Scrapie is a degenerative, fatal disease of the CNS of sheep and goats. Clinical signs, when they are present, often include ataxia and recumbency. Scrapie cannot be treated and is best diagnosed by microscopic postmortem exam. Selective breeding for genetic resistance, surveillance, and depopulation are the primary means of controlling this disease.
Scrapie is a disease of sheep and goats classified as a transmissible spongiform encephalopathy (TSE). TSEs are infective, fatal neurodegenerative disorders due to prions (misfolded copies of naturally occurring proteins) that convert normally folded proteins into abnormally folded duplicates of the prion. Other TSEs include human Creutzfeldt-Jakob disease (CJD), bovine spongiform encephalopathy (BSE), and cervid chronic wasting disease (CWD). Descriptions of classical scrapie have been documented for three centuries in western Europe. Today classical scrapie can be found worldwide, except for in Australia and New Zealand, which are widely considered free of the disease. Atypical scrapie has been identified worldwide, including in both Australia and New Zealand.
Etiology, Transmission, and Pathogenesis of Scrapie
Etiology
Scrapie results from misfolding of the cellular prion protein (PrPc) into an abnormal state (PrPsc) and subsequent aggregation of the abnormal protein in tissues. PrPc is a cell surface protein encoded by the PRNP gene, which is highly conserved in mammals and birds. PRNP is strongly expressed in the central and peripheral nervous systems; in neurons and glial cells; and in lymphoid, reproductive, and gastrointestinal tissues. Proposed functions of PrPc include myelin maintenance, memory formation, circadian rhythm stabilization, calcium homeostasis, and neuroprotection. However, a complete determination and understanding of the physiological roles of PRPc remain elusive. Removing the PRNP gene in mice is not fatal; however, spatial cognitive abnormalities are noted in these animals. Genetically unrelated PrPc-deficient goats have been found in multiple flocks in Norway. These goats exhibit no detectable differences from flockmates with PrPc; however, they do not develop scrapie when experimentally inoculated with PrPsc.
Since 1998, two types of scrapie have been identified: classical scrapie and atypical scrapie. Classical scrapie is the historically documented disease that occurs in young and old animals and exhibits vertical and horizontal transmission. Genotypic resistance to this type of scrapie has been documented in both sheep and goats. Atypical scrapie was originally described in Norwegian sheep in 1998; however, retrospective analysis showed the presence of atypical scrapie in the United Kingdom a decade earlier.
Atypical scrapie is a spontaneously occurring prion disease that affects single, older animals in a flock or herd. It can occur in animals considered genetically resistant to classical scrapie. PrPsc deposition and spongiform changes are different from those of classical scrapie. Atypical PrPsc is present in lymphoid tissue, but it cannot be detected by traditional immunohistochemical techniques. Atypical scrapie has been found in countries considered free of classical scrapie. Because of the spontaneous nature and poor transmissibility of this disease, however, detection of the atypical disease does not change a country’s scrapie status for international trade.
Resistance and Susceptibility
Resistance to scrapie occurs at a genetic level. Changes to the nucleotide sequence of the PRNP gene (polymorphisms) that subsequently alter the amino acid sequence of the PrPc protein influence an animal’s susceptibility or resistance to classical scrapie. These genetic markers are the basis for the genetic screening tests performed in many countries.
In sheep, three codons—136, 154 and 171—play important roles in determining susceptibility to classical scrapie. Of the three, codon 171 is the most influential. Sheep with valine (V) at codon 136, arginine (R) at codon 154, and glutamine (Q) at codon 171 are considered the most susceptible to scrapie. By contrast, sheep with alanine (A) at codon 136, histidine (H) at codon 154, and arginine (R) at codon 171 are considered resistant to scrapie. A lysine (K) at codon 171 occurs infrequently but may impart partial resistance. Overall, the most common haplotypes found in sheep are ARR, ARQ, AHQ, ARH, and VRQ. The ARR/ARR genotype is considered highly resistant to scrapie. Heterozygous ARR and AHQ genotypes display partial resistance. The VRQ/VRQ, ARQ/VRQ, and ARQ/ARQ genotypes are the most susceptible to classical scrapie.
In goats, genetic susceptibility to classical scrapie is less well understood; however, recent studies and genetic tests have made initial genetic screening exams possible. Isoleucine (I) at codon 142 may increase susceptibility, and methionine (M) at the same codon prolongs the incubation period for classical scrapie in goats but does not confer resistance. Asparagine (N) at codon 146 increases susceptibility, but serine (S) at the same codon increases resistance. Histidine (H) at codon 154, glutamine (Q) at codon 211, or lysine (K) at codon 222 all appear to impart some resistance to scrapie in goats. Current commercial tests evaluate codons 146 and 222. Goats with an NQ/NQ genotype are considered susceptible to scrapie. The NQ/NK, NK/NK, NQ/SQ, SQ/SQ, and NQ/SK genotypes are all considered to have increased resistance. In one outbreak of classical scrapie in a herd of 2,500 goats, no goats that carried an allele of the PRNP gene that is believed to confer resistance (S146, Q211, or K222) were found to be scrapie positive.
Resistance/susceptibility to atypical scrapie also appears to have a genetic component. In sheep, the VRQ haplotype, which is very susceptible to classical scrapie, is more resistant to atypical scrapie. The four other common sheep haplotypes are more susceptible to atypical scrapie, including the ARR haplotype, which is highly resistant to classical scrapie. Codon 154 appears to be important in both sheep and goats, given that histidine at that position has been linked to greater susceptibility in both species.
With classical scrapie, age also affects susceptibility. The younger the animal is when exposed to the disease, the more susceptible it is. To develop disease, lambs >4 months old at the time of exposure require inoculation with 30 times more PrPsc than lambs exposed at 12 hours old require.
A historical belief in breed-related susceptibility to classical scrapie has been invalidated. In the US, black-faced breeds have traditionally been considered more susceptible to scrapie than white-faced breeds; however, scrapie is found in many white-faced breeds worldwide. Furthermore, through selective breeding programs highlighting resistant genotypes, as well as culling of scrapie-infected animals, the prevalence of scrapie among white, mottled, and black-faced sheep is now equal in the US.
Transmission
Classical scrapie is transmitted predominantly through oral exposure in a contaminated environment. Experimental studies suggest that the disease may also be transmitted when PrPsc contacts conjunctiva, nasal passages, and abraded skin. Environmental contamination results principally from the shedding of PrPsc in the placenta and placental fluids. However, PrPsc is shed also in feces, saliva, and urine.
Once in the environment, PrPsc can be found on multiple surfaces (particularly metal items), in airborne dust samples, and on pasture. PrPsc persists for years in the environment. However, its durability can be influenced by local climate conditions. Cycles of moisture and desiccation may decrease the amount of PrPsc in the environment; in cool, stable climates such as Iceland, however, environmental infectivity has reportedly persisted at least 16 years. Scavengers and predators may further disseminate environmental PrPsc, because it remains infective as it passes through the GI tracts of these animals when they are exposed experimentally. PrPsc has been found in semen, but so rarely and in amounts so small, that breeding is still considered to pose minimal risk of transmission. Blood and tissue concentrations of PrPsc rise in infected animals before the onset of clinical signs, so iatrogenic transmission is possible, although documented accounts of such transmission events are rare.
PrPsc occurs at very high concentrations in placental tissue and fluids and is also shed in colostrum and milk, making the periparturient period the riskiest time for the transmission of classical scrapie. Most animals are thought to be infected from their dam at or near birth. Horizontal transmission to other members of the herd or flock that contact fetal membranes and fluids increases during this time as well. Evidence for in utero transmission has been documented. In one study of experimentally infected pregnant ewes, even delivering lambs via cesarean section and removing them immediately from the dam did not break the transmission cycle.
Goats have lower amounts of PrPsc in their placentas than sheep do; however, the amount is still sufficient for transmission between animals, especially via an oral route. Although transmission of classical scrapie in goats historically has most commonly occurred in animals that have commingled with sheep, transmission also occurs within goat herds that have had no contact with sheep.
No evidence of natural transmission of atypical scrapie in either species has been documented.
Pathogenesis
The incubation period for classical scrapie is 2–7 years, and most sheep show clinical signs between the ages of 2 and 5. The incubation period has three phases of pathogenesis: entry into the gut-associated lymphoid tissue (GALT), lymphoid dissemination, and neuroinvasion. Once ingested, PrPsc enters the palatine tonsil and crosses the intestinal wall, passing into the lymph and blood with the help of M cells (specialized intestinal epithelial cells found in GALT). PrPsc begins accumulating in the GALT. Peyer’s patches are considered a crucial point of entry for the prion. This site of early accumulation may explain aspects of age susceptibility to scrapie, because GALT tissue decreases in amount as animals age, making prion entry in older animals much less efficient.
After accumulation in the GALT, PrPsc travels to other lymphoid tissues and other organs throughout the body by lymph drainage and hematogenous spread. After wide tissue dissemination of PrPsc, neuroinvasion begins in the enteric nervous system. PrPsc moves in retrograde fashion along sympathetic and parasympathetic neurons until it reaches the spinal cord and eventually the brain. Once in the brain, PrPsc begins to cause neurodegeneration; however, the exact pathway of pathogenesis is unknown. Direct neurotoxicity is unlikely, but indirect toxic effects and loss of neuroprotection may play roles.
Relationship of Codon Changes in the PNRP Gene to Scrapie Susceptibility or Resistance in Sheep and Goats.
Genotype | Susceptible | Partially Resistant | Resistant |
|---|---|---|---|
Sheep (codons 136, 154, 171) | VRQ/VRQ ARQ/VRQ ARQ/ARQ Commonly known as QQ | ARR / heterozygous AHQ / heterozygous | ARR/ARR, commonly known as RR |
Goats (codons 146, 222) | NQ/NQ | SQ/SQ NQ/SQ NQ/SK NQ/NK NK/NK | Unknown |
Abbreviations are the standard one-letter abbreviations for amino acids: A, Alanine; H, Histidine; K, Lysine; N, Asparagine; Q, Glutamine; R, Arginine; S, Serine; V, Valine. Codons are listed in numerical order. For example, V136R154Q171 is written as VRQ. | |||
Clinical Findings of Scrapie
Courtesy of Dr. Joan S. Bowen.
Courtesy of Dr. Joan S. Bowen.
Clinical signs of classical scrapie are variable but progressive, and they are indicative of changes to the CNS. Classical scrapie in both sheep and goats has multiple strains, which are distinguished by differences in their molecular profile on Western blot, incubation period, clinical signs, postmortem lesions, and other characteristics. Depending on the infective strain, early clinical signs may be only behavioral, with infected animals isolating themselves from the herd or flock. Aggression is possible but rare. Ataxia, gait changes (high-stepping or hopping), and hypersensitivity to stimuli may also occur. The eponymous "scraping" (pruritus) is documented in a majority of sheep; however, it is not as common in goats. Goats are reported to nibble at body parts rather than rubbing them, and they may be more difficult to milk. In animals exhibiting pruritus, scratching of the tail head may lead to nibbling or other rhythmic body motions. Goats are less likely than sheep to respond in this way. Tremors of the head may be evident, as well as a decreased menace response and fixed stare.
Most animals begin to lose condition early in the disease process, in spite of a healthy appetite. Weight loss often progresses to emaciation. In some reports of scrapie-infected goats, weight loss and listlessness are the only clinical signs noted. Over the course of a few months, the affected animals become recumbent, unable to rise on their own, and die. With some strains of scrapie, death may occur within days of recumbency; with other strains, recumbent animals may survive for weeks.
Atypical scrapie may show similar clinical signs in both sheep and goats; pruritus, however, is uncommon with this form of the disease. Some animals with atypical scrapie may display no clinical signs and the disease is detected only through routine testing at slaughter.
Diagnosis of Scrapie
Postmortem microscopic tissue examination and immunohistochemical evaluation
No specific clinical signs occur with scrapie; however, many animals with the disease become ataxic and recumbent. Antemortem tests exist but have poor sensitivity. Because of the lack of consistent clinical signs, any sheep or goat that dies mysteriously or after exhibiting weight loss or CNS signs should have a postmortem necropsy in which tissues are collected for further scrapie testing. Common differential diagnoses include neurologic diseases such as meningitis, listeriosis, CNS abscesses, and rabies, as well as diseases associated with chronic wasting, such as caseous lymphadenitis, paratuberculosis, lentivirus infection (ovine progressive pneumonia or caprine arthritis and encephalitis, dentition abnormalities, and cancer.
No specific gross pathologic lesions are generally evident on necropsy. Lesions consistent with chronic weight loss and pruritus are the most common findings. Microscopic lesions of classical scrapie include neuronal vacuolation and noninflammatory spongiform changes in the CNS. Often these changes are found in the brainstem with classical scrapie; animals suffering from atypical scrapie, however, have lesions more rostral in the cerebellum and cerebral cortex and in the basal ganglia, if they have lesions at all.
Until the late 20th century, diagnostic testing for scrapie relied on finding vacuolization in brainstem tissue. Advancements in molecular testing since then have made the detection of PrPsc in tissue possible through the use of immunohistochemical (IHC) techniques, Western blotting, and ELISA. Testing availability depends on the country, and not all tests may be approved for use across national boundaries.
IHC analysis of brain tissue is the favored postmortem diagnostic test for scrapie. Sampling the medulla oblongata at the level of the obex and the cerebellum is ideal for detecting both classical and atypical scrapie. Atypical-scrapie PrPsc has not yet been detected by the use of IHC in tissues outside of the CNS. Classical-scrapie PrPsc, however, can be found in lymph tissue through IHC; the medial retropharyngeal lymph node or palatine tonsil are suggested sampling sites.
The presence of IHC-detectable classical-scrapie PrPsc in lymph tissue has made possible the development of antemortem tests for this disease. Samples of the rectal mucosa, tonsils, or conjunctiva of the third eyelid contain lymph tissue that can be infected by PrPsc. The sensitivity of these antemortem tests, however, is not ideal, and negative test results should be evaluated many times.
Murine bioassays and in vitro assays are also available for detecting PrPsc. These assays can be used to amplify very low concentrations of PrPsc and to discriminate various strains.
Control of Scrapie
Surveillance
Depopulation of infected herds
Tracing back of exposed animals
Selective breeding of naturally genetic-resistant animals
Scrapie cannot be cured and is resistant to most decontamination techniques. Control of scrapie must rely on surveillance, depopulation of infected herds, tracing back of exposed animals, and selective breeding of naturally genetic-resistant animals. Such control measures have already substantially decreased the incidence of scrapie in the US.
Decontamination for scrapie is extremely challenging. PrPsc is not degraded by detergents, nor does it break down during rendering, and it can survive ultraviolet light, ionizing radiation, many disinfectants, and heat sterilization. Combinations of chemical and physical decontamination methods are recommended. For disinfection, equipment can be soaked in 2.5N sodium hydroxide (NaOH) overnight, followed by porous-load autoclaving. Carcasses can be incinerated or digested in NaOH.
Because there is no treatment for scrapie, whether classical or atypical, and decontamination is challenging, the control of scrapie centers on surveillance, quarantining, depopulation of infected herds, official identification and tracing of infected animals, import restrictions, and selection of genetically resistant animals. According to the USDA Animal and Plant Health Inspection Service, these measures have decreased the incidence of classical scrapie in culled sheep in the US by 99% between 2003 and 2019. Scrapie is a reportable disease in many countries, and national/local reporting guidelines should be followed if it is detected.
Trans-species Transmission
Classical scrapie has been found naturally in mouflons (breed of wild sheep).1 Experimentally, it has been transmitted to voles, raccoons, and pigs.2,3,4. Although it has been hypothesized that the late-20th-century bovine spongiform encephalopathy (BSE) outbreak in the United Kingdom was due to cattle feeding on PrPsc-infected products, oral transmission of PrPsc to cattle has not been demonstrated under experimental conditions. When PrPsc is inoculated intracranially in cattle, the resulting disease is characteristically different from BSE. Chronic wasting disease (CWD) in cervids has also been hypothesized to come from exposure to classical scrapie. Of white-tailed deer experimentally exposed oronasally or intracranially to the classical-scrapie prion, 100% developed disease with a CWD-like Western blot pattern.5 However, continued research must be completed before any natural trans-species transmission can be proved. Experimental inoculations of elk show them to be more resistant to the classical-scrapie agent than are white-tailed deer.
A modified classical-scrapie prion has been successfully transmitted to macaques; to date, however, no zoonotic transmission of scrapie has been documented.
References
Jeffrey M, Gonzalez L. Classical sheep transmissible spongiform encephalopathies: pathogenesis, pathological phenotypes and clinical disease. Neuropathol Appl Neurobiol. 2007 Aug;33(4):373-94. doi: 10.1111/j.1365-2990.2007.00868.x
Nonna R, Di Bari M, Cardone F, et al. Efficient Transmission and Characterization of Creutzfeldt–Jakob Disease Strains in Bank Voles. PLoS Pathog. 2006 Feb; 2(2): e12. doi: 10.1371/journal.ppat.0020012
Hamir AN, Miller JM, O'Rourke KI, et al. Transmission of transmissible mink encephalopathy to raccoons (Procyon lotor) by intracerebral inoculation. J Vet Diagn Invest. 2004 Jan;16(1):57-63. doi: 10.1177/104063870401600110
Marin B, Lugan S, Espinosa JC, et al. Classical BSE prions emerge from asymptomatic pigs challenged with atypical/Nor98 scrapie. Sci Rep 11, 17428 (2021). doi.org/10.1038/s41598-021-96818-2
Cassmann ED, Frese RD, Greenlee JJ. Second passage of chronic wasting disease of mule deer to sheep by intracranial inoculation compared to classical scrapie. J Vet Diagn Invest. 2021 Jul;33(4):711-720. doi: 10.1177/10406387211017615
Key Points
Scrapie is a fatal degenerative prion disease of the CNS that occurs in classical and atypical forms.
The classical form is transmissible and found in every country of the world except Australia and New Zealand. The atypical form is spontaneous, nontransmissible, and found around the world, including in Australia and New Zealand.
Classical scrapie transmits vertically and horizontally and cannot be cured, and decontamination is extremely difficult.
Clinical signs of scrapie are vague and vary by species. Carriers can be clinically normal.
Control measures such as national surveillance and depopulation of infected herds have proved successful.
Certain haplotypes in both sheep and goats confer natural genetic resistance to scrapie.
For More Information
Center for Food Security & Public Health: Scrapie
World Organisation for Animal Health: Scrapie
USDA Factsheet: National Scrapie Eradication Program
