Scrapie is a neurodegenerative disorder affecting sheep and goats, and it is definitively classified as a prion disease. Understanding this unique infectious agent, its mechanism, and its spread is important for animal health and for grasping the broader context of similar diseases in other species.
What is a Prion?
Prions are infectious agents composed entirely of protein, making them distinct from bacteria or viruses which contain genetic material like DNA or RNA. The term “prion” is a shortened form of “proteinaceous infectious particle.” This agent is a misfolded version of a normal protein, the cellular prion protein, which is naturally present in the brains of all mammals.
The normal, harmless version is designated PrPC, while the disease-causing, misfolded version is called PrPSc (with ‘Sc’ referencing Scrapie). When a PrPSc prion enters a healthy host, it acts as a template, forcing the normal PrPC molecules to change their shape into the abnormal, pathogenic form. This change involves the normal protein’s flexible coils refolding into flat structures known as beta strands.
This process creates a chain reaction where the number of misfolded prions increases exponentially within the central nervous system. Unlike the normal protein, the PrPSc form is highly resistant to breakdown by cellular enzymes, leading to its accumulation. The buildup of these proteins ultimately destroys neurons, causing the brain tissue to become riddled with microscopic holes, which gives the diseases their characteristic “spongiform” appearance. These diseases are progressive and fatal.
Scrapie: A Transmissible Spongiform Encephalopathy
Scrapie is the oldest known example of a Transmissible Spongiform Encephalopathy. The name of the disease is derived from one of its most common clinical signs: an intense, persistent itch that causes affected animals to compulsively rub or scrape their bodies against stationary objects. This chronic itching, known as pruritus, often leads to patches of wool loss and skin lesions.
Beyond the characteristic scraping behavior, Scrapie causes a progressive decline in neurological function. Clinical signs typically appear in animals between two and five years of age, following a long incubation period that can last for years. Affected sheep and goats display changes in gait, such as ataxia or incoordination, and may struggle to keep up with the rest of the flock.
Behavioral changes, head tremors, and excessive lip smacking are also observed as the neurodegeneration advances. Once clinical signs emerge, the disease progresses rapidly over a period of one to six months and is always fatal.
How Scrapie Spreads Among Sheep and Goats
Scrapie is highly contagious within ovine and caprine populations, with transmission occurring primarily through direct contact and environmental contamination. The most important route of spread is from mother to offspring, which is considered a form of vertical transmission. This occurs when susceptible young animals ingest infectious material, often immediately after birth.
Infected ewes shed high concentrations of prions, particularly within the placenta and birth fluids, which heavily contaminate the immediate environment during the birthing process. Lambs and kids can become infected by licking or ingesting this contaminated material, or even by consuming milk or colostrum from an infected dam. Lateral transmission, or spread between unrelated animals, also occurs when healthy animals graze on pastures or live in buildings previously used by infected flocks.
A challenge in controlling Scrapie is the remarkable resistance of the prion agent, which can persist in the soil and environment for several years. Prions are difficult to inactivate, resisting common sterilization methods, heat, and many chemical disinfectants. This environmental persistence means that a farm remains infectious long after infected animals have been removed, making eradication and decontamination extremely difficult.
Scrapie and the Link to Other Prion Diseases
Scrapie is part of a family of Transmissible Spongiform Encephalopathies that includes diseases like Bovine Spongiform Encephalopathy (BSE) in cattle, Chronic Wasting Disease (CWD) in deer and elk, and Creutzfeldt-Jakob Disease (CJD) in humans. The primary factor governing the transfer of a prion disease between different species is the “species barrier,” which relates to the difference in the amino acid sequence between the host’s normal prion protein and the invading prion. A greater difference in protein structure usually means a higher barrier to transmission.
Unlike BSE, which was conclusively linked to a new variant of CJD in humans, classical Scrapie is not known to be naturally transmissible to people. Despite Scrapie being endemic in many parts of the world for centuries, no credible epidemiological evidence connects it to any human prion disease. This historically negligible risk is generally attributed to the strong species barrier between sheep prions and human prion protein.
However, the scientific understanding of this barrier is complex, and some experimental studies have suggested that certain Scrapie strains could theoretically convert human prion protein. These findings have prompted a reassessment of the zoonotic potential, but the consensus remains that the risk to human health from Scrapie through consumption is extremely low, contrasting sharply with the known zoonotic nature of BSE. Control programs for Scrapie focus primarily on animal health and breeding for genetic resistance within sheep flocks.