Chronic wasting disease strain diversity, distribution and transmission

2021 Summer.Includes bibliographical references.Chronic wasting disease (CWD) is an invariably fatal prion disease affecting captive and free-ranging cervids, including white-tailed deer, mule deer, moose, elk and reindeer. Since the initial discovery of the disease in the 1960's, CWD has spread across the US and Canada, South Korea, and, most recently, Europe. While some outbreaks of CWD were caused by transport of infected animals from endemic regions, the origin of CWD in other epizootics is unclear and not all outbreaks have been characterized. Previous studies have shown that there are multiple strains of CWD; however, the continuous spread and the unclear origin of several outbreaks warrant continued surveillance and further characterization of strain diversity. Moreover, studies implicating extraneural prions as more zoonotic motivated us to examine within-host prion strain diversity. The overarching goal of the work presented here was threefold: 1) address CWD strain differences between lymphoid and brain tissue from the same animal, 2) assess if there are any differences in CWD from either within or between contiguous and non-contiguous outbreaks and 3) address aspects of plant-vectored CWD transmission. The work presented here has important implications for understanding strain diversity within and between deer, as well as identifies samples that appear to be novel strains that warrant follow up assessment. Finally, we show how plants may be playing a role in vectoring infectious prions shortly after exposure. This research has important implications for our understanding of prion strain diversity and distribution as well as adds insight to plant-vectored prion transmission. First, we assessed differences between lymph node-derived and brain-derived prions from within the same animal to characterize strain differences within a single animal. To do this, we assessed isolates using biochemical techniques including electrophoretic mobility, glycoform ratio and conformational stability. Interestingly, we found that there were biochemical differences between lymph node and brain isolates, novel intermediate conformations of the prions in the brain (but not the lymph node) and increased variability in the lymph node-derived prions. Collectively, these results suggest that there are more diverse prion strains in the periphery and are distinct from neurological prions. The research discussed here advances our understanding of the differences between lymph node-derived and brain-derived prions. In addition to within-host strain comparisons, we also wanted to assess biochemical strain differences from naturally infected cervid species. Numerous studies have examined CWD strains upon passage into transgenic mouse models. For the purposes of our research, we wanted to examine CWD strains from the natural host for a number of reasons: 1) bioassay is expensive and time consuming, making strain characterization challenging, 2) research indicating that host factors other than PrPC may be influencing strain characteristics and 3) to determine if we could detect dramatic biochemical differences in strains, thereby providing an easier method to determine CWD strain prevalence in cervid populations without bioassay. Because the origin of CWD is unknown and some outbreaks of CWD have no clear exposure/connection to ongoing CWD outbreaks, this research would provide insight into the evolution and origin of CWD. Here, we show that there are some cases of CWD that present with novel biochemical characteristics that distinguish them from other CWD isolates. These instances suggest a new strain has emerged or that there is differential evolution in these subpopulations. Importantly, this work highlights that there is a lot more variability CWD biochemical characteristics than previously described. As a part of the strain typing project, two samples were received from captive white-tailed deer in Texas. These samples immediately proved to be a challenge to work with because they were behaving in an unusual way in our biochemical strain typing assays. In short, these isolates behaved in strange ways depending on the detergent class with which they were being digested. Because there was no known introduction of CWD to this captive herd, we were suspicious that we were seeing a novel strain of CWD. Isolates were passaged into cervid and human PrP mice. Upon passage, these isolates looked like classical CWD in Tg33 mice and, fortunately, don't appear to have any zoonotic transmission potential into human PrP mice. Importantly, this work highlights that CWD can present in a unique way in a cervid host but cause a classical-type disease in transgenic animals. Finally, we examined the role of plants to transmit CWD. Previous research implicated plants as having a possible role as a vector in prion transmission. We built upon this previous research by using CWD prions rather than hamster prions and a different plant model. The research presented here will show that plants are able to uptake prions shortly after exposure, but that these prions are no longer detected by 72 h. The work presented here implicates plants as potential CWD vectors in the short term