Efforts to control brucellosis in cattle around Yellowstone National Park may be focusing on the wrong wildlife suspects, according to new DNA research on the disease.
The study suggests elk are the most likely source of brucellosis outbreaks in domestic cattle. That complicates the work of officials around Yellowstone charged with controlling the spread of brucellosis. Suspicion that bison were the main spreaders of the disease to cattle prompted extensive restrictions on bison trying to migrate out of the park into grazing lands of Montana, Idaho and Wyoming. Those restrictions have included hazing the herds back into the park, and hunting, butchering or quarantining thousands of bison that could not be driven back into Yellowstone.
“Of all the cases we had, we found no direct links from bison to livestock,” said Pauline Kamath, U.S. Geological Survey ecologist and lead author of the study. “That’s suggesting there’s little transmission from bison to animals in other areas in the Greater Yellowstone.”
Brucellosis causes infected females to abort their calves. Its presence in an area may require ranchers to quarantine their herds and incur expensive testing and vaccinations before the animals can be sold or moved. The Greater Yellowstone area is the last reservoir of the disease in North America, and about 20 private herds of cattle or bison have reported infections in Montana, Idaho and Wyoming since 2002.
The study was published in the journal Nature Communications on May 11. It traced 30 years of Brucella abortus’ genomic history in samples of cattle, bison and elk tissue. It found five distinct strains of the bacteria, all likely from historic cattle introductions. Four of those five strains are now mainly in elk herds that use Wyoming state winter feeding grounds.
“This study provides the most definitive evidence to date that brucellosis is now self-sustaining in Montana elk and has spread at an increased rate in elk populations outside of the feeding grounds,” Kamath said. While the bacteria strain most often found in bison has shown slow spread outside of Yellowstone, the elk strains have been radiating 2 to 4 miles a year.
“Any attempt to control the rate of spread in wildlife must be evaluated at the ecosystem scale and include an effective strategy to address infection in elk across the greater Yellowstone area,” said study co-author Rick Wallen, lead wildlife biologist for the bison program in Yellowstone National Park. “Focus on bison alone, as was suggested in the past, will not meet the disease eradication objective and conserve wildlife.”
Montana livestock and wildlife officials face big challenges in confronting brucellosis in elk. Although bison weigh two to four times as much as domestic cattle and are considered wildlife, they can be herded, fenced and managed similar to cattle. Elk, on the other hand, are allowed to roam free throughout the state. Because of elk’s popularity as a big-game animal, the state has extensive rules preventing landowners from interfering with elk populations.
The study found “a substantial increase in the documented transmission events from elk to livestock (17 incidents), standing in contrast to none recorded in the previous decade.” It also found that elk and bison pass the disease back and forth between species, complicating eradication efforts.
Changes in animal management also affected the spread of the disease. Brucellosis was originally detected in Yellowstone bison in 1917. A 1992 report by the Government Accountability Office found brucellosis present in just 12 percent of Yellowstone bison and no evidence of the disease in 151 elk studied. It noted that “The National Park Service and Montana wildlife officials are unaware of any documented cases of brucellosis transmission from wildlife, including bison and elk, to livestock in the wild.”
The study notes that the National Park Service stopped large-scale removals of bison and elk in the late 1960s from Yellowstone. The result was bison populations expanded from 400 to 3,500 and elk populations went from 3,000 to 18,000 between 1968 and 1990. Bison access to elk feeding grounds in Wyoming also resulted in increases in disease transfer.