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ARS Home » Pacific West Area » Pullman, Washington » Animal Disease Research » Research » Research Project #431730

Research Project: Genetic Impact and Improved Diagnostics for Sheep and Goat Transmissible Spongiform Encephalopathies

Location: Animal Disease Research

Project Number: 2090-32000-035-00-D
Project Type: In-House Appropriated

Start Date: Oct 18, 2016
End Date: Oct 17, 2021

Objective:
Objective 1: Determine the effects of the PRNP genotype on current diagnostic test assay accuracy in sheep and goats with scrapie. Subobjective 1.1: Determine the association of M112T polymorphism with the density and distribution of PrP-Sc in an archived set of brain and lymphoid tissues of sheep from U.S. surveillance program. Subobjective 1.2: Determine the effect of G127S polymorphism on the temporal spread of PrP-Sc from the gut to the brain in goats. Objective 2: Develop improved methods for antemortem detection of PrP-Sc in sheep and goats with scrapie. Subobjective 2.1: Determine the effect of prior biopsy on the kinetics and distribution of PrP-Sc accumulation in the RAMALT of sheep and goats. Subobjective 2.2: Develop a sensitive, high-throughput assay (immuno-quantitative PCR; immuno-qPCR) suitable for use in veterinary diagnostic laboratories for detection of PrP-Sc in sheep with classical scrapie. Subobjective 2.3: Determine the suitability of the immuno-qPCR for detection of PrP-Sc(Nor98) in brain, peripheral tissues, and placentas from sheep with Nor98.

Approach:
Objective 1 will support eradication efforts by addressing the unknown effects of specific prion protein gene (PRNP) polymorphisms on current diagnostic test performance. Previous work with chronic wasting disease demonstrates that certain PRNP polymorphisms prolong disease incubation and negatively impact diagnostic detection in white-tailed deer. In the current project, two polymorphisms that prolong scrapie incubation in small ruminants and which are common in U.S. livestock will be studied: M112T in sheep and G127S in goat. For sheep, a large validated tissue archive is available to test the hypotheses that the M112T polymorphism (1) affects the probability of detecting PrP-Sc in tissues collected during postmortem surveillance, and (2) the relative quantity and distribution of PrP-Sc accumulating within positive tissues. A similar archive does not exist for goats, thus an inoculation study will be conducted using goats of known genotypes to determine if the G127S polymorphism affects the kinetics of PrP-Sc accumulation in peripheral lymphoid tissues and brain. Objective 2 aims to improve upon methods of scrapie detection in small ruminants by addressing the unknown effects of previous biopsy on subsequent diagnosis by biopsy of the rectal mucosa, and by producing a higher throughput assay with improved diagnostic sensitivity that might expedite eradication of classical scrapie in the U.S., be adapted to blood-based detection, and improve etiological understanding of atypical (Nor98) scrapie. With regard to rectal biopsy, data from deer suggests prior biopsy may reduce disease detection in subsequent biopsies. This knowledge gap in sheep and goats will be addressed by determining the effect of first biopsy at 1 year of age on the diagnostic quality of the lymphoid tissue remaining after 1 and 2 years healing time. Development of a higher throughput, higher sensitivity diagnostic will be based on detecting total PrP-Sc (proteinase-sensitive and proteinase-resistant) using methods already in use in veterinary diagnostic laboratories in the U.S. The hybrid assay to be developed (immuno-qPCR) couples the specificity and convenience of a well validated, proteinase-free plate binding assay with the high sensitivity and rapid turnaround of real-time PCR. The hybrid assay will be first adapted to tissues collected during postmortem surveillance and sensitivity compared to prion titer as determined by transgenic mouse assay. The hybrid assay will then be applied to the components of blood to which prions are most frequently associated. Finally, this project aims to adapt the immuno-qPCR assay to enhance detection of PrP-Sc(Nor98) and to apply immuno-qPCR and standard transgenic mouse bioassay to determine the infection status of progeny born to Nor98-infected ewes.