Green River -- Brucellosis appears to be increasing in several elk herds in northwestern Wyoming, according to a new U.S. Geological Survey study.

The study's data suggests that enhanced elk-to-elk transmission in free-ranging populations may be occurring because of denser elk clusters during winter.

The study also said that elk populations inside and outside of the Greater Yellowstone area that traditionally did not maintain brucellosis may now be at risk because of population increases.

The new study on brucellosis -- a bacterial infection of cattle, elk and bison -- appeared in the March publication of Ecological Applications, said USGS spokeswoman Suzanna Carrithers.

The magazine is a publication of the Ecological Society of America.

Brucellosis causes miscarriages in cattle. The disease is present in many elk and bison in the greater Yellowstone region, known as the last reservoir for brucellosis in the United States. The disease can be transmitted from wildlife to cattle.

The presence of the disease within livestock herds has resulted in additional testing requirements and trade restrictions in past years in both Wyoming and Montana.

Both states have had to institute strict testing and quarantine procedures to regain federal brucellosis-free status.

Paul Cross, USGS disease ecologist and lead author of the study, said elk-to-elk transmission of brucellosis may be increasing in new regions around Yellowstone.

He noted several cattle herds have been infected in Wyoming, Idaho and Montana since 2004.

Cross said recent cases of brucellosis in cattle are thought to have come from elk considering the lack of contact between bison and cattle.

Positive brucellosis tests in free-ranging elk increased from 0-7 percent in 1991-92, according to the study. The rate went to from 8-20 percent in 2006-07 in four of six herd units around Yellowstone.

The study said biologists have known that brucellosis in the greater Yellowstone area is being sustained by "abnormal densities" and restricted distribution of elk that congregate on the 20-plus feedgrounds in western Wyoming during the winter.

Bison populations also maintain brucellosis irrespective of population size, the study said.

More elk

Cross said the new research shows that brucellosis may also be increasing in some elk populations that are distant from supplemental feeding grounds and bison herds.

He said researchers looked at a number of hypotheses for why increases in brucellosis in the region were observed.

"Two seemed most probable," said Cross.

"Either brucellosis transmission among elk is becoming more frequent as elk densities increase, or the diagnostic tests are cross-reacting with another pathogen that is increasing in prevalence," he continued.

The authors noted that some elk populations were five to nine times larger in 2007 than they were in the 1970s. Elk have tended in recent years to take refuge for prolonged periods on lands with limited or no hunting, which creates a situation similar to feedgrounds.

Some elk groups outside the region are as large as those on the supplemental feeding grounds in Wyoming, the study said.

"Addressing the unintended consequences of these increasing populations is complicated by limited hunter access to private lands, which places many ungulate populations out of administrative control," Cross said.

"Agency/landowner hunting access partnerships and the protection of large predators are two management strategies that may be used to target high ungulate densities in private refuges to reduce the current and future burden of the disease," he added.

The study noted the available brucellosis tests indicate whether an individual has produced circulating antibodies to the brucella infections, but not whether they are currently infected with the disease.

Outbreaks of the disease in domestic cattle can be very expensive to the ranching industry.

Although other free-ranging animals such as elk can carry the disease, ranchers have long thought that bison play the major factor in the spread of brucellosis in the region.

The brucellosis research was conducted at the USGS's Northern Rocky Mountain Science Center in partnership with the Wyoming Game and Fish Department; Montana Fish, Wildlife and Parks, U.S. Fish and Wildlife Service, National Park Service, Princeton University, and the universities of Wyoming and Montana.