Scrapie: a transmissible neurodegenerative disease of sheep and goats

Clinical features and course

Animals affected by scrapie develop behavioral changes, pruritus (intense itching), abnormal gait, tremors and eventually profound neurological dysfunction. Signs typically progress slowly over months to years. Producers commonly notice wool loss from rubbing and wasting as late-stage signs. Key affected tissues and systems are detailed in reviews of the nervous system.

Cause and transmission

Current evidence implicates misfolded prion protein as the infectious agent. These prions induce normal host proteins to adopt an abnormal conformation, accumulating in central nervous system tissue and lymphoid organs. More on prion biology is available at prion resources. Transmission can occur horizontally between animals and vertically via placenta or milk under some circumstances.

History and relationship to bovine diseases

Scrapie has been recognized for centuries and was described in the 18th century. It is often compared with bovine spongiform encephalopathy (BSE, “mad cow disease”) because both involve prions; see comparative material at BSE comparisons. Although related, scrapie and BSE differ in species affected and in some epidemiological features; BSE primarily affects cattle.

Diagnosis, control and significance

Definitive diagnosis requires laboratory testing of brain or lymphoid tissue. Control relies on surveillance, selective breeding for resistant genotypes, culling of affected flocks, and movement controls. Because prions are unusually resistant to standard disinfectants, strict biosecurity and regulatory measures are important to limit spread and protect flocks.