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Brucellosis reduces pregnancy rates in greater Yellowstone elk

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Image of antlerless elk (Cervus canadensis) at a winter feedground in Wyoming. Credit: Mark Gocke/Wyoming Game and Fish Department
Results signal that brucellosis has previously hidden consequences for wild elk population.

For decades, researchers have known that brucellosis in elk, bison and cattle in the greater Yellowstone ecosystem causes periodic abortions in these animals and that the disease poses a financial concern for dairy producers and cattle ranchers, but its effects on the wild elk population have generally been considered minor, according to an announcement from Utah State University.

In recent years, however, elk pregnancy rates have become the subject of controversy, with various researchers claiming that stress caused by fear of wolves and nutritional deficiencies caused by drought can explain low pregnancy rates in specific elk herds. Until now, though, the effects of brucellosis on elk pregnancy rates have not been scrutinized, Utah State said.

The Greater Yellowstone Ecosystem encompasses a vast landscape including portions of Montana, Idaho and Wyoming. Brucellosis in the region has long been contentious. Recently, Montana adopted new rules on vaccinating and testing cattle in its designated surveillance area (DSA), while Wyoming reported a new case in a cattle herd in its DSA.

A 2017 report from the National Research Council lays out brucellosis control options for the Greater Yellowstone Ecosystem and linked the transmission of the disease to elk instead of bison. Brucellosis is endemic in elk and bison in the Greater Yellowstone Ecosystem, which is the last known reservoir of the Brucella abortus pathogen in the U.S.

Utah State researchers Gavin Cotterill and Johan du Toit reported that by midwinter, elk that test positive for brucellosis are less likely to be pregnant than healthy elk, independent from the abortions caused by that disease later in the year.

Cotterill and du Toit, along with colleagues from the Wyoming Game & Fish Department, the U.S. Geological Survey Northern Rocky Mountain Science Center and the University of California-Berkeley, discussed their findings in an article published Oct. 28 in Ecology & Evolution.

"Midwinter elk pregnancy rates are often seen as an indicator of an elk herd's health and viability," said Cotterill, a doctoral candidate at Utah State and lead author of the paper. "If we're interested in figuring out the effect that predators or climate are having on elk, we need to also account for disease."

The researchers analyzed pregnancy and disease data collected over the last 20 years at Wyoming's supplemental winter feeding grounds for elk and ran additional pregnancy tests on stored blood samples.

"We found that the disease causes a substantial decline in the probability of pregnancy among young adult elk, and the effect is weaker in older animals, but it's still unclear what the mechanism is that's causing this to happen," du Toit noted.

The researchers said the results are not a cause for alarm but are a signal that brucellosis has previously hidden consequences for the wild elk population. Quantifying this effect is important, particularly as brucellosis is spreading through the elk population, so the effects of other factors influencing elk reproduction may be estimated more accurately, they said.

"Elk numbers in most of the region are high, and we don't expect that to change because of brucellosis," Cotterill said. "It's one more factor that researchers and managers need to keep in mind moving forward."

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