By Amy Conner
June 2005 -- There are many unanswered questions about the Sierra Nevada Bighorn sheep, along with them, opinions that support every possible angle. In an effort to put an end to the questions, the University of Nevada, Reno sponsored a Sierra Nevada Bighorn sheep roundtable discussion in Reno, Nev., on Feb. 25. Several leaders and decision makers from various agencies attended the meeting, along with a number of sheep producers, to discuss disease transmission from domestic sheep to bighorn sheep and the genetic make-up of the Sierra Nevada Bighorn sheep.
Hudson Glimp, Ph.D., extension sheep specialist and coordinator of the Rafter 7 Ranch Merino sheep breeding and education program at University of Nevada, Reno and organizer of the roundtable, says that he pulled this meeting together because he, along with many others, are concerned about a possible 14-mile barrier around each Sierra Nevada Bighorn sheep herd, being called for by the Sierra Nevada Recovery Team. In addition to the 60,000 domestic sheep already removed from federal, state and private lands, another 60,000 domestic sheep may also be removed because of the increased barrier and which will potentially impact the U.S. sheep industry by more than $100 million.
The Recovery Team is asking for this extended barrier because they are concerned that domestic sheep will transfer pneumonia to the bighorn sheep and cause a die-off; however, Glimp disagrees, stating that pneumonia is a stress-related disease.
?Bighorn sheep, along with other mammals, are all natural carriers of pasteurella haemolytica,? Glimp said. ?If they are stressed, with things such as predation, relocation, hunting and nutrient deficiency, they will die.?
Disease Transmission from Domestic Sheep to Bighorn Sheep
The roundtable discussion began on the topic of disease transmission among domestic and bighorn sheep. Presenters covered the following topics:
? the transmission of pneumonia-complex diseases based on the literature and recent field studies;
? interpretations of the literature and recent field studies; and
? the risk of disease transmission to the Sierra Nevada Bighorn sheep, with the context of other potential stressors, such as climate, recreation, low body condition and adequacy of habitat.
There was no shortage of controversy on this issue during the roundtable. In addition, the research that is available is outdated and poorly conducted. However, some believe that domestic sheep and bighorn sheep should not be held in proximity because of the fatal consequence of bighorns from pneumonia.
One of those believers is Ben Gonzales, DVM, from the California Department of Fish and Game in Rancho Cordova, Calif., who said, ?The most essential step in this sheep-specific process is to reach a common understanding among all involved that incompatibility between domestic sheep and bighorn sheep exists and mixing the two species will eventually result in a bighorn sheep die-off.?
Gonzales? opinion is based on a couple different research studies. The first was conducted by W.J. Foreyt, DVM, who researched disease transmission between domestic and bighorn sheep in captive studies. He concluded that the bighorn sheep, after being held in captivity and being vaccinated with pasteurella haemolytica, all suffered from acute hemorrhagic pneumonia on days four through 71.
In addition to the vaccination of pasteurella haemolytica as a confounder in these results, Gonzales said, ?Whatever errors in technique or assumption he (Foreyt) made, for me, that is the bottom line.?
In other studies published in 1992 and 1996, researchers looked at bighorn sheep outbreaks of pneumonia that they believed were linked to the presence of domestic sheep. However, Gonzales admitted there was circumstantial evidence that these outbreaks were initiated by contact with domestic sheep.
?The problem with these field outbreaks is that there are many potentially confounding factors. You have lungworm, nutrition, winter kill, in other words, some other initiating factor that could just make for that normal flora to become a pathogen,? said Gonzales.
Although Gonzales admits that respiratory disease outbreaks can occur in bighorn sheep without known contact from domestic sheep, there is no technology available that can identify which contact between domestic sheep and bighorn sheep will be safe.
Believing the studies conducted in the early ?90s were damaging to the issue of whether domestic sheep transferred respiratory diseases to bighorn sheep was Anette Rink, DVM, from the Animal Disease and Food Safety Laboratory at the Nevada Department of Agriculture.
?In the ?90s, there were a series of publications that took what I would call a tunnel-vision approach, and I really think that is very detrimental,? she said. ?We continue to review the literature and there is a long list of publications, which represent the bigger picture. And when I talk about the bigger picture, I am talking about those that actually acknowledge that there are multiple factors that lead to disease.?
Those other factors include items such as inclement weather, predation, wildfires, inadequacy of habitat and lack of nutrients, specifically, selenium in the Sierra Nevada Mountain Range.
In order for a decision to be accurately-based, Rink believes that comprehensive diagnostic work-ups need to be initiated, that the results need to be derivative in context and also recognize the vulnerability of completely susceptible populations, such as the bighorn sheep.
?There is no zero-risk obtainable here, and if every domestic sheep were to leave the range in the western United States right now, I am not sure we would get rid of any of the disease problems,? Rink said.
Nevada and California are not the only states dealing with this issue. Keith Aune, DVM, from the Montana Department of Fish, Wildlife and Parks spoke about the bighorn scenario in Montana.
In Montana, Aune has helped build up the bighorn herd, which runs on a great deal of privately owned land in that state. In one instance, a herd of bighorn and domestic sheep had been running on the same piece of land for more than 20 years; however, no major epidemic of pneumonia had occurred until 1994 when a severe epizootic hit that herd.
?If we characterize the last four die-offs, where we had evidence of die-offs, some diagnostic evidence to chase, I think there is a lot of variation in terms of factors out there,? said Aune. ?Are domestic sheep involved in all bighorn die-offs in Montana? No. Could they be involved in some? Yes.
?I think the transmission (from domestic to bighorn sheep) of disease is probabilistic ? not certain ? in field conditions, and we don?t know all the factors that relate to those probabilities,? Aune said.
It is really unknown how the disease-transmission factor became the major issue of the recovery of Sierra Nevada Bighorn sheep, since there have been no bighorn deaths attributed to the pneumonia complex.
Speaking on his experience, John Weyhaysen, Ph.D., from the University of California-San Diego, said, ?Having studied the issue for 30 years?I have never myself seen any evidence of disease in the Sierra Bighorn. I have never seen a snotty nose; I have never seen any of those things that we associate with pasteurella and so on. So, we have no evidence that there have been any disease problems within the last 30 years.?
Predation is the biggest, individual death loss that the Sierra Nevada Bighorn sheep population currently faces; it is not pneumonia
?It is known that many of bighorn sheep are actually victims of predation, which is 54.5 percent,? said Rink.
Domestic sheep can actually assist in the recovery of the Sierra Nevada Bighorn sheep in this instance, because the brush that predators hide in can be taken away by grazing management with domestic sheep.
?It has turned out that domestic sheep really seem to be the only tool that you can use in an environmentally friendly way to control those weeds and to move deteriorated habitat back into a way where it is more nutritious,? said Rink.
Regarding the selenium deficiency in the Sierra Nevada Range, Hank Vogler, Nevada sheep producer and sportsman, said that he experienced die-offs of adult bighorn sheep, attributed to a lack of selenium, in the Northwest Territory of the Artic Circle on a hunting trip.
?They had die-offs; they had every problem that we have discussed here today,? Vogler said. ?Fifteen-hundred-plus miles to the nearest cow or sheep or any domestic livestock, they still had the same problems.?
One thing that was agreed upon during the roundtable discussion was that the recovery of the Sierra Nevada Bighorn sheep is multi-factorial. There are many factors that weigh into the disease transmission issue.
?To hang the total hat for recovery of this population on the total absence of domestic sheep is, I think, a little bit nearsighted,? said David Thain, DVM, from the Nevada Department of Agriculture.
Genetic Diversity of the Sierra Nevada Bighorn Sheep
The second portion of the roundtable discussion dealt with the controversy of whether the Sierra Nevada Bighorn sheep is actually a sub-species of the California Bighorn sheep. The presenters covered the following topics:
? genetic information that would indicate Sierra Nevada Bighorn sheep as a genetically unique subspecies;
? adequate documentation to justify listing the Sierra Nevada Bighorn sheep as a distinct population segment;
? sufficient genetic diversity to sustain this population segment; and
? ideas of how scientists can work together to share data, genetic samples and other valuable information.
There is great debate on the genetic information available indicating Sierra Nevada Bighorn sheep are indeed a genetically unique subspecies and a distinct population segment. The reason for this debate is because most information supporting this theory is based on mitochondrial DNA studies, instead of nuclear DNA studies.
Through mitochondrial DNA testing, conducted by Rob Ramey, Ph.D., chair of the Department of Zoology at the Denver Museum of Nature & Science, Wehausen said, ?There is a clear separation between desert sheep and Rocky Mountain sheep. What was found was the so-called California/British Columbia Bighorn were not separable from Rocky Mountain sheep, but were very separable from those in the Sierra Nevadas.?
Weyhaysen followed this study up with morphometric work and measured 1,200 skulls of bighorn sheep. He discovered the same patterns as Ramey. The bighorn sheep found in British Columbia and Washington were not distinguishable from those found in the Rocky Mountains; however, the bighorn in British Columbia and Washington and those found in the desert are quite separable from the Sierra Nevada Bighorn sheep.
?We have a fairly large Rocky Mountain subspecies, we have a desert subspecies that goes all the way up into Oregon?and strangely, we have this little subspecies here in the southern Sierra,? said Weyhaysen.
Rink?s opinion differs as she believes using mitochondrial DNA testing can create an artificial result of separate populations. The method she prefers to use for population studies is microsatellite markers.
?They (microsatellite markers) have a low mutation, there is no selection pressure on them unless they are linked to a trait, and they are highly polymorphic, so that is the one way you get a maximum amount of information with a relatively small amount of input of money and resources,? Rink said.
Through her microsatellite research, Rink has compared 22 microsatellites of the Sierra Nevada Bighorn sheep with other bighorn sheep, discovering that Sierra Nevada Bighorn sheep group closely with other desert bighorn sheep from the state of Nevada. She has not found that the Sierra Nevada Bighorn sheep are a distinct population.
?Eleven microsatellites have the power to actually differentiate between every single individual that is right now on this planet, so 22 microsatellites, I think, give a pretty good idea of how related or how different any individual is from any other individual or from any other population,? said Rink.
?My personal impression is that microsatellite genotyping should have been performed prior to actually getting the listing (as an endangered species) done since only the nuclear genome tells the whole story,? said Rink.
The future of the Sierra Nevada Bighorn sheep is dependent on the genetic diversity of the herd. Heterozygosis tests have been conducted to measure the degree of genetic diversity in these animals. Results of these tests indicate that the Sierra Nevada Bighorn sheep?s heterozygosity levels are low, which means the animal may be somewhat inbred.
Through her microsatellite work, Rink has tested rams, discovering they are homozygous at 62 percent of its loci and heterozygous at 30 percent of its loci.
?Whether that is sufficient genetic diversity (or not)?there is an indication that this animal is a little bit inbred,? Rink said. ?Whether that is going to be significant, I can?t tell you. I have never been able to find a specific threshold level anywhere that describes at what point of loss of heterozygosity a population becomes vulnerable, because that is really dependent on the environment they live in.?
Rink further explained that if these animals were well taken care of, their heterozygosity level would probably not make a difference on their survival rate. However, Sierra Nevada Bighorn sheep have a variety of challenges to deal with, so in order for a high survival rate, they need a higher heterozygosity level.
?There is a distinct line of thought in that if you lose heterozygosity at the MCH level, the population becomes incredibly susceptible to disease,? Rink said. ?Without heterozygosity, you basically compromise the population or the species; it won?t be able to survive in the long-term.?
On the other hand, Weyhaysen stated, ?Large mammals in general, show indications that low genetic diversity doesn?t seem to pose much of a problem. Animals with very low genetic diversity have shown very good recoveries in their populations.?
He pointed out an example of a herd of bighorn sheep in the Mojave Desert that has a genetic diversity similar to that of a herd in the Sierra Mountains. This herd is thriving, and there is no indication that genetic diversity is a problem at this point.
One thing that was agreed upon during this day-long meeting was that more communication is needed from both sides to assist in this issue. Working together on collecting data and determining research methods will help put an end to some of the many unanswered questions about this animal and how it is affected by domestic sheep.
As stated by Doug Busselman, executive vice president of the Nevada Farm Bureau, ?I would hope that down the road we could somehow plan to come together in a more collaborative fashion to discuss how we might be able to live together and manage and conserve this species, as well as the livelihoods of agricultural producers who are threatened by the management prescriptions that have been discussed.?