A Unique Approach
Studying Patterns of Disease
From the beginning, we wanted to step back from focusing on the question “What is the microbe causing bighorn sheep pneumonia?” and answer other questions that could help us understand pneumonia outbreaks in bighorn sheep. The data we work with — including numbers of sheep in each herd, ages and sex, movement patterns of individual animals and of herds, individual-level information about which bighorn sheep and which herds are affected by disease, and environmental factors surrounding outbreaks such as season, food quality, contact with domestic sheep, and other variables — comprise the most comprehensive long-term dataset on bighorn sheep health in the United States.
Using Technology to Understand Disease Spread
We study bighorn sheep by monitoring radio-collared individuals. More than 500 bighorn sheep have been radio-collared since 1997, some with GPS collars that upload locations to satellites. The satellite transmitter sends data about an individual’s location to a web site or via email. We also use visual observation on the ground, or from a fixed-winged aircraft, to track the animals. Lambs are especially susceptible to pneumonia so we intensively monitor ewes and lambs from birth to weaning. These data are the basis of our research. We use information about survival, location, reproduction, and habitat to improve understanding of pneumonia transmission dynamics across the landscape of Hells Canyon.
Questions that Integrate Disciplines
The Consortium brings together specialists from many fields to share their expertise: disease ecologists, microbiologists, wildlife biologists, veterinarians, parasitologists, modelers, landscape ecologists, and science communicators. We ask questions that haven't been asked before, and investigate them using as many perspectives as possible.
We want to understand how the microbes that cause pneumonia are transmitted among bighorn sheep, and between bighorn sheep populations. We also seek to understand why pneumonia persists in some populations but fades out of others. We study transmission and persistence by analyzing patterns of bighorn sheep mortality and morbidity (how many sheep die and how many recover, if any) across populations over time. Information like which bighorn sheep die, how old they are when they die, and how much past exposure to pneumonia they have had (that is, whether they have built up immunity) gives us important clues about how pneumonia microbes circulate and persist within bighorn sheep populations. We also study whether bighorn sheep brought in from other geographic areas have more or fewer pneumonia outbreaks. Our analyses show that when pneumonia is first introduced into a population with no previous exposure (and hence no immunity), the disease often progresses through several stages. First, there is often an all-age die off in which 30-90% of the herd can die over weeks or months. After the initial die off, surviving adults have some immunity to the disease and rarely die, but pneumonia outbreaks may occur in new lambs every summer. Lamb pneumonia outbreaks can continue for years or decades. Rarely, a pulse of adult mortality occurs after the herd has been infected for years. Read our 2013 Journal of Animal Ecology paper describing these phases of pneumonia persistence.
Although it's fully understood that bighorn pneumonia is highly fatal, no one knows if animals that survive a pneumonia outbreak are immune to future infection or for how long. Our analyses suggest that ewes exposed to pneumonia develop a degree of immunity that protects them during future epidemics. However, this immunity does not protect their lambs. Our research also suggests that pneumonia in bighorn sheep changes the age structure of herds, because adults tend to survive pneumonia outbreaks better than lambs. Bighorn sheep that have been translocated into areas with infected populations appear to have a high risk of developing pneumonia. Read a paper on these subjects published in PloS One.
Although finding out what microbe causes pneumonia was not our focus, one consortium member, Tom Besser, and his colleagues at the Washington Animal Disease Diagnostic Lab, have worked extensively on this question. Their work provides strong evidence that Mycoplasma ovipneumoniae is the most likely cause of outbreaks of bighorn sheep pneumonia. M. ovipneumoniae causes illness by disrupting the mucociliary system (hair-shaped structures surrounded by mucus that line the airways and stop foreign particles and microorganisms from getting into the lungs). Other pathogens once thought to be the primary cause of outbreaks of bighorn sheep pneumonia, such as Mannheimia haemolytica and other Pasteurellaceae, are likely secondary pathogens that usually live harmlessly in the nose or throat, but descend into the lungs and contribute to lung damage once the mucociliary system is disrupted. Many other types of bacteria, even gut bacteria that reach the mouth during rumination, also contribute to the lung damage following M. ovipneumoniae infection. See an animation of how M. ovipneumoniae spreads through populations.
Connectivity and Habitat Modeling
We set out to understand how bighorn sheep populations are connected through the movements of individuals, and how individuals within populations form subgroups. Before our project, no one had studied bighorn sheep interactions at this level of detail. Our models use location data, gathered using transmitters on radio-collared sheep and accompanied by observations, to understand how sheep are using the Hells Canyon landscape. We have more than 70,000 location points, collected from 1997 to the present, from more than 500 individual sheep. Our models can be used to guide bighorn sheep management over very large landscapes, and to predict potential routes for gene flow and risk of disease transmission in Idaho, Washington, Oregon, and throughout the west.
More About Connectivity Modeling
Using location data, we are estimating annual and seasonal home-range size; relating these estimates of space-use intensity to habitat and landscape variables through a hierarchically structured mixed-model approach, and extending these model outcomes to predict population centers, habitat conductance, and regional connectivity within a circuit-theoretic analytical framework. When we understand how the entire Hells Canyon population (the metapopulation) uses this habitat, we will be able to analyze relationships between movements of bighorn sheep and disease occurrence. Read more about Brett's work and connectivity modeling.
We created mathematical and simulation models to examine key elements of pneumonia disease dynamics. We wanted our models to answer these questions: (1) How does pneumonia disappear from populations? This is important because if we can understand what drives disease to extinction, we may be able to find management strategies to help bighorn sheep populations recover from pneumonia. (2) Why does pneumonia persist in some populations for so many years, even when the populations have become very small? If we can understand differences between populations that are able to rebound from pneumonia and populations that are not, we may be able to help guide management strategies to help infected populations recover.
We are analyzing data on movement and disease patterns from bighorn sheep translocated into or around Hells Canyon. Bighorn sheep were initially translocated into Hells Canyon to restore populations after their extirpation a century ago. Translocation efforts continue today to try to boost bighorn sheep numbers. However, naïve translocated animals are highly susceptible to pneumonia if they are moved into or near infected herds. We want to understand the effects translocated animals have on pneumonia dynamics within Hells Canyon, such as whether translocated animals are changing population and disease dynamics in established resident herds.
Field and Modeling Studies of Chronic Carriage
Some animals that survive an initial outbreak of pneumonia are able to carry M. ovipneumoniae for long periods without apparently developing the disease. These 'silent' carriers can maintain pneumonia within populations for years or sometimes decades. We don't know if particular individuals are always carriers, or whether carrier status shifts between individuals over time. If we can understand what risk factors predispose individual sheep for becoming carriers, we may be able to target interventions to these higher-risk individuals. One of our current field research efforts focuses on identifying carriers in bighorn populations, and monitoring their carrier status over time to understand the role of these individuals in maintaining and transmitting pneumonia. We are incorporating our data, collected over two years from repeated nasal swabs and blood samples from individuals, into mathematical models that allow us to understand the population-level implications of individual carrier biology.
Lamb Contact Networks
In summer 2013 we initiated an intensive field monitoring effort in two populations in the northern part of the Hells Canyon system. We are observing ewe-to-lamb, and lamb-to-lamb contact networks during lamb-pneumonia outbreaks to improve our understanding of the severe summer lamb mortality that cripples many infected bighorn sheep populations.