Scrapie (prion disease)

Scrapie is a fatal, degenerative neurological disease of sheep and goats caused by abnormal prion proteins that belongs to the family of transmissible spongiform encephalopathies (TSEs), which also includes bovine spongiform encephalopathy in cattle and Creutzfeldt-Jakob disease in humans. First recognized in sheep in Great Britain more than 250 years ago, scrapie is the oldest known TSE and has been studied extensively as a model for understanding prion diseases. The disease derives its common name from the characteristic compulsive scraping behavior affected animals exhibit as they rub against fences, posts, and other objects in response to intense pruritis, though this sign is not universally present in all cases.

Scrapie occurs worldwide with the exception of Australia and New Zealand, which have remained free of the disease through strict import controls and biosecurity measures. The disease affects both sheep and goats, though sheep are more commonly diagnosed due to their greater susceptibility to classical scrapie strains and larger global population. Prevalence varies significantly between countries and regions, with active eradication programs having substantially reduced scrapie incidence in many developed countries. Atypical scrapie, a distinct form of the disease discovered more recently, appears to occur spontaneously at low rates even in populations free of classical scrapie.

The economic and welfare impact of scrapie extends beyond individual animal losses to encompass flock-level consequences and trade implications. Diagnosis of scrapie in a flock triggers regulatory responses that may include quarantine, testing requirements, and in some cases mandatory depopulation under eradication programs. International trade in sheep and goat genetics is significantly impacted by scrapie status, with importing countries often requiring freedom from scrapie or specific genetic testing of exported animals. The prolonged incubation period means that infected animals may be sold and distributed widely before disease becomes apparent, complicating traceback and control efforts.

Scrapie is a reportable disease in virtually all countries, requiring notification of veterinary authorities when diagnosed. No treatment or vaccine exists for scrapie, and the disease is invariably fatal. Control relies on genetic selection for resistance, surveillance and testing programs, and management practices to reduce environmental contamination. Understanding of prion genetics has enabled development of breeding programs that can significantly reduce or eliminate scrapie susceptibility in sheep flocks, offering a pathway to disease control not available for most infectious diseases.

Scrapie is caused by prions, which are abnormal misfolded forms of a normal cellular protein called PrP (prion protein). Unlike conventional infectious agents such as bacteria, viruses, or parasites, prions contain no nucleic acid and consist solely of protein. The abnormal prion protein (designated PrPSc for scrapie) acts as a template that converts normal cellular prion protein (PrPC) into the abnormal form, creating a chain reaction that progressively accumulates abnormal protein in nervous tissue. This accumulation causes the characteristic spongiform degeneration of brain tissue that gives the disease family its name.

Genetic susceptibility to scrapie is determined primarily by variations in the prion protein gene (PRNP), which codes for the normal cellular protein that becomes misfolded in disease. In sheep, polymorphisms at codons 136, 154, and 171 of the PRNP gene significantly influence susceptibility to classical scrapie, with certain combinations conferring high resistance while others confer high susceptibility. Animals with the ARR/ARR genotype at these positions are highly resistant to classical scrapie, while those with VRQ alleles are highly susceptible. Goats have different PRNP polymorphisms that influence their susceptibility to scrapie infection.

Environmental factors play a crucial role in scrapie transmission because the prion agent is remarkably stable in the environment and resistant to conventional disinfection procedures. Contaminated pastures, lambing areas, and housing can serve as sources of infection for susceptible animals for years after infected animals have been removed. The placenta and birth fluids of infected ewes contain high concentrations of infectious prions and represent the primary source of environmental contamination. Persistence of prions in soil, on equipment, and in facilities creates ongoing infection risk that is extremely difficult to eliminate.

Risk factors for scrapie infection include genetic susceptibility, exposure to contaminated environments, and contact with infected animals or their birth products. Lambs born to infected dams face high transmission risk both through ingestion of contaminated placental materials and through later environmental exposure. Introduction of infected animals into a flock, even if they do not show clinical signs during their time in the flock, can establish environmental contamination that persists long after the infected animal is gone. The long incubation period, typically two to five years, means animals may be infected long before diagnosis is possible.

The pathophysiology of scrapie involves initial prion replication in lymphoid tissues following oral exposure, followed by spread to the central nervous system where progressive neurodegeneration occurs. After ingestion, prions accumulate in gut-associated lymphoid tissue and spread to other lymphoid organs including spleen and lymph nodes. Transit to the nervous system occurs along peripheral nerves, eventually reaching the brain and spinal cord. Progressive accumulation of abnormal prion protein in neurons causes cell death and the characteristic spongy appearance of affected brain tissue visible on histological examination.

Early warning signs of scrapie are subtle and easily missed without careful observation of individual animal behavior. Initial changes may include subtle alterations in temperament, with previously calm animals becoming nervous or previously social animals isolating themselves from the flock. Slight changes in gait or coordination may be apparent only to observers very familiar with the individual animal's normal movement. Minor weight loss despite adequate nutrition can precede more obvious neurological signs by weeks to months. These early changes are often attributed to other causes or dismissed as individual variation.

Clinical presentation of scrapie in sheep and goats shares core features but can vary significantly between individuals and between classical and atypical forms of the disease. Classical scrapie typically presents with a combination of pruritis causing wool loss and skin trauma, progressive neurological dysfunction, and weight loss despite maintained appetite. Goats infected with scrapie may show less pronounced pruritis than sheep and more prominent neurological signs. Atypical scrapie often presents primarily with ataxia and neurological dysfunction without the characteristic pruritis of classical scrapie.

Behavioral changes in scrapie-affected animals reflect progressive brain dysfunction and are among the most consistent disease features. Affected animals often become hyperexcitable and may startle exaggeratedly at normal stimuli. Separation from the flock and apparent apprehension or anxiety are commonly observed. Lip smacking, teeth grinding, and abnormal postures may develop. Changes in voice may be noticed by attentive caretakers. Some animals develop a characteristic high-stepping gait or bunny-hopping hindquarter movement when moved quickly.

Physical signs of scrapie become more obvious as the disease progresses. The characteristic pruritis leads animals to rub, scrape, and bite at their wool or hair, causing wool loss in patches particularly over the hindquarters, flanks, and head. The nibble reflex, where light scratching of the back elicits lip smacking and head elevation, is highly suggestive of scrapie when present. Progressive incoordination affecting primarily the hindquarters develops, with swaying, stumbling, and difficulty navigating terrain. Tremors of the head and neck may be observed, particularly when the animal is excited or stressed.

Symptom progression in scrapie follows a gradually worsening course over weeks to months once clinical signs become apparent. Early subtle signs give way to obvious behavioral abnormalities and pruritis. Neurological dysfunction progresses from mild ataxia to severe incoordination and difficulty rising. Weight loss accelerates as feeding behavior is impaired by neurological dysfunction. Eventually affected animals become recumbent and unable to rise. Death occurs typically within two weeks to six months of first obvious clinical signs, though the overall disease course from infection to death spans years.

Emergency symptoms in scrapie relate to welfare concerns requiring intervention rather than treatable conditions. Animals that become recumbent and unable to rise face suffering from inability to access food and water, exposure to environmental conditions, and pressure sores from prolonged lying. Severe wool loss and skin trauma from compulsive rubbing can lead to secondary infections and fly strike. These welfare concerns typically prompt euthanasia decisions rather than treatment attempts, as scrapie cannot be treated and clinical deterioration is inevitable.

Clinical examination of animals suspected of scrapie should document neurological abnormalities, pruritis, body condition, and behavioral changes consistent with the disease. Testing the nibble reflex by scratching along the back can elicit the characteristic lip-smacking and head elevation response. Neurological examination assesses gait, coordination, reflexes, and mental status. Documenting wool loss patterns and skin lesions from rubbing supports clinical suspicion. However, clinical signs alone cannot definitively diagnose scrapie, and laboratory confirmation is required.

Laboratory diagnostic testing for scrapie has advanced significantly with the development of sensitive immunological and biochemical methods for detecting abnormal prion protein. Definitive diagnosis requires examination of brain tissue, traditionally through histopathology revealing spongiform changes and immunohistochemistry detecting PrPSc accumulation. Third eyelid lymphoid tissue biopsy provides an ante-mortem diagnostic option for classical scrapie in sheep with certain genotypes, as prions accumulate in lymphoid tissue before reaching the brain. Rapid screening tests using ELISA or Western blot techniques enable large-scale surveillance testing of brain samples from slaughter or fallen stock.

Differential diagnosis of scrapie must consider other conditions causing neurological signs and behavioral changes in sheep and goats. Listeriosis produces neurological dysfunction but typically has more acute onset and may affect multiple animals simultaneously. Rabies causes progressive neurological disease but follows a much shorter clinical course. Pregnancy toxemia affects late-pregnant animals with distinct metabolic derangements. Polioencephalomalacia causes neurological signs that may respond to thiamine treatment. External parasites can cause pruritis and wool loss without neurological involvement. Maedi-visna can cause weight loss and weakness but has a different clinical pattern.

Flock-level diagnostic approaches for scrapie include genotyping to assess genetic susceptibility, surveillance testing of cull and fallen animals, and epidemiological investigation following positive diagnosis. PRNP genotyping characterizes the genetic risk profile of the flock and identifies individuals suitable for resistant breeding programs. Testing brain samples from all adult sheep that die or are culled enables detection of subclinical cases before environmental contamination spreads. When scrapie is diagnosed, traceback investigation identifies source flocks and trace-forward identifies flocks that may have received infected animals.

There is no treatment for scrapie in any animal. The abnormal prion protein that causes disease cannot be eliminated or neutralized by any known therapeutic intervention, and the neurological damage caused by prion accumulation is irreversible. This fundamental untreatable nature of scrapie has been consistent since the disease was first described and despite extensive research into prion diseases generally. Animals diagnosed with scrapie will invariably progress to death from the disease.

Medical management cannot alter the course of scrapie but may address welfare concerns during the clinical phase. Providing easily accessible food and water accommodates animals with impaired coordination. Protection from environmental extremes reduces additional stress on compromised animals. Management of secondary conditions such as skin infections from rubbing may provide temporary comfort. However, these measures cannot prevent disease progression, and euthanasia is typically appropriate once significant clinical signs develop.

Surgical intervention has no role in scrapie management. The disease process occurs within the central nervous system in ways that cannot be addressed surgically. No biopsy, debulking, or other surgical procedure affects disease outcome. Ante-mortem biopsy of lymphoid tissue serves diagnostic purposes only and does not represent treatment.

Supportive care for clinically affected scrapie animals focuses on maintaining comfort and quality of life during the terminal phase of disease. Providing soft, dry bedding helps prevent pressure sores in animals with reduced mobility. Ensuring feed and water are easily accessible supports nutrition despite neurological impairment. Separating affected animals from the flock reduces stress from competition and may reduce environmental contamination. These measures support humane management during the period before euthanasia becomes necessary.

Flock-level management following scrapie diagnosis focuses on preventing further transmission and reducing future disease occurrence rather than treating affected individuals. Removal and euthanasia of clinically affected animals eliminates active sources of environmental contamination. Genotyping remaining animals identifies those at highest risk for future disease and those suitable for resistant breeding programs. Enhanced monitoring enables early detection of additional cases. These population-level interventions represent the only meaningful response to scrapie diagnosis.

Decisions regarding individual animals with scrapie must balance animal welfare against diagnostic and regulatory requirements. Clinically affected animals should be humanely euthanized when quality of life deteriorates to unacceptable levels. Brain tissue should be collected for confirmatory testing and strain characterization. Carcass disposal must follow regulations for potentially infectious material, typically incineration. Owners should be counseled about the lack of treatment options and the importance of euthanasia for animal welfare.

Recovery from scrapie does not occur. The disease is invariably fatal once the neurodegenerative process begins, and no animal has been documented to survive clinical scrapie. The abnormal prion proteins continue accumulating in the brain until neurological function is sufficiently compromised that the animal can no longer survive. This absolute fatality is characteristic of all prion diseases across species and reflects the inability of any known biological process to clear or neutralize misfolded prion proteins once they begin accumulating.

Post-diagnosis management of scrapie-affected flocks focuses on controlling disease spread and reducing future occurrence rather than recovery of affected individuals. Following diagnosis, regulatory authorities typically impose movement restrictions on the affected flock pending investigation. Testing and genotyping programs characterize the scope of infection and genetic susceptibility within the flock. Decisions about flock depopulation versus selective culling depend on regulatory programs, genotype distribution, and owner circumstances.

Prognostic factors for individual animals with scrapie relate only to the timeline of disease progression rather than survival prospects. Animals with early subtle signs may survive for several months before clinical deterioration requires euthanasia. Animals with advanced neurological dysfunction or severe welfare compromise have short survival times. Genotype influences incubation period, with more susceptible genotypes typically showing earlier disease onset, but all infected animals regardless of genotype eventually develop fatal disease.

Flock recovery from scrapie occurrence is possible through genetic improvement and management changes over time, though individual animals cannot recover. Selective breeding for resistant PRNP genotypes progressively reduces the proportion of susceptible animals in subsequent generations. Combined with appropriate environmental management and sourcing of resistant replacement stock, flocks can rebuild with substantially reduced scrapie risk. This recovery process typically spans multiple years as resistant genetics are established throughout the flock.

Vaccination against scrapie is not possible because prions do not trigger immune responses in the conventional way that bacteria and viruses do. The abnormal prion protein is derived from a normal host protein, and the immune system does not recognize it as foreign. Research into potential vaccine approaches continues, but no practical scrapie vaccine is available or anticipated in the near term. Prevention must rely on genetic selection, biosecurity, and surveillance rather than immunization.

Biosecurity measures for scrapie prevention focus on excluding the agent from naive flocks and reducing environmental contamination in affected flocks. Avoiding introduction of animals from scrapie-affected or unknown-status flocks prevents introduction of infection. Quarantine of new animals with observation before integration provides a safety period, though the long incubation period limits effectiveness. Management of lambing to reduce contamination from potentially infected birth products includes immediate removal of placentas and thorough cleaning of lambing areas.

Nutritional factors do not influence scrapie susceptibility, as the disease results from prion exposure in genetically susceptible animals rather than nutritional deficiency or imbalance. Maintaining good nutrition supports general animal health but cannot prevent scrapie in exposed susceptible animals. No dietary intervention alters prion susceptibility or disease progression.

Genetic selection represents the most effective prevention strategy for scrapie in sheep flocks. Selection of breeding rams with resistant PRNP genotypes, particularly ARR/ARR at codons 136, 154, and 171, produces offspring with substantially reduced scrapie susceptibility. Many countries have established genotyping and recording systems that facilitate selection for scrapie resistance. Progressive genetic improvement can effectively eliminate classical scrapie susceptibility from flocks within several generations of selective breeding. Similar genotyping approaches are being developed for goats, though the genetics are more complex.

Surveillance and testing programs enable early detection of scrapie and inform control efforts. Active surveillance through testing of fallen stock and cull animals identifies affected flocks before clinical disease becomes widespread. Genotyping of breeding stock identifies susceptible animals for management decisions. Participation in national scrapie eradication programs supports progress toward reduced national prevalence and enables certification of flock status. These programs provide the information needed for effective genetic selection and biosecurity implementation.

Daily management and monitoring for scrapie prevention involves awareness of disease signs and careful observation of animal behavior and condition. Watching for subtle behavioral changes, loss of body condition, or pruritic behavior enables early detection of affected animals. Observing animals during quiet periods when characteristic behaviors are most likely to be displayed improves detection sensitivity. Prompt investigation of any animal showing compatible signs ensures timely diagnosis and management. Regular body condition scoring identifies weight loss that might indicate early scrapie.

Housing and environmental management for scrapie prevention addresses the persistence of prions in contaminated environments. Lambing areas represent the highest contamination risk and should be managed to minimize environmental contamination. Prompt removal and disposal of placentas and birth fluids reduces contamination. Bedding from lambing areas should be disposed of appropriately rather than spread on pastures. In known-affected premises, environmental decontamination is extremely difficult due to prion resistance to conventional disinfection, and pasture rest periods of several years may be needed.

Flock health programs addressing scrapie should incorporate genetic assessment, surveillance, and breeding strategies. Initial genotyping of the flock characterizes the genetic risk profile and identifies animals suitable for breeding programs. Developing a genetic improvement plan with progressive selection for resistance reduces future disease risk. Establishing protocols for testing cull and fallen animals supports ongoing surveillance. Participating in official scrapie control programs enables certification of flock status and demonstrates commitment to disease control.

Record keeping and monitoring systems for scrapie management document genotypes, lineage, and disease status. Individual animal identification linked to PRNP genotype results enables informed breeding decisions and traceback if disease occurs. Recording parentage allows prediction of offspring genotypes and tracking of genetic improvement progress. Documenting any deaths or clinical abnormalities creates a record for investigation if scrapie is later diagnosed. These records support both routine genetic improvement and emergency response.

Economic considerations for scrapie management include genotyping costs, testing fees, and the impact of disease status on animal values. Initial whole-flock genotyping requires substantial investment but enables targeted genetic improvement. Ongoing genotyping of replacement stock maintains program progress. Rams with documented resistant genotypes command premium prices due to their value in scrapie control programs. Certification of scrapie-free or scrapie-resistant status enhances marketability of breeding stock. These investments typically generate returns through improved market access and reduced disease risk.

All breeds of sheep can develop scrapie, but susceptibility varies significantly based on PRNP genotype frequency within breeds. Some breeds have been found to have higher frequencies of susceptible genotypes, though active selection programs are changing genotype distributions in many populations. Suffolk sheep in some countries historically had higher scrapie incidence, potentially related to genotype frequencies, though selection has reduced susceptibility in many Suffolk populations. Other meat breeds with historically higher susceptibility have similarly benefited from genetic selection programs.

Production type considerations influence how scrapie impacts different sheep operations. Breeding stock operations face significant consequences from scrapie diagnosis due to trade restrictions and reduced marketability of genetics from affected flocks. Commercial operations producing lambs for meat experience direct production losses but may have less long-term business impact if genetic stock is not the primary product. Dairy sheep operations face both production losses and potential concerns about milk from infected animals, though scrapie prions are not known to be present in milk from clinically normal animals.

Genetic selection and testing form the foundation of scrapie risk management in modern sheep production. PRNP genotyping identifies individual animal susceptibility and enables targeted breeding decisions. Selecting rams that are ARR/ARR at the critical codons produces offspring with significantly enhanced resistance to classical scrapie. National genotyping schemes in many countries facilitate genetic improvement by providing standardized testing and recording systems. Over time, consistent selection for resistance can effectively eliminate classical scrapie susceptibility from a flock while maintaining other desirable production characteristics.

Scrapie shares fundamental disease mechanisms with other transmissible spongiform encephalopathies, creating a family of related prion diseases across species. Bovine spongiform encephalopathy (BSE) in cattle caused significant concern when first recognized due to its transmissibility to humans causing variant Creutzfeldt-Jakob disease. Chronic wasting disease in cervids represents an expanding wildlife prion disease with implications for both wild and farmed deer populations. Understanding relationships between these prion diseases informs both research and regulatory approaches to scrapie control.

Conditions with similar clinical presentations to scrapie must be differentiated for accurate diagnosis and appropriate management. Listeriosis causes neurological signs in sheep and goats but typically has acute onset and may respond to early antibiotic treatment. Maedi-visna causes progressive weight loss and weakness but without the characteristic pruritis of classical scrapie. Rabies produces neurological disease but follows a much shorter clinical course. External parasite infestations can cause pruritis and wool loss without neurological involvement. Sarcoptic mange, lice, and other ectoparasites should be ruled out in animals showing pruritis without neurological signs.

Complications of scrapie relate primarily to welfare consequences of the progressive neurological and dermatological manifestations rather than secondary disease processes. Severe wool loss and skin damage from compulsive rubbing predispose to secondary bacterial skin infections and fly strike during warm weather. Progressive incoordination leads to injuries from falls and difficulty accessing feed and water. Recumbency in terminal cases causes pressure sores and exposure. These complications typically prompt euthanasia decisions before they become severe, as the underlying scrapie cannot be treated and clinical deterioration is inevitable.