BSE can propagate in sheep co-infected or pre-infected with scrapie

Angela Chong,Matthew J O’Connor,E Fiona Houston,Martin Jeffrey,Keith Bishop,Kevin C Gough,Ben C Maddison,Nora Hunter,James D Foster,Lorenzo Gonzalez,Wilfred Goldmann

doi:10.1038/s41598-021-91397-8

Abstract

To understand the possible role of mixed-prion infections in disease presentation, the current study reports the co-infection of sheep with bovine spongiform encephalopathy (BSE) and scrapie. The bovine BSE agent was inoculated subcutaneously into sheep with ARQ/ARQ or VRQ/ARQ PRNP genotypes either at the same time as subcutaneous challenge with scrapie, or three months later. In addition, VRQ/VRQ sheep naturally infected with scrapie after being born into a scrapie-affected flock were challenged subcutaneously with BSE at eight or twenty one months-of-age. Sheep were analysed by incubation period/attack rate, and western blot of brain tissue determined the presence of BSE or scrapie-like PrPSc. Serial protein misfolding cyclic amplification (sPMCA) that can detect very low levels of BSE in the presence of an excess of scrapie agent was also applied to brain and lymphoreticular tissue. For VRQ/ARQ sheep challenged with mixed infections, scrapie-like incubation periods were produced, and no BSE agent was detected. However, whilst ARQ/ARQ sheep developed disease with BSE-like incubation periods, some animals had a dominant scrapie western blot phenotype in brain, but BSE was detected in these sheep by sPMCA. In addition, VRQ/VRQ animals challenged with BSE after natural exposure to scrapie had scrapie-like incubation periods and dominant scrapie PrPSc in brain, but one sheep had BSE detectable by sPMCA in the brain. Overall, the study demonstrates for the first time that for scrapie/BSE mixed infections, VRQ/ARQ sheep with experimental scrapie did not propagate BSE but VRQ/VRQ sheep with natural scrapie could propagate low levels of BSE, and whilst BSE readily propagated in ARQ/ARQ sheep it was not always the dominant PrPSc strain in brain tissue. Indeed, for several animals, a dominant scrapie biochemical phenotype in brain did not preclude the presence of BSE prion.

Highlights

To understand the possible role of mixed-prion infections in disease presentation, the current study reports the co-infection of sheep with bovine spongiform encephalopathy (BSE) and scrapie
The prion protein (PrP) is central to the pathogenesis of Transmissible spongiform encephalopathies (TSEs) and exists in two main isoforms. PrPC is a normal host protein which is expressed in many cell types especially in brain, and misfolded PrPSc which is so closely associated with TSE infectivity that it is believed to form part, or all, of the infectious agent[13]
Direct evidence for mixed infection within single sheep has been reported for classical scrapie and atypical scrapie[27, 28], and the report of PrPSc in natural sheep scrapie with features akin to both bovine spongiform encephalopathy (BSE) and classical scrapie could be the result of mixed infection or a hybrid strain phenotype[29]

Summary

Introduction

To understand the possible role of mixed-prion infections in disease presentation, the current study reports the co-infection of sheep with bovine spongiform encephalopathy (BSE) and scrapie. Whilst ARQ/ARQ sheep developed disease with BSE-like incubation periods, some animals had a dominant scrapie western blot phenotype in brain, but BSE was detected in these sheep by sPMCA. VRQ/VRQ animals challenged with BSE after natural exposure to scrapie had scrapie-like incubation periods and dominant scrapie PrPSc in brain, but one sheep had BSE detectable by sPMCA in the brain. Because of the indirect methods used to identify strains, it is necessary to use two strains with identifiable features, for example short and long incubation periods, different patterns of brain vacuolation (lesion profiles) and those producing PrPSc with distinguishable patterns on western blots and/or differential antibody binding. Further studies in hamsters have shown that following some mixed infections all clinical disease and pathology seem to result from the faster replicating strain, more sophisticated tests (PrPSc stability) suggest that the slower replicating strain is still present, perhaps at low levels[23]

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Conclusion