Test vaccine shows promise for bighorn sheep

May 6, 2011

Concerns over pneumonia transmission between wild bighorn sheep and bands of domestic sheep grazing western rangelands have plagued land managers in the Northwest for the last several years. The conflict reached a fever pitch last year when Idaho’s Payette National Forest reduced domestic sheep grazing by 70 percent in an attempt to reduce interactions between the two species. That decision has led to considerable friction between sheep ranchers, who worry that their livelihoods are now in peril, and environmental groups, many of whom contend that sheep grazing should be removed entirely from public rangelands. Compounding the issue, increases in bighorn die-offs due to pneumonia throughout the West in recent years have led to concerns that other public lands may soon follow the Payette’s lead. In the midst of this debate, researchers at Washington State University’s (WSU) College of Veterinary Medicine have recently begun making strides towards a resolution that may give both species the opportunity to coexist.

In February, researchers, led by WSU professor Dr. ‘Sri’ Srikumaran, began testing on an experimental vaccine designed to boost the immune systems of the bighorns, allowing them to fight off pneumonia-causing bacteria. Though Sri cautions that there are still many obstacles to be overcome before any type of practical use is possible, the new vaccine does show early signs of promise. In the most recent experiment, four bighorns were treated with the new vaccine, essentially a killed version of the pneumonia bacteria, followed by a massive dose of those same organisms. An additional four were dosed with pneumonia, but left untreated. According to Sri, the results were undeniable.

"All four vaccinated bighorns survived, and all four of the untreated ones died within two days," he said. "What enabled those four to survive was the bacteria that we put in."

Although small numbers were used, Sri points out that a 100 percent success rate is significant.

"I am certain that if you gave this vaccine to 100 bighorns, 75-90 percent of them would survive infection, maybe more," he said.

Sri began his research in this field several years ago, initially to determine whether transmission of pneumonia-causing bacteria between the two species was even a possibility, a theory that has been the subject of considerable debate. In that experiment, bighorns were exposed to domestic sheep that had been infected with a genetically "tagged" form of bacteria that was fluorescent and readily identifiable under a microscope. Following exposure, the bighorns did die of pneumonia, and subsequent testing proved that the tagged bacteria had transferred from the domestic to the wild species. Although there are still those who contend that transmission is highly unlikely, Sri feels that the debate on that subject has largely been put to rest.

"I think that, without any doubt, we have proved that (transmission occurs)," he said. Following that experiment, USDA researchers, who had collaborated in the effort, presented the findings to representatives of the sheep industry. "(USDA personnel) told them ‘Let’s not fight over it, let’s find a solution,’" said Sri.

The result was a reduction in the skepticism that had previously surrounded the transmission issue from many of the ranchers, and a commitment from several to assist in continuing research efforts.

Despite his recent success, Sri cautions that there is a very long road between vaccinating captive bighorns and treating wild populations on the range.

"We know very well that needle immunization is not practical in wild sheep," he says, "that is not the question we were trying to answer."

Instead, he explains, the purpose of the recent experiment was to determine if a vaccine could be effective at all. Previous research has shown that domestic sheep carry a high level of antibodies for these disease-causing organisms, antibodies that the bighorns lack.

"What we have shown is that if, by some means, we can raise the antibody levels for these organisms in bighorns, they can resist a challenge," says Sri. "The next phase, of course, is to come up with a method that need not be given several times per year, and to find a way to administer it in a feed supply."

Winter feeding of pellets to struggling bighorn populations is a normal practice performed by federal agencies in several areas around the West. Sri has observed bighorn sheep coming in bunches of 100 or more to feed on pellets in the mountains near Yakima, WA, and points out that this could be a reliable delivery method once the vaccine is fully developed.

Although estimates of a timeline for a usable vaccine range from three to five years and beyond, Sri is hesitant to provide assurance that such a speedy development is possible. Instead, he counsels patience, pointing out that in the field of human medicine, vaccines often spend 20 years or more in the development phase. For the immediate future, he intends to begin researching methods to make the vaccine deliverable in an oral form, rather than the injection process currently necessary, and to extend the useful life of the vaccine, which must currently be given multiple times per year to remain effective.

"We don’t want to rush into this; we need to take our time," he says.

Despite the obstacles ahead, Sri remains hopeful that the new vaccine will eventually provide resolution to the debate surrounding domestic sheep and bighorn interactions on public land.

"The domestic sheep industry is an important part of our economy," he says. "Nevertheless, some bighorn advocates, they don’t care about the sheep industry at all. Likewise, some sheep ranchers would gladly see all bighorns wiped from the earth. Neither view is good, so we are working for some solution so that both industry and bighorns can survive." — Jason Campbell, WLJ Correspondent