Efficient transmission of US scrapie agent by intralingual route to genetically susceptible sheep with a low dose inoculum

Authors: MAMMADOVA, NAJIBA - Oak Ridge Institute For Science And Education (ORISE); CASSMANN, ERIC - Orise Fellow; Greenlee, Justin

Submitted to: Research in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2020
Publication Date: 10/20/2020
Citation: Mammadova, N., Cassmann, E., Greenlee, J.J. 2020. Efficient transmission of US scrapie agent by intralingual route to genetically susceptible sheep with a low dose inoculum. Research in Veterinary Science. 132:217-220. https://doi.org/10.1016/j.rvsc.2020.06.010.
DOI: https://doi.org/10.1016/j.rvsc.2020.06.010

Interpretive Summary: Scrapie is a fatal disease of sheep and goats that causes damaging changes in the brain. The infectious agent is an abnormal protein called a prion that has misfolded from its normal state. Previous work has defined two distinct isolates of classical scrapie, x124 and No. 13-7, that have differences in incubation periods and susceptibility in sheep with different genetic backgrounds. It has been shown that after inoculation with US isolate x124, susceptibility and incubation period are associated with valine at codon 136 (V136) of the prion protein: sheep with two V codons (VV136) have the shortest incubation periods, followed by sheep with one V codon and one A codon (AV136), while sheep with two A codons (AA136) only developed disease after inoculation via the intracerebral route. Sheep scrapie can be transmitted orally from ingestion of prions and it is possible that oral lesions may pose a risk for scrapie infection. In this study, we investigated infectivity of decreasing doses of the x124 scrapie agent (100mg, 50mg, 20mg, and 10mg) on incubation time and attack rate after experimental intralingual inoculation into susceptible AV136 sheep. Our results indicate that the lowest inoculum dose tested in this study effectively transmitted the x124 scrapie agent in AV136 sheep, with a 100 percent attack rate, and no significant difference in incubation times among sheep inoculated with varying doses. This study provides a starting point for future studies to assess the minimum infectious dose of the x124 scrapie agent in sheep. This information is important for determining the potential risk of scrapie occurrence in sheep flock and could be used by veterinarians and regulatory agencies.

Technical Abstract: Scrapie is a naturally occurring transmissible spongiform encephalopathy (TSE) in sheep and goats that results in accumulation of the misfolded prion protein (PrPSc) and progressive neurodegeneration. To date, two distinct US isolates of classical scrapie, x124 and No. 13-7, have been described with differences in incubation periods, neuroanatomical deposition profiles of PrPSc, as well as genotype susceptibilities. It has been shown that after inoculation with US isolate x124, susceptibility and incubation period are associated with valine at codon 136 (V136) of the prion protein: VRQ/VRQ had the shortest incubation periods, followed by VRQ/ARQ sheep, while ARQ/ARQ sheep only developed disease after inoculation via the intracerebral route. Additionally, sheep scrapie can be transmitted from ingestion of prions shed in the environment and/or bodily fluids, as well contaminated environmental sources. Therefore it is possible that oral lesions may facilitate susceptibility to scrapie transmission. In this study, we aim to investigate the infectivity of decreasing doses of the x124 scrapie agent (100mg, 50mg, 20mg, and 10mg) on incubation time and attack rate after experimental intralingual inoculation into susceptible VRQ/ARQ sheep. Our results indicate that the lowest inoculum dose tested in this study effectively transmitted the x124 scrapie agent in VRQ/ARQ sheep, with a 100% attack rate, and no significant difference in incubation times among sheep inoculated with varying doses. Moreover, immunohistochemistry and western blot analysis revealed similar biochemical and immunohistochemical features among the four cohorts of sheep irrespective of inoculum dose. This study provides a starting point for further investigation to determine the minimum infectious dose of x124 scrapie in sheep and its effect on attack rate and incubation time, central for assessing the potential risk of scrapie occurrence in sheep flock.