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Scientists make strides toward restoring bighorn sheep in the American West

An interdisciplinary research team with several scientists from the Huck Institutes has made key discoveries elucidating dynamics of pneumonia in bighorn sheep as mortality from the disease continues to limit recovery of the once nearly extirpated bighorn population in the western United States.
Bighorn lamb. Credit: Tony Bynum

Bighorn lamb. Credit: Tony Bynum

By: Seth Palmer

At the beginning of the 19th Century, bighorn sheep in the western United States, Canada, and northern Mexico numbered an estimated two million or more; but by 1900, there were only several thousand bighorns left – a result of extensive sport hunting, expanding ranch lands encroaching on their habitat, and disease.

Conservation efforts – including a drastic reduction in bighorn-hunting permits, establishment of natural parks and wildlife refuges, and reintroduction programs – have stemmed the bighorn's rapid decline, but disease, particularly pneumonia, continues to be a primary factor limiting the iconic animal's recovery across western North America.

But an interdisciplinary research team that includes Huck Institutes Director Peter Hudson and scientists Raina Plowright and Kezia Manlove from the Center for Infectious Disease Dynamics at Penn State has recently published in the journal PLOS ONE and in the Journal of Animal Ecology several crucial discoveries about the dynamics of pneumonia in bighorn sheep, which may help to inform ongoing conservation and management efforts.

“Our findings suggest that the impacts of pneumonia on bighorn sheep populations are much worse than previously reported,” said Plowright, lead author of the PLOS ONE paper. ”The initial epidemic can cause high mortality, but subsequently, adults – even chronic carriers – survive well, obscuring our detection of the disease. However, an average of 80% of the lambs die each year, and some populations continue to lose their lambs for decades, even to the point of extinction. Lamb pneumonia mortality is very difficult to detect, but it may be an explanation for the poor growth rates of many populations across the West.”

The team, which also includes scientists Frances Cassirer from the Idaho Department of Fish and Game, Tom Besser and Kathleen Potter from the Department of Veterinary Microbiology and Pathology at Washington State University, Andrew Dobson from the Department of Ecology and Evolutionary Biology at Princeton University, and Paul Cross from the U.S. Geological Survey's Northern Rocky Mountain Science Center in Bozeman, Montana, represents the first multidisciplinary study of bighorn sheep pneumonia – a study which involves the largest extant dataset on the disease, spanning 15 years and 16 interconnected bighorn populations in Washington, Idaho, and Oregon.

“Our multidisciplinary approach – which involves specialists in wildlife biology, epidemiology, mathematical modeling and quantitative ecology, statistics, disease ecology, microbiology, immunology, and landscape ecology – enables us to examine the disease from the pathogen scale through to the landscape scale, hoping ultimately to understand why the disease persists in some herds, and why some herds are able to recover,” said Plowright.

Bighorn ram and ewe on a cliff face. Credit: Tony Bynum

High challenges

Any wildlife biologist will tell you that studying wildlife is a challenge – with ever-changing field conditions, moving subjects, and no experimental controls – and the high mountain habitat of bighorn sheep makes for an especially challenging study.

Pneumonia presents its own unique challenge to researchers: the disease – initially spread to bighorns through contact with domestic sheep – is now endemic to many bighorn populations, and most observed cases die from secondary bacterial infections which mask the identity of the primary pathogen.

“For decades the bighorn sheep research community was stymied by debate about which pathogen was causing the outbreaks of pneumonia,” Plowright explained, “hence the pathogen’s means and pathways of transmission were exceptionally difficult to investigate. Only in the last few years, with the availability of new DNA techniques, has the primary pathogen at last been identified as Mycoplasma ovipneumoniae.”

“We started this study without knowing the identity of the pathogen,” Plowright continued. “Because of the uncertainty regarding the pathogen, very little progress had been made in understanding pneumonia, so we stepped back and asked what we could learn from broad-scale spatial and temporal patterns of infection. While we made some important findings, our inability to track a specific pathogen presented some challenges. For example, we were unable to distinguish between exposed animals that were resistant and those that were tolerant – i.e., silent carriers of disease – and some of our observations that suggested development of immunity could be alternatively explained by other epidemiological processes.”

Raina Plowright with bighorn rams

Dynamics, immunity, and mortality

Analyzing the data for patterns of pneumonia-induced mortality, the researchers identified naïve adults and newborn lambs as the most vulnerable groups whose mortality is driving the bighorns' overall population decline.

Naïve (previously unexposed) individuals are usually at the greatest risk of infection and of dying from a disease, but those that survive – in the case of diseases such as bacterial pneumonia – develop some immunity to subsequent infections; the data appeared to confirm this for the bighorns, as mortality among adults rose and fell from year to year, but the lambs were dying at a troubling rate year after year.

“These lambs die a horrible death as their lungs fill with a soup of bacteria, leading to suffocation and septicemia. As well as being a conservation issue, this is an animal welfare issue – but one that is hidden in most populations because predators quickly scavenge dead lambs. It takes enormous effort by Dr. Cassirer and colleagues in the Idaho, Oregon, and Washington wildlife agencies to monitor lambs intensively enough to detect this problem” said Plowright.

Digging deeper, the researchers examined the relationship between the bighorns' exposure history and their survival, and confirmed that the survivors must be developing immunity to the pneumonia-causing pathogen.

Usually, a degree of immunity is then conferred to the offspring of the surviving individuals – as is the case in domesticated sheep – but the bighorn lambs displayed a striking lack of this inherited, or passive, immunity.

“We hypothesized that previously exposed ewes would protect their lambs from pneumonia through maternal immunity,” said Plowright. “Paradoxically, we saw the opposite effect: the higher the maternal exposure to pneumonia, the more likely her lamb was to die. This is in contrast to the protective effect of maternal immunity in domestic sheep, to which M. ovipneumoniae is well adapted. Because of this, the burden of mortality in the bighorn populations we've been studying falls heaviest on the lambs, leading to poor recruitment and aging adult populations – and this is a critical problem. The survival of the bighorn sheep is absolutely dependent on the survival of their lambs, and right now most of them aren't surviving.”

Changing the future

As the primary pathogen causing bighorn sheep pneumonia has only recently been identified, an effective vaccine against bighorn sheep pneumonia has not been developed, and so researchers currently in charge of bighorn conservation efforts have focused primarily on translocating healthy bighorns to reestablish herds or boost declining herds' numbers; but these most recent findings by Dr. Plowright and her colleagues suggest that if bighorn sheep are translocated into or near infected populations – as has happened within their study region – such methods may have little or no practical utility.

“Naïve animals translocated into our study area succumbed to pneumonia within a few years of translocation,” said Plowright. “Our results show that introducing naïve animals into areas with infected animals is not productive for conservation or recovery. The recent discovery of M. ovipneumoniae as the probable primary pathogen causing bighorn sheep pneumonia opens up many more opportunities to test our hypotheses in the field and lab. We have begun intensively sampling populations to understand the long term carrier state, and we are combining these field studies with mathematical modeling in order to find potential strategies of breaking this cycle of disease.”

“In publishing these most recent results,” Plowright continued, “we really are fortunate to have had many years of hard work already done for us by our colleagues who collected much of the data; in particular, Frances Cassirer, the lead author of the Journal of Animal Ecology paper, has spent the past 15 years recording mortality patterns in Hells Canyon bighorn sheep, and Tom Besser led the efforts that resulted in the discovery of M. ovipneumoniae as the probable cause of bighorn sheep pneumonia endemics. We owe Frances and Tom a great debt in all of this, and we'd like to thank them – along with Kathleen Potter, Andy Dobson, and Paul Cross – for their part in making our discoveries possible.”

Raina Plowright (foreground) and Andy Dobson (background) with bighorn ewes and ram. Credit: Peter Hudson

This research was supported by funding from the Morris Animal Foundation; New York Community Trust; EcoHealth Alliance and Pamela M. Thye; the Cedar Tree Foundation's Smith Fellows program; the Research and Policy for Infectious Disease Dynamics (RAPIDD) program of the Science and Technology Directorate, Department of Homeland Security and Fogarty International Center, National Institutes of Health; Idaho Department of Fish and Game; Oregon Department of Fish and Wildlife; Washington Department of Fish and Wildlife; Bureau of Land Management; United States Forest Service; Wild Sheep Foundation National, chapters, and affiliates; Shikar-Safari Club International; Oregon Hunters Association; Wildlife Forever; Cabela’s; Federal Aid to Wildlife Restoration Project W-160-R administered through the Idaho Department of Fish and Game; and Idaho Wildlife Disease Research Oversight Committee.

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