Wyoming sees spike in brucellosis cases

Dec 10, 2010

Brucellosis, an infectious bacterial disease which commonly leads to abortion in livestock and wildlife, may be on the rise in northwest Wyoming. Animal health officials in that state confirmed infections in two separate herds in Park County in recent weeks, and have identified a possible third outbreak in nearby Sublette County.

Once a widespread disease risk, mandated vaccinations and aggressive management of outbreaks have effectively eradicated the infection throughout the majority of the nation. The last known reservoir for the disease is in the region surrounding Yellowstone National Park, where it is endemic in wild elk and free-roaming bison populations, both of which are capable of transmitting the disease to area livestock. For ranchers within the Greater Yellowstone Area (GYA), the risks are well understood, and the occasional outbreak, while it can be devastating, is not entirely uncommon. However, with one case each in Idaho and Montana since December of 2009, confirmation of a third case in Wyoming would bring the total count of cases in the three-state GYA to five within the last year, a number that has producers and health officials wondering whether prevalence of the disease may be on the rise.

For Assistant State Veterinarian Dr. Bob Meyer, the first indication came in late October in the form of three positive brucellosis blood tests on cattle coming through the sale barn in Worland. Under Wyoming’s brucellosis rules, the area around Yellowstone is labeled as a "Designated Surveillance Area" (DSA) for the disease. "The DSA is a statute designed to increase scrutiny in the area where livestock are at greater risk for disease," says Meyer. "Any livestock that changes hands or moves out of the DSA must be tested." If an animal tests positive, it is classified as a reactor. Tissue samples are then cultured to determine whether or not the animal actually has the disease. "In this case," said Meyer, "the reactors all sourced from one herd in Park County. Subsequent testing of the rest of the herd found one more reactor out of 300-plus animals tested." According to Meyer, tissue samples were taken, and brucellosis was cultured from those samples, confirming the infection.

The second case, which occurred shortly before Thanksgiving, involved 12 captive bison that were tested prior to sale. "We euthanized one reactor, and took tissue samples to confirm the disease, which we did," said Meyer. Subsequent testing yielded 18 more reactors out of the 200 animal group the bison had sourced from. "At this point, we are developing a plan to test the remaining bison in the herd," said Meyer, adding that the total number in the herd is roughly 1,400 head. "Based upon those results, we’ll make a decision on how to proceed." Currently, both herds are quarantined.

One year ago, how to proceed would have been dictated by federal law. At that time, USDA restrictions would have called for a total depopulation, or the wholesale slaughter, of both affected herds, resulting in untold losses to afflicted ranchers. Additionally, two cases in separate herds would have meant a loss of Wyoming’s "brucellosis free" status, resulting in increases in costly testing procedures and other economic impacts statewide until the state’s status could be upgraded, a process which typically took at least a year. Beginning at the end of 2009, however, USDA began a process of lessening the restrictions concerning the disease, recognizing that statewide sanctions were having unnecessary negative effects on producers outside the DSA in each of the three affected states. Until a permanent rule is drafted, USDA is operating under an interim rule which allows each state to manage the affected area. Under this rule, the entire state is not subject to a damaging status change, as it would have been in the past. "As long as we’re managing the cases and doing the necessary testing to respond appropriately, they will not downgrade the whole state," says Meyer. "We don’t have that noose tightening around our neck."

According to Meyer, necessary testing also includes testing, and if necessary quarantining, neighbors surrounding the affected herds. In the first case, 10 surrounding herds were initially quarantined pending testing. Some 4,000 head were tested, all of which came back negative. These findings allowed Wyoming to lift the quarantine on those herds. Though the original affected herd remains under quarantine, USDA rules indicate that depopulation will not be necessary if three consecutive tests fail to find more positive animals. "That herd has now completed one negative test," says Meyer. "They are hopeful that, by repeated testing, we will be able to clear it from quarantine." In the case of the bison herd, three adjacent herds are undergoing testing. One herd has been cleared, while results on the other two are still pending.

Investigation of one more case, this one in west central Wyoming’s Sublette County, is still underway. In that case, one animal out of 180 tested came back positive. According to Meyer, the animal has been euthanized, and tissue samples sent in for further testing, a process that is expected to take a week. "Testing is still underway, so we’re not calling that an infected herd yet," says Meyer.

As with many cases in the region, the disease is thought to have been brought in by roaming elk herds. "Our preliminary epidemiology strongly suggests that the infection likely came from infected elk, which somehow commingled with domestic beef and bison herds," says Meyer. He adds that, while these results are not conclusive, a domestic source for the disease is not apparent. According to Meyer, the most common method of transmission is via the actual aborted fetus, and the accompanying amniotic fluid, both of which are heavily infected with brucella bacteria. It is common, he says, for curious livestock to sniff and lick these leavings, picking up the disease in the process. They then abort as well, and a cycle of infection begins.

As for the seemingly unusual number of cases over the past year, Meyer says that it is difficult to determine if the disease is on the rise or not. "Are we just starting to see a period of increased transmission? Has the prevalence grown to a point that we are going to see more? I can’t answer those questions; time will tell," he says. One thing, he says, is certain. "I don’t think anybody expects this will end tomorrow. Until something is done with the reservoir, which is largely infected elk and bison in the GYA, we can expect to see spillovers into other susceptible species."

How to handle infected wild populations has become a politically charged issue in recent years, particularly in the case of bison in the GYA, where lethal removal of potentially infected animals has raised outcry from wildlife activists. Compounding the issue, area ranchers have pointed out that increased pressure from protected predators, particularly wolves, seems to be forcing the elk’s annual calving season into lower country, where the risk of exposure to domestic livestock is increased. To address this, the Montana Department of Fish, Wildlife, and Parks has recently launched a five-year study designed to determine whether increased pressure from predators and human hunters is potentially increasing the instance of brucellosis outbreaks in elk due to increasing concentrations of the animals on private ground where hunting is not allowed, a situation that could easily translate to increased outbreaks among livestock.

For an effective solution, Meyer points out that fish and game personnel in all three states are going to have to play a major role. "They’ve got to help us find a solution," he says. "Otherwise, we can expect to have outbreaks for a long time to come." — Jason Campbell, WLJ Correspondent