Tactics analyzed in fighting remaining brucellosis infections

Published on January 18, 2011
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By Greg Cima

New federal rules for responding to brucellosis outbreaks are intended to increase flexibility and decrease impacts on livestock owners and government agencies.

An interim rule published Dec. 27 by the Department of Agriculture's Animal and Plant Health Inspection Service and effective that day will also reduce brucellosis control requirements for most states while allowing them to maintain the highest status and lowest level of restrictions related to the bacterial disease.

Lingering cases connected with wildlife exposure

The changes include the following provisions:

  • Allow states to reduce testing and keep their class-free status if they have had that status for five years and have no Brucella abortus infection in wildlife. States can maintain their status if affected herds are quarantined, plans are enacted to prevent disease spread, quarantined herds are tested as requested by the USDA, animals without negative test results are removed and destroyed, and states conduct adequate surveillance.
  • Eliminate automatic reclassification for states and areas on discovery of two herds with brucellosis within two years or single herds not depopulated within 60 days.
  • Reduce to six months the age for including sexually intact cattle and bison in herd blood tests.
  • Require brucellosis management plans for states with B abortus infection in wildlife that want to maintain class-free status.
  • Provide more testing options for dairy herds.

Class-free status is given to states with no known brucellosis cases in cattle for 12 months. All U.S. states and territories held that status simultaneously for the first time in February 2008.

The interim rule was enacted as APHIS officials continued development of new regulations for brucellosis and tuberculosis. The USDA published in September 2009 concept papers about regulations for each disease, and an APHIS Veterinary Services working group that first met in September 2010 is expected to provide input on the new joint regulations by spring 2011. The USDA intends to publish the regulations in 2011.

"Eradication depends on finding the last remaining brucellosis-reactor animal, the last remaining brucellosis-affected herd, and eliminating the disease from wildlife reservoirs," the 2009 brucellosis concept paper states. "All potential risks for exposure and transmission of brucellosis from infected wildlife populations must be mitigated and eliminated as well."

A Federal Register notice on the interim regulations indicates previous brucellosis regulations forced APHIS and states to expend scarce resources on regaining class-free status following reports of infections, rather than working on disease management and elimination. Livestock owners also endured additional costs as a result of testing and movement requirements, regardless of the disease risk connected with their herds. 

Progressing since 1934

When the Cooperative State-Federal Brucellosis Eradication Program began in 1934, about 11.5 percent of cattle tested for brucellosis had a positive reaction, USDA information states. About 124,000 U.S. herds were found to be infected in 1956, 700 were infected in 1992, and only six were reported in 2000.  

Disease reservoirs in wild bison and elk are the likely sources of the remaining sporadic infections in domestic ruminants, according to state and federal agriculture authorities. Wild elk infected with B abortus are the suspected sources of infections in Wyoming cattle and bison in the past year. Montana state authorities know brucellosis-positive elk had been on the grounds of a ranch where domestic bison were found to be infected during 2010, and the B abortus organism isolated from an infected bison was a genetic match for the organism isolated from an elk killed in 2009. Idaho's last known brucellosis infections were discovered in a cattle herd and reported in December 2009, but state officials could not be reached in time for this article.

"When you get a disease like this in wildlife that are free-ranging, it is very, very difficult to completely eradicate it unless the political and public will is there to take the steps that would be necessary to do it," Dr. Jim Logan, Wyoming state veterinarian, said. He has worked on brucellosis eradication about 20 years.

Wyoming, Montana, and Idaho share the Greater Yellowstone Area, which holds the nation's last known wildlife reservoirs of B abortus.

While APHIS has paid livestock owners for herds depopulated following the discovery of brucellosis-infected animals—an incentive given to encourage depopulation rather than quarantine—the agency now recommends removing infected animals. The change is intended to reduce costs, public outrage, environmental consequences of animal disposal, and loss of animal proteins, APHIS information states.

Feeding wildlife, controlling movement

Dr. Martin A. Zaluski, Montana state veterinarian, said decreasing the degree of brucellosis infection in wildlife will depend in part on wildlife management, particularly in changing practices that can cause wildlife to congregate. He cited winter feeding operations as an example of a state practice that needs to change, and he noted that the U.S. Animal Health Association this past November adopted a resolution that urges wildlife agencies in the Greater Yellowstone Area not to establish new public or private elk and bison feeding grounds and to consider phasing out existing winter feeding.

Wyoming has established feeding grounds and Idaho has an emergency feeding program used during weather emergencies, but Dr. Zaluski noted that Montana doesn't have a winter wildlife feeding program.

USAHA information states that evidence suggests supplemental winter feeding of elk and bison "creates abnormal animal densities and distributions" connected with increased transmission of diseases including brucellosis. That information notes that the now-inactive Greater Yellowstone Interagency Brucellosis Committee found that wildlife feeding was "contrary to effective disease elimination and control" and was opposed to establishing new winter feeding grounds.

Dr. Logan said Wyoming's state-operated elk feeding grounds were established in the 1920s in areas where deep snow can hinder wildlife movement for forage, and they are intended to prevent wildlife starvation or movement to private lands. They effectively separate many elk from cattle, but they provide excellent opportunities for brucellosis transmission among elk, he said.

Dr. Logan also said decreases in wildlife populations, along with disease testing and the removal or slaughter of infected wildlife, are among the practices needed to eliminate the wildlife reservoirs of brucellosis. He thinks hunting could be used to help control wild elk populations, but he noted that some landowners in the Yellowstone area give elk safe havens from hunters, sometimes on land containing or near livestock. 

New vaccines needed  

Dr. Dave Hunter, veterinarian for Turner Enterprises Inc. and Turner Endangered Species Fund, said current brucellosis vaccines don't effectively protect domestic cattle and bison against infection or abortion. He thinks federal authorities should remove B abortus from the list of select agents—a designation that restricts access to certain biological agents and toxins—and allow more research on developing a vaccine that could better protect livestock and effectively reduce concerns about wildlife.

"If we had that good vaccine for livestock that protected them against infection, we wouldn't care if the bison and elk in Yellowstone carried brucellosis for the next generations," Dr. Hunter said. "Brucellosis is not a population-limiting disease in bison or elk."

In fall 2010, a pregnant bison on a Turner ranch in Montana was found to be infected with brucellosis through contact with an infected elk, Dr. Hunter said. The infection was found during annual testing of the breeding herd.

Two other suspect animals were negative for the disease, but all three bison were removed from the herd. Bison calves on Turner ranches are vaccinated against brucellosis, but the vaccine is less efficacious against abortion in bison than it is against abortion in cattle and largely ineffective against infection, he said.

Among the 55,000-plus bison on Turner ranches, about 8,000 are in areas close to infected elk.

Dr. Zaluski thinks a highly effective livestock-use vaccine or an effective wildlife-use vaccine paired with a delivery system could end interspecies transmission of brucellosis. Like Dr. Hunter, he thinks better vaccines could be available if more laboratories were able to work with the B abortus agent.

"Brucella abortus is designated as a select agent by federal agencies, and that makes it very difficult to make progress on the science of brucellosis just because of the requirements for facilities, paperwork, laboratory documentation, etc.," Dr. Zaluski said.  

Studies ongoing

Dr. Zaluski said brucellosis transmission from wildlife to livestock appears to have increased, despite eradication efforts. Wildlife officials in his state announced Dec. 3 that it appeared elk were increasingly exposed to brucellosis, and a federally funded, five-year, $1.5 million study would examine elk exposure to the disease. About 8,000 elk have been tested for brucellosis exposure over the past 30 years, and exposure rates have risen from between zero percent and 2 percent in tests performed during the early 1990s to between 5 percent and 16 percent in recent surveys. 

"We want to determine if a cow elk that aborts in one year continues to do so, or if, as some suspect, it only occurs with the first pregnancy," Ken McDonald, the state agency's wildlife bureau chief, said in an announcement. "We also want to see if elk that test positive in one year continue to do so."

Testing through the study is intended to both indicate disease prevalence and evaluate testing methods, the release states.