Final answer:
Multiple challenges existed in proving sheep-to-sheep transmission of scrapie because it is caused by prions, non-conventional infectious agents without DNA or RNA. This ran contrary to the scientific consensus of the time, leading to initial skepticism. However, the theory was ultimately accepted, which acknowledged Stanley Prusiner's groundbreaking work on prions.
Step-by-step explanation:
Proving Sheep-to-Sheep Transmission of Scrapie
Proving sheep-to-sheep transmission of scrapie was a significant challenge because this disease is caused by prions, which are proteinaceous infectious particles. Unlike bacteria or viruses, prions do not contain DNA or RNA, which made the concept of prions as infectious agents controversial. Stanley Prusiner's discovery in 1982 that scrapie was caused by prions was met with skepticism, in part because it contradicted the established understanding that all infectious agents must contain genetic material. However, Prusiner's concept was eventually accepted, and his work earned him a Nobel Prize. The difficulty in understanding the transmission of scrapie lies in the fact that prion diseases, or transmissible spongiform encephalopathies (TSEs), involve a structural variant of a normal cellular protein, PrP, turning into an infectious form, PrPSc. This process leads to the conversion of the normal protein into the infectious one, causing the disease.