Brucellosis takes toll on Greater Yellowstone elk

A study of elk on winter feeding grounds in Wyoming suggests that the likelihood that 2-year-old cows exposed to brucellosis will get pregnant is less than those animals that don’t carry the bacteria.

Although increased predation by grizzly bears, cougars and wolves along with drought have been blamed for the decline in the Greater Yellowstone Ecosystem’s elk populations, a recent study said scientists have overlooked the effect the disease brucellosis is having on elk reproduction.

“If we’re interested in figuring out the effect that predators or climate are having on elk we need to also account for disease,” said Gavin Cotterill, a Utah State University researcher, in a news release.

According to a study co-authored by Cotterill, in an average year exposure to brucellosis reduced pregnancy rates of 2-year-old elk captured in mid- and late winter an average of 40 percent, compared to only 9 percent for 2-year-old elk not infected with the bacteria.

“Mid-winter elk pregnancy rates are often seen as an indicator of an elk herd’s health and viability,” Cotterill said.

Elk ages 3 to 9 years old were only “7 percentage points less likely to be pregnant” than elk of the same age that weren’t exposed to brucellosis.

“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,” said Johan du Toit, a USU researcher and study co-author, in a press release.

Feedground elk

The data is based on blood samples taken from more than 6,000 elk captures since 1990 on Wyoming elk feedgrounds. More than 1,000 of the records included the elk’s age and pregnancy status.

Taken all together, the scientists said brucellosis “reduces the reproductive output of exposed female elk by 24 percent, which affects population dynamics to a similar extent as severe winters or drought.

“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.”

Disease spread

Brucellosis was brought to the GYE by infected cattle. The disease then jumped to bison and now elk. In humans the bacteria causes undulant fever. Brucellosis can cause infected animals to abort. The disease is believed to be spread through contact with infected birthing material.

Montana officials have limited Yellowstone bison to wander only into specific zones outside the park’s north and west entrances in the winter out of fear that they may spread brucellosis to cattle in the region. Those that range farther are hazed or killed.

No such restriction is placed on elk, since their travels are more difficult to control. Instead, the state has created designated surveillance zones around Yellowstone where the rate of brucellosis is monitored and managers attempt to keep elk and cattle separated on the landscape through hazing and kill permits.


Montana fish and wildlife officials have long called for Wyoming to eliminate its 23 winter elk feedgrounds to reduce the chances of disease transmission, which now includes chronic wasting disease, which is spread by contact with an infected animal’s urine, feces and saliva.

If GYE elk infected with brucellosis also contract CWD, “could selection for resistance to one disease disrupt selection for resistance to the other?” the researchers asked.

The scientists also questioned whether elimination of the Wyoming feedgrounds would have the desired results of reducing disease prevalence, since winter feeding of elk makes those animals healthier, “offsetting disease costs.” They further noted that because feedgrounds have made studying animals easier — facilitating “handling and sampling opportunities” — to understand the full effects of brucellosis similar testing of animals not on feedgrounds is necessary to see what the differences are between the herds.

Cotterill and du Toit, along with colleagues from the Wyoming Game and Fish Department, U.S. Geological Survey Northern Rocky Mountain Science Center, and University of California Berkeley, discuss their findings in the Oct. 28 edition of “Ecology and Evolution.”

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