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United States Department of Agriculture

Agricultural Research Service

Related Topics


Location: Virus and Prion Research

2015 Annual Report

1a. Objectives (from AD-416):
1. Investigate the pathobiology of atypical transmissible spongiform encephalopathies (TSEs) in natural hosts. A. Investigate the pathobiology of atypical scrapie. B. Investigate the pathobiology of atypical bovine spongiform encephalopathy (BSE). 2. Investigate the horizontal transmission of TSEs. A. Assess the horizontal transmission of sheep scrapie in the absence of lambing. B. Determine routes of transmission in chronic wasting disease (CWD) infected premises. C. Assess oral transmission of CWD in reindeer. 3. Investigate determinants of CWD persistence. A. Determine CWD host range using natural routes of transmission. B. Investigate the pathobiology of CWD.

1b. Approach (from AD-416):
The studies will focus on three animal transmissible spongiform encephalopathy (TSE) agents found in the United States: bovine spongiform encephalopathy (BSE); scrapie of sheep and goats; and chronic wasting disease (CWD) of deer, elk, and moose. The research will address sites of accumulation, routes of infection, environmental persistence, and ante mortem diagnostics with an emphasis on controlled conditions and natural routes of infection. Techniques used will include clinical exams, histopathology, immunohistochemistry and biochemical analysis of proteins. The enhanced knowledge gained from this work will help mitigate the potential for unrecognized epidemic expansions of these diseases in populations of animals that could either directly or indirectly affect food animals.

3. Progress Report:
Research efforts directed toward meeting objective 1 of our project plan include work in previous years starting with the inoculation of animals for studies designed to address the pathobiology of atypical scrapie, atypical bovine spongiform encephalopathy (BSE), as well as a genetic version of BSE. Post-mortem examination of the animals inoculated with atypical scrapie has been initiated and laboratory analysis of the tissues is ongoing. Atypical BSE animals have developed disease and evaluation of the samples is currently underway. Animals inoculated with a genetic version of BSE have developed disease with a manuscript reporting these results was published (2012), and additional laboratory comparisons of genetic BSE to atypical and classical BSE are ongoing. In addition, we have investigated the possibility that atypical scrapie was present earlier than previously detected in the national flock by analyzing archived field isolates using methods that were unavailable at the time of original diagnosis. Sample quality was sufficiently degraded that modern methods, beyond those applied to the tissues at the time the tissues were archived, were not suitable for evaluation. In research pertaining to objective 2, "Investigate the horizontal transmission of TSEs", we have initiated a study to determine if cohousing non-lambing scrapie inoculated sheep is sufficient to transmit scrapie to neonatal lambs. At this time, scrapie free ewes have lambed in the presence of scrapie inoculated animals and the lambs are cohoused with these inoculated animals.

4. Accomplishments
1. Changes in retinal function in cattle can be used to identify different types of bovine spongiform encephalopathy (BSE). BSE belongs to a group of fatal, transmissible protein misfolding diseases known as transmissible spongiform encephalopathies (TSEs). Like other protein misfolding diseases including Parkinson's disease and Alzheimer's disease, TSEs are generally not diagnosed until the onset of disease after the appearance of unequivocal clinical signs. As such, identification of the earliest clinical signs of disease may facilitate diagnosis. The retina is the most accessible part of the central nervous system. ARS scientist in Ames IA described antemortem changes in retinal function and thickness that are detectable in BSE inoculated animals up to 11 months prior to the appearance of any other signs of clinical disease. Differences in the severity of these clinical signs reflect the amount of PrPSc accumulation in the retina and the resulting inflammatory response of the tissue. These results are the earliest reported clinical signs associated with TSE infection and provide a basis for understanding the pathology and evaluating therapeutic interventions. Further, this work shows that High-type BSE and classical BSE can be differentiated by eye examination alone, the first time BSE strains have been differentiable in a live animal.

2. Sheep genetics influences the susceptibility of sheep to scrapie. Sheep scrapie is a transmissible spongiform encephalopathy that can be transmitted between affected animals resulting in significant economic losses in affected flocks. The prion protein gene (PRNP) profoundly influences the susceptibility of sheep to the scrapie agent and the tissue levels and distribution of PrPSc in affected sheep. In this study, sheep of 3 different prion genetic types (denoted VRQ/VRQ, VRQ/ARR and ARQ/ARR) were inoculated and subsequently euthanized upon onset of disease. Disease aspects were uniform across genotypes and consistent with manifestations of classical scrapie. Mean survival time differences were associated with the genetic type such that VRQ/VRQ sheep survived 18 months, whereas VRQ/ARR and ARQ/ARR sheep survived 60 and 56 months, respectively. Microscopic evaluation revealed similar accumulations in central nervous system tissues regardless of host genetic type. PrPSc in lymphoid tissue was consistently abundant in VRQ/VRQ, present but confined to tonsil or retropharyngeal lymph node in 4/5 VRQ/ARR, and totally absent in ARQ/ARR sheep. The results of this study demonstrate the susceptibility of sheep with the ARQ/ARR genotype to scrapie by the intracranial inoculation route with PrPSc accumulation in CNS tissues, but prolonged incubation times and lack of PrPSc in lymphoid tissue. These results are important for science based policy with regard to testing of sheep for scrapie where some live animal testing is conducted using lymphoid tissues which would not detect scrapie in some specific genetic types which could limit the national scrapie eradication program.

Review Publications
Greenlee J.J. 2014. The prion diseases of animals. In: McManus, L.M., Mitchell, R.N., editors. Pathobiology of Human Disease. San Diego: Elsevier. p. 1124-1133.

Greenlee, J.J., Kunkle, R.A., Richt, J.A., Nicholson, E.M., Hamir, A.N. 2014. Lack of prion accumulation in lymphoid tissues of PRNP ARQ/ARR sheep intracranially inoculated with the agent of scrapie. PLoS One. 9(9):e108029.

Munoz-Gutierrez, J.F., Schneider, D.A., Baszler, T.V., Dinkel, K.D., Greenlee, J.J., Nicholson, E.M., Stanton, J.J. 2015. hTERT-immortalized ovine microglia propagate natural scrapie isolates. Virus Research. 198:35-43.

Nicholson, E.M. 2015. Detection of the disease-associated form of the prion protein in biological samples. Bioanalysis. 7(2):253-261.

West Greenlee, M.H., Smith, J.D., Platt, E.M., Juarez, J.R., Timms, L.L, Greenlee, J.J. 2015. Changes in retinal function and morphology are early clinical signs of disease in cattle with bovine spongiform encephalopathy. PLoS ONE. 10(3):e0119431.

Greenlee, J.J., West Greenlee, M.,H. 2015. The transmissible spongiform encephalopathies of livestock. ILAR Journal. 56(1):7-25.

Comoy, E.E., Mikol, J., Luccantoni-Freire, S., Correia, E., Lescoutra-Etchegaray, N., Durand, V., Dehen, C., Andreoletti, O., Casalone, C., Richt, J.A., Greenlee, J.J., Baron, T., Benestad, S., Brown, P., Deslys, J. 2015. Transmission of scrapie prions to primate after an extended silent incubation period. Scientific Reports. 5:11573.

Last Modified: 08/17/2017
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