested once again. Those who are pregnant receive a vaginal implant transmitter that allows researchers to track the location of an abortion or birth. Between March and June, technicians gather samples from the afterbirth to determine if the female is shedding Brucella.

Following the five study years, the elk are euthanized and taken to the Montana Veterinary Diagnostic Laboratory for biological sampling, though Proffitt said cows are often harvested by hunters or die of natural causes before the five years are up.

“They need the actual tissues from a dead animal,” she said, adding that about 30 samples are taken from various lymph nodes throughout the body. “That’s the real determination of whether that animal was infectious or not.”

Within the Greater Yellowstone, Mill Creek south of Livingston is one of several very high-risk areas where the disease presence is high and interactions with livestock are very likely. This spring, seven pregnant elk will be monitored. A single seropositive elk in the Madison Valley north of Ennis is being tracked through her pregnancy as well.

Liska said that this study has been incredibly helpful, as segregation of cattle and livestock is the best way to prevent brucellosis transmission. “Timely separation from potentially infected wildlife is ideal. So we need to know who’s contagious, where they are, and what livestock are at risk,” he added. “Unfortunately, livestock are completely at the will of wildlife.”

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