Investigating Distributions and Risk Factors of Treponeme-associated Hoof Disease in Elk (Cervus canadensis)

Steven N. Winter

doi:10.7273/000007500

Treponeme-associated hoof disease (TAHD) is a transmissible disease of free-ranging elk (Cervus canadensis) in the northwestern United States. The epidemiology and ecology of TAHD are poorly understood and need to be elucidated to appropriately guide disease management efforts. The objective of my dissertation was to provide foundational information on distributional patterns and risk factors of TAHD occurrence in free-ranging elk. My investigations used available datasets from different surveillance programs across the known distribution of TAHD. First, I described and compared distributional patterns of disease in space and time using community science and diagnostic case surveillance strategies in western Washington, where TAHD was first discovered. I found that hoof abnormalities in elk that were reported by hunters occurred in statistically significant spatial clusters of game management units (GMUs) in southwestern Washington, and that observations of limping elk from the general public have expanded from this TAHD endemic area to other regions in the state. In the subsequent parts of my dissertation, I gained valuable insights into environmental and host-related risk factors for TAHD. These investigations were centered on the knowledge that disease risk in wildlife is modified by factors in host and pathogen populations, as well as environments where infections can take place. For the first group of risk factors investigated, I hypothesized that TAHD cases would be associated with low trace mineral levels in elk, based on the importance of minerals in immune function and hoof health, thereby influencing elk susceptibility to infection. Using mineral concentrations in hair and liver, I found limited direct support for this hypothesis; however, findings suggest that environments known to have forage containing low availability of selenium and copper may be areas with higher risk for TAHD. Further, the management practice of supplementing minerals to elk cannot yet be recommended because any possible benefits from supplementation may be offset by congregating elk and inadvertently promoting pathogen transmission. In the second group of risk factors investigated, I hypothesized that TAHD would be associated with environmental factors, based on their potential to affect host susceptibility, exposure to conditions that promote pathogen survival, or contact rates between hosts. I examined how hoof abnormalities identified by hunters were associated with soil characteristics, topographic features, land cover types, and precipitation before the harvest season. Of the variables evaluated, I found strong evidence that TAHD was associated with higher clay content in soil and greater amounts of agricultural land use. My findings suggest that areas with soil properties that promote moisture retention and land use that can contribute to elk congregation are important locations for TAHD risk by possibly contributing to density-dependent transmission, the survival of TAHD pathogens, and/or the susceptibility of elk to infection. Management interventions that reduce the number of TAHD-affected elk and elk congregation in these high-risk areas may have beneficial effects on reducing TAHD transmission. Overall, my work can be used by wildlife disease managers to support existing or deploy new disease surveillance and control efforts in Washington and other areas where TAHD occurs or may spread to new elk populations.

Investigating Distributions and Risk Factors of Treponeme-associated Hoof Disease in Elk (Cervus canadensis)

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