The breadth of scientific literature in the field of prion diseases is overwhelming; however, the preponderance of that literature is published in journals that are typically not readily accessible by practicing veterinarians. In fact, a comprehensive review of the numerous intricacies of scrapie has not been published in a journal readily accessible to veterinary practitioners in quite some time. The purpose of this article is to provide an up-to-date review on the pathogenesis, detection, and control of scrapie.
The review begins with the historical context of prion diseases to provide readers with a foundation for understanding the establishment of the protein-only theory. Current understanding of scrapie and other prion diseases presupposes that those diseases are caused by a protein. Next, the functions of PrPC are reviewed along with a discussion of how misfolding of that protein leads to PrPSc accumulation and neurodegeneration. The current state of knowledge surrounding the transmission and pathogenesis of classical scrapie and the genetic determinants that control susceptibility of sheep to classical scrapie are described, including recent evidence regarding new genotypes with a high degree of resistance to the scrapie agent. This is especially important because, historically, eradication of classical scrapie has been largely dependent on selective breeding of animals with resistant genotypes.
Many different strains of scrapie have been described with unique incubation periods and molecular profiles on western blot analysis. Although an indepth review of the theories surrounding prion strain genesis is beyond the scope of this article, we intend to discuss different scrapie strains within a practical and clinical context. Additionally, it is important to understand the differences between classical and atypical scrapie. These 2 variants of the disease have unique characteristics that require different approaches to disease management. Consequently, we have dedicated a separate section in this article to the features of atypical scrapie with an emphasis on highlighting its disease phenotype, transmission, and management characteristics.
It is not unusual for US veterinary professionals to be faced with difficult questions from producers, clients, and the public regarding the diagnosis and epidemiology of prion diseases such as scrapie and CWD. This review addresses current knowledge regarding scrapie including updates on interspecies transmission (ie, the host range) of the scrapie agent and experimental attempts to transmit nonscrapie prion agents to sheep.
The primary goal of the NSEP is to eradicate classical scrapie from the US sheep and goat populations. The program has had great success to date, and this review outlines the NSEP's successes and addresses the challenges it faces moving forward. The USDA APHIS updated regulations in the NSEP final rule on March 25, 2019, and we highlight several of the important changes made.