Brucellosis Fever

Ranchers fear it–but do wild bison really give cattle the dread disease?


It’s no wonder livestock producers hate it. Brucellosis causes cows to abort their first calves; some will continue to suffer abortions, and later calves may be born unhealthy. If brucellosis were left unchecked in cattle, the expenses of lost calves and brucellosis testing could pose a serious threat to the livelihood of many American cattle ranchers and dairy farmers—APHIS estimates that annual beef and milk production costs could rise by $80 million a year in less than ten years if brucellosis-eradication efforts were halted.

But thanks to APHIS’ successful eradication program, there’s no longer any likelihood of brucellosis causing a livestock meltdown, which is perhaps why APHIS now is turning its attention to the Yellowstone bison. The trouble is, the agency’s contention that wild bison can give brucellosis to cattle is based on just one scientific study and a half-dozen anecdotal reports—all of which have been largely discredited.

Epidemiologists speculate that brucellosis came to the U.S. with European cattle in the 1800s, gradually spreading to American cattle and swine and eventually to wildlife populations, including bison and elk. The disease can also infect humans, usually via unpasteurized milk, and although it may cause a chronic, debilitating fever, it is infrequently fatal and is now considered rare. Fewer than 100 cases worldwide were reported in 1996, most of them contracted in foreign countries.

Since 1934, APHIS’ Cooperative State Federal Brucellosis Eradication Program has labored to eliminate the disease from domestic livestock. It’s been a big success: After a $3.5 billion campaign by private interests and state and federal authorities, APHIS expects the nation’s cattle herds to be entirely brucellosis-free by the end of this year. The agency has awarded all but seven states the coveted “brucellosis class-free status,” which allows them to ship livestock to other states without performing expensive testing. For its part, Montana claims to save $1-2 million a year by not having to test its cattle.

But there is extensive controversy over APHIS’ assumption that wild bison and elk can transmit brucellosis to livestock. Much of the evidence cited by livestock regulators is circumstantial, because almost no controlled research in wild populations has been conducted. These suppositions are based on conjecture about similarities between bison and cattle, or inferred from knowledge of the disease in farm animals, or largely anecdotal. The clinical, pathological, and population effects of brucellosis in wild bison are poorly understood due to a lack of research, and may differ significantly from cattle.

Ironically, experts believe Yellowstone’s bison originally caught brucellosis from cattle, perhaps by ingesting infected cow’s milk. In cattle, the disease is spread primarily by contact with infected birthing tissues or aborted fetuses. In bison, however, the very limited research available suggests that transmission may occur via contaminated milk, and indicates that infected Yellowstone bison have a much lower abortion rate than cattle.

So why does APHIS think cows will catch it from bison on the open range? APHIS bases its theory of bison-cattle brucellosis transmission mainly on a 1990 Texas A&M study, which the agency’s Patrick Collins calls “the key scientific initiative” establishing transmissibility between the species. The Texas research project—funded in part through a cooperative agreement with APHIS—confined infected bison and healthy cattle in artificial conditions; the cattle became infected and aborted, an outcome the agency claims demonstrates that “bison infected with [brucellosis] could spread the disease to cattle through contact.” However, in order to contract brucellosis from bison in the wild, cattle would have to come into direct contact with infected bison cows, their afterbirths, or their aborted tissues—a situation which is plausible in controlled conditions, but of unknown likelihood in the wild.

Independent brucellosis authorities dispute the study’s applicability to bison in a wild setting, among them Dr. Paul Nicoletti, a professor at the University of Florida’s College of Veterinary Medicine who spent 20 years with APHIS’ brucellosis eradication program. Nicoletti says the Texas A&M researchers “wrote the conclusions and then did the work,” noting that the proximity of the bison and cattle in the study did not reflect natural conditions and that Texas A&M researchers “overdosed” the bison with a highly virulent cattle strain of brucellosis.

The Texas study “tells you virtually nothing about the established risk in a field situation,” says Wayne Brewster, who supervised Yellowstone’s contribution to the DEIS.

While Nicoletti says that “No one questions that under some conditions bison can infect cattle” with brucellosis, he is convinced that “Yellowstone National Park bison have never been the source of brucellosis in the area.” The support for his conclusion, he says, is obvious: In 25 years of close surveillance, there has never been a documented case of interspecies transmission from the Yellowstone herd. Instead of scientific evidence, Nicoletti says, it is “APHIS’ policy of eradication that is driving the pressure for NPS to do something about brucellosis.”

APHIS also relies on a small group of Yellowstone-region epidemiological studies of alleged bison-to-cattle and elk-to-cattle transmissions that are based principally on circumstantial evidence. After reviewing the data, the National Research Council concluded that “wildlife cannot be determined to be the source of brucellosis infection in these six cases.”

While APHIS’ policies clearly have been successful in eradicating brucellosis in cattle, its approach to the disease in bison suggests that bad science is leading to bad policy.