S52 Vol. 23, No. 4 April 2001 Email comments/questions to compendium@medimedia.com CE Article #9 (1.5 contact hours) Refereed Peer Review KEY FACTS Clinical signs of scrapie have been 100% correlated with convergence of the normal protein (PrP C ) to the abnormal form (PrP SC ), a disease-related isoform in neurons. Genetic testing of sheep is now being used to identify and eliminate scrapie-susceptible flocks. Identification of the PrP SC protein in lymphoreticular tissue is used as a premortem test to identify sheep with scrapie before they show clinical signs. Scrapie: Deciphering Its Pathophysiology and Cause Tuskegee University Margo R. Machen, DVM, PhD ABSTRACT: Scrapie belongs to a group of fatal neurodegenerative diseases known as transmissible spongiform encephalopathies. Molecular research into the cellular pathophysiology of scrapie has identified the presence of a normal glycoprotein (PrP C ) and an abnormal form (PrP SC ) within neurons of affected sheep. The rate and ability of PrP C to be converted into PrP SC is dependent on the genetic susceptibility unique to each breed of sheep. Three codon positions (136, 154, and 171) determine genetic susceptibility. The combination of amino acids that occurs at these loci conveys resistance to scrapie infections and influences incubation periods. Scrapie is one of a group of fatal neurodegenerative diseases known as transmissible spongiform encephalopathies (TSEs). Although primarily considered an ovine disease, scrapie has been reported in goats. Other species that can be infected by TSEs are humans (e.g., kuru, Creutzfeldt-Jacob disease [CJD], Gerstmann-Straussler syndrome, fatal familial insomnia disease); mule deer and elk (e.g., chronic wasting disease); cattle (e.g., bovine spongiform encephalopathy [BSE]); domestic and zoo cats (e.g., feline spongiform encephalopathy); and mink (e.g., transmissible mink encephalopathy). All of the diseases in these species are slowly progressive, cause vacuolation in neurons leading to neurodegeneration, and are fatal. 1,2 Most TSEs have been shown to be transmissible experimentally and have a strong genetic component. There are several hypotheses concerning the cause of scrapie, and each attempts to link genetic susceptibility and the concept of an infectious agent. Researchers currently believe there are different strains of scrapie that vary in the breeds of sheep they affect and in the incubation period. 3 6 Variances in breed affected and incubation period are directly related to the genotype of the sheep. Recent molecular data have identified the presence of an abnormal glycoprotein (PrP) within neurons of sheep affected with scrapie. The normal form of this protein is denoted by PrP C and the abnormal form by PrP SC. Previous literature has indicated the presence of a gene (Sip) identified as affecting scrapie incubation time. It is now believed that the PrP gene and the Sip gene are the same, and current literature reflects the change in nomenclature. 7 This article reviews the current molecular research data on scrapie and its implications in deciphering the pathophysiology and cause of the disease.
Compendium April 2001 Food Animal S53 HISTORY Scrapie was first reported in sheep in England in 1732. 8 It became a major worldwide problem shortly after World War II because of greater sheep movement internationally. Only New Zealand and Australia are currently considered scrapie-free. 1,2,9 More attention is being given to TSEs because of the recent outbreak of BSE (mad cow disease) in Europe. BSE may be linked to a zoonotic form of TSE that has been identified in humans. In 1995, 10 human cases of a new variant of CJD (nv-cjd) were identified in England. The average age of onset of clinical signs for this new variant is 27 years, whereas the previously identified form, sporadic CJD (s-cjd), primarily affected elderly patients. 10 One of the causes of the outbreak in Europe was believed to be the practice of feeding processed sheep protein to cattle. 9 As a result, there are many new laws and restrictions in Europe concerning the elimination of waste products from sheep farming and banning the use of ruminant protein in feed for other ruminants. In the United States, the Animal and Plant Health Inspection Service has restricted the importation of live ruminants and ruminant products from other countries since 1989. Before this restriction, 496 cows were imported from England; however, all but 10 of these cows have been accounted for the majority have been slaughtered, and 17 are being monitored closely. 10 In 1997, the FDA also took steps to prevent the amplification and establishment of BSE by banning the use of mammalian protein (with the exception of pure pork or pure horse protein, blood and blood products, gelatin, milk products, and plate waste) in ruminant feeds. 10 In the United States, scrapie was first reported in a sheep flock in Michigan in 1947. 11 Scrapie is considered a sporadic disease in the United States, with the primary economic losses resulting from import/export restrictions on live sheep, semen, and ova and the individual loss of genetic material in affected sheep that die or are culled. Approximately 84% of sheep affected with scrapie in the United States are Suffolks raised in farm flocks, which implies that they are managed as a confined or semi-confined flock. 8 Flock morbidity can range from 10% to 50%, with a 100% mortality rate in affected animals. EPIDEMIOLOGY Scrapie occurs most frequently in sheep between the ages of 2 and 5 years. Disease occurrence before 18 months of age is rare; however, in flocks with high morbidity rates, the average age of clinical presentation can decrease to 18 months. 9 The majority of sheep presenting with clinical signs of scrapie are 3.5 years of age. 9 The progression of clinical signs is usually slow, resulting in death approximately 6 months after the initial onset; however, some animals have been reported to die suddenly. 1,2 Because it is generally believed that most sheep are infected at birth, the age of clinical presentation is a reflection of the incubation period. 8 CLINICAL SIGNS Clinical signs of scrapie in sheep vary according to breed and strain of scrapie. Scrapie is a nonfebrile disease that does not produce alterations in an animal s clinical biochemistry. 3,8,9 As a result, no immunologic response can be used to identify diseased sheep. Clinical signs reflect the damaged locations in the central nervous system. Animals often initially show subtle behavioral alterations that may include separation from the flock, hypersensitivity, aggressive behavior, dementia, and head pressing. 8 Some sheep will demonstrate intense pruritus, resulting in self-inflicted skin lesions and wool loss along their sides and flanks. When these lesions are rubbed, sheep extend their necks and elicit a characteristic nibbling response known as the provocative scrapie test. 8 Locomotive abnormalities include muscle tremors, ataxia, incoordination, and progressive hindlimb weakness that causes a loss of balance. If these animals are encouraged to run, they have a high-stepping gait with their forelimbs and a bunny-hop movement with their hindlimbs. Often, affected sheep lose weight despite a good appetite. Several diagnostic differentials must be considered when animals present with the aforementioned clinical signs. These include external parasites, environmental and iatrogenic toxins, nutritional deficiencies, viral diseases, and other infectious agents. 8 TRANSMISSION AND PATHOGENESIS The natural transmission of scrapie, both horizontal and vertical, among or within sheep flocks is well documented. It has been shown conclusively that the offspring of ewes and rams affected with scrapie are more likely to become infected than are the offspring of unaffected animals or those housed in a common area. 9 It also is believed that rams play a lesser role in the transmission of scrapie than do ewes because of the lack of detection of the agent in the testes and seminal vesicles of affected rams. 9 It is currently not known how scrapie is transmitted from ewe to lamb, although there is clear evidence of hereditary susceptibility. A lower incidence of disease has also been demonstrated in the offspring of scrapie-infected ewes that are removed immediately from their dams. 9 These observations imply that an infectious agent may be responsible for transmission of the disease and that transmission occurs postpartum, not in utero. 1,2 Horizontal transmission has been documented via shedding of the infectious agent in such
S54 Food Animal Compendium April 2001 TABLE 1 Physiochemical Variations Between the Wild Type and Mutant Form of PrP Protein Characteristic PrP C (Wild Type) PrP SC (Mutant) Proteinase K Degrades Partially resistant Detergent Soluble Insoluble Length ~250 Amino acids ~250 Amino acids Protein structure α-helix and loops β-pleated sheets Molecular weight 33 35 kd 33 35 kd Antigenicity Bind to same antibody Bind to same antibody Localization Cell surface Fibrils, deposits in cell Expression Many tissues Brain, central nervous system, lymph nodes, spleen, tonsil Expression in disease Protein levels constant Protein levels increase Turnover Rapid Slow bodily secretions as feces, uterine fluids, and placenta that were orally inoculated into naive hosts. 12 The infectious agent of scrapie is extremely resistant to desiccation by dry heat, ultraviolet light, ionizing radiation, and physical and chemical treatments. 8 Current recommendations for decontamination of infected environments include the use of humidified heat sources and molar sodium hydroxide solutions. The primary natural mode of infection is believed to be oral. 9 After entry into the host, a silent or zero phase ensues in which no infectivity can be detected in any host tissue; this period may last for over 8 months. 9 Replication initially begins in the lymphoreticular system (LRS) during the silent phase, but there is no detectable level of the agent in the nervous system at this time, which is considered to be the incubation period. The incubation period can last up to 2 years, during which the agent continues to replicate in the LRS. 9 The end of the incubation period is marked by the presence of the infectious agent in the nervous system. How the infection spreads from the LRS to the nervous system is not known. Once it enters the nervous system, the molecular conversion of the naturally occurring PrP C protein which is anchored to the neuronal cell surface by a glyocophosphatidylinositol (GPI) anchor to the PrP SC form is detectable within neuronal cells. 3 There is much debate and confusion as to how the mutant form of the protein causes degenerative neural disease and whether the PrP SC form of the protein itself is the transmissible infectious agent. 3,13 What is known is that the mutant form of the protein accumulates in large quantities in a very short period within the neuron and that there is a loss of normal PrP C protein in that cell. 3 PATHOLOGY Diagnosis of scrapie usually occurs at postmortem examination with histologic examination of formaldehyde-fixed brain tissue for disease-specific spongiform changes. 14 This information, along with the farm and clinical history of infected animals, is used to confirm the diagnosis. Examination of brain tissue should reveal characteristic neuron degeneration with the formation of vacuoles, proliferation of astroglial cells, and absence of demyelination or overt inflammatory changes. 8 The glial reaction precedes the vacuolization of neurons, deposition of amyloid, and neuronal loss that is characteristic of TSE diseases. 15 The histologic changes that occur in scrapie-infected sheep are believed to be caused by changes in the production of certain cytokines and an increase in the number of enzymes and transport proteins present in neurons. 15 The majority of lesions occur in the gray matter of the brain stem. The areas of the brain most often affected are the medulla, pons, midbrain, and thalamus. 9 Evidence indicates that the pattern of vacuolation and clinical signs present in scrapie-infected sheep vary with the breed of sheep and strain of scrapie. 16 With the discovery of the PrP SC protein in affected sheep, certain alterations in the physiochemical properties that occur between the wild type and mutant form of the protein have allowed for additional pathologic markers to be exploited diagnostically (Table 1). The PrP SC protein appears as rods under electron microscopy and has been referred to as scrapie-associated fibrils (SAFs). 16 SAFs are unique because they are not appreciably affected by autolysis of tissue. SAFs can also be recovered from fresh-frozen or formaldehyde-fixed tissue. 14 RELATIONSHIP BETWEEN GENETICS AND THE INFECTIOUS AGENT With the discovery that the PrP SC in neural tissue is almost 100% correlated with the occurrence of clinical
Compendium April 2001 Food Animal S55 TABLE 2 The Three Most Important PrP Gene Polymorphisms Related to Scrapie in Sheep Codon Position Amino Acid Substitution 136 Valine (V 136 ) Alanine (A 136 ) 154 Arginine (R 154 ) Histidine (H 154 ) 171 Arginine (R 171 ) Glutamine (Q 171 ) scrapie in sheep, there has been a significant amount of speculation about the role of this protein in the transmission and pathogenesis of the disease. Convergence of PrP C to PrP SC Protein The PrP is approximately 250 amino acids in length, depending on whether it is glycosylated. The variation that occurs between the PrP C and PrP SC proteins results in the removal of 30 to 50 amino acids of host cellular protein at the N-terminus. 17 This causes a structural change in which PrP C, which is normally about 40% α-helix and 3% β-pleated sheets, transforms into PrP SC, which is 45% β-pleated sheets with much less α-helix formation. 17 The convergence of PrP C to PrP SC is believed to be a posttranslational event. 5 In a cellfree system, experiments have demonstrated that the conversion of PrP C to PrP SC can occur without the synthesis of new proteins. This indicates that the PrP C protein can be directly altered by the presence of PrP SC, causing a change in the protein content of the infected neuron. 13 The findings regarding PrP C and PrP SC have led researchers to hypothesize that PrP SC is the infectious agent responsible for scrapie. 3,12,18,19 Because PrP SC is a protein that may act as a virus and is capable of causing an infection, it has been called a prion. The PrP SC protein is thought to infect normal cells and somehow act as a catalyst for the conversion of PrP C proteins to more PrP SC, thus destroying normal cell function. 3,16,18 This hypothesis has merit but does not explain the familial appearance of natural scrapie in sheep flocks. Genetic Susceptibility of Sheep to Scrapie Molecular data have indicated that there is a genetic susceptibility related to the ability of the PrP C protein to be converted to the PrP SC form. This susceptibility tends to vary according to the strain of scrapie infecting the sheep and the breed of sheep infected. 3 5,13 The genotype of an animal is a combination of maternal and paternal genetic material; at a single locus the dam and sire each contribute one genetic determinant. The phenotypic expression of that determinant is dependent on whether it has dominant or recessive characteristics TABLE 3 Distribution of PrP C Alleles in Common Sheep Breeds and Their Susceptibility to Conversion to PrP SC Susceptibility Amino Acids at to PrP C Positions Convergence Breed 136, 154, 171 a to PrP SC Cheviot VRQ/VRQ Most susceptible Shetland ARQ/VRQ Most susceptible Swaledale Welsh Mountain Cheviot ARR/VRQ Resistant Shetland AHQ/VRQ Resistant Swaledale Welsh Mountain Suffolk ARQ/ARQ Most susceptible ARR/ARQ Resistant a A single-letter code in the order of the position codon numbers 136, 154, and 171 indicates each amino acid. The combined pairing of the two alleles (one maternal and one paternal) are denoted by the three amino acids separated by a slash. Thus AAR/AHQ indicates that one gene has an A at position 136, A at 154, and R at 171, and the other gene has an A at 136, H at 154, and Q at 171, making the animal a homozygote A at 136 and heterozygote AH at 154 and RQ at 171. (phenotype indicates what is actually expressed by the gene that can be clinically observed). In sheep, eight different polymorphisms have been identified, with the potential for nine allelic variations (polymorphisms are nucleotide variations that occur at positions along the genome that may or may not cause an amino acid substitution and are considered normal genetic variances within a species). 7,13 Studies have confirmed that the eight polymorphisms identified result in six amino acid variations at three different codon positions: 136, 154, and 171 in the sheep genome (Table 2). Two of these codon positions, 136 and 171, are associated with a high susceptibility to scrapie and short survival time in certain breeds of sheep. 6,7,12 The combination of amino acid substitutions that occur at these three loci lend a specific susceptibility profile and variance in incubation period to each breed. This discovery has prompted genetic testing of different breeds of sheep to determine what combinations of amino acids at these three positions affect the clinical course of scrapie. Researchers have found that certain combinations of genotypes increase the susceptibility of different sheep breeds to scrapie and decrease the incubation period of the disease (Table 3). 12 Not all breeds of sheep have the same genotype at the
S56 Food Animal Compendium April 2001 three different codon positions, and certain genotypes appear to be more prevalent among specific breeds. 4 For example, Suffolk sheep more commonly have an A 136 and R 154 than do Cheviot, Swaledale, and Shetland breeds, which more commonly have V 136 and are known as the valine breeds. 7 Certain similarities can be drawn from a comparison of the different genotypes and their susceptibility to PrP conversion and the length of the incubation period. All breeds appear to be more susceptible to scrapie if they have a genotype encoding of QQ 171. 3,4 The valine breeds also are more susceptible to scrapie if they have a VV 136 -encoded genotype. 3,4,13 In valine breeds of sheep, it is an absolute requirement to have a haplotype of VQR for clinical scrapie to develop (haplotype is the genotype of one strand of DNA). Heterozygosity at these alleles tends to alter the susceptibility profile to scrapie as well as the incubation period. In the valine breeds, genetic alleles that encode for AHQ/VRQ and ARR/VRQ tend to be more resistant to developing scrapie, despite having the haplotype of V 136. 4 Incubation periods are also affected by amino acid variations at the three alleles and the strain of scrapie. Cheviot sheep with a VV 136 allele are linked to a 167- day incubation period when infected with a laboratory strain of scrapie known as SSBP/1. 3 However, infections with the same strain of scrapie in Cheviot that have a VA 136 allele have an incubation period of 322 days. 3,6 There also seems to be specific targeting of TSE strains to alleles in the sheep genome. For instance, BSE strain CH1641 appears to target codon 171 rather than codon 136 in the case of SSBP/1. Sheep with the QQ 171 codon have a shorter incubation period when infected with CH1641 than do those with a QR 171 codon. 3 Hypotheses About the Pathogenesis of Scrapie In light of the information concerning the genetic susceptibility of sheep to scrapie, researchers have hypothesized that there is an interaction between the genetic components of scrapie and the infectious nature of its transmission. The most current theory attempts to commingle the two aspects of the disease. It is believed that the infectious agent causing scrapie is the PrP SC mutant protein. 3,13,15 Hosts are probably inoculated with the agent orally and depending on their genotype at alleles 136, 154, and 171 are either resistant to the conversion of PrP C to PrP SC or are susceptible to the conversion. 4 The incubation period is dependent on the strain of scrapie infecting the sheep and the animal s genotype at the specific codon the strain targets. 12 The PrP SC protein is believed to be toxic to cells, and as it accumulates within the cells it causes neurodegenerative lesions histologically observed in scrapie-affected sheep. 3,14,16 Although the details of how the PrP SC protein acts as a catalyst for the conversion of PrP C is not known, two alternative theories have been presented. The first involves a chaperone protein termed protein X, which is necessary for the seeding process of PrP SC to occur in the neurons. 3,15 In the second theory, the PrP SC protein acts as a receptor for the infectious agent causing neuron degeneration; as more PrP SC is formed, it allows the cell to become more susceptible to infection by the agent. 3 One consideration that has alarmed researchers and sheep producers is that all sheep are susceptible to scrapie. There is some evidence that sheep considered genetically resistant to scrapie may simply have incubation periods that are longer than their life span. 4,15 There is concern that these animals may be shedding infectious agent throughout their lives. This hypothesis has been supported by reports of sheep in Japan that have developed clinical disease despite having genotypes that are supposed to infer resistance to scrapie. 4,6,15 Although these reports are rare, several have been confirmed. GENOTYPE CLASSIFICATION With the discovery of the unique genetic predisposition to scrapie in different sheep breeds, veterinarians, sheep producers, and regulatory agencies around the world are developing breeding strategies to eliminate scrapie from flocks. The United States first recognized scrapie as a problem in 1952 when an identification, quarantine, and depopulation program was established. 9,11 Because of the economic hardship on producers and loss of valuable genetic breeding stock, this program was abandoned in 1983. At that time a bloodline/surveillance program was initiated in which animals identified with scrapie along with their bloodline relatives were depopulated from the flock. 11 This program was ineffective because it did not establish measures to eliminate the horizontal transmission of scrapie. The Scrapie Flock Certification program is currently in place and relies on the identification of scrapie-free herds that are followed for 5 years to obtain a scrapie-free certification status. 11 This program consists of four phases; as a herd is elevated from one phase to the next, it is considered to have a lower risk of incidence of scrapie. The benefit of maintaining the fourth phase is that it allows for the free export and sale of animals interstate and internationally. Many state programs have been initiated in the past 3 years that integrate the genetic information of the herd along with herd records to help eliminate scrapie from flocks. Genetic testing of sheep is easily accom-
Compendium April 2001 Food Animal S57 plished using buccal swabs to collect mucosal cells, or blood samples can be used to extract DNA from the buffy coat. England has a very advanced voluntary program that classifies the susceptibility of sheep to scrapie into five gene classes specific for different breeds. 5 Flocks have a classification based on this information and the history of disease on the farm. As research continues into the mechanisms responsible for the transmission and pathogenesis of scrapie, these programs are reviewed frequently to ensure that they are using all the currently available information to obtain scrapie-free flocks. PRECLINICAL TESTING FOR SCRAPIE-INFECTED SHEEP The most conclusive way to diagnose scrapie has been to identify the presence of the PrP SC protein histopathologically in brain tissue after death. 20,21 Because of the long incubation before clinical signs develop in sheep and the possibility that infected animals are a reservoir for scrapie, it is very important to be able to detect presymptomatic carriers as early as possible and eliminate them from the flock. The discovery of the PrP SC protein has allowed for the development of premortem immunoassays that use biopsies of tissues from the LRS. (Replication of the infectious agent initially begins in the LRS during the incubation period, which can last up to 2 years.) Lymphoreticular tissues such as lymph nodes, the third eyelid, tonsils, and lymphocytes from venous blood can be harvested, and antibodies designed specifically to distinguish PrP C from PrP SC can be used for immunoblotting. 21,22 This technique has been employed successfully by a group of researchers who were able to detect PrP SC proteins in the tonsils of infected sheep 1 year before the animals developed clinical disease. 23 However, this technique has not proven successful in the preclinical diagnosis of BSE in cattle because the agent is not as widely distributed in tissue outside the central nervous system. 23 CONTINUED RESEARCH Future research needs to address many unanswered questions about scrapie. What is the nature of the infectious agent causing scrapie? Does the disease result simply from the conversion of PrP C to PrP SC, or is another factor involved? What is the significance of PrP gene variation and its association with disease incidence in different breeds of sheep? Do different strains of naturally occurring scrapie exists? As this information becomes available, it will increase understanding of all TSE-related diseases and make possible the treatment of affected animals or elimination of the disease by breed selection. REFERENCES 1. Woolhouse MEJ, Stringer SM, Matthews L, et al: Epidemiology and control of scrapie within a sheep flock. Proc R Soc Lond B Biol Sci 265:1205 1210, 1998. 2. Hoinville LJ: A review of the epidemiology of scrapie in sheep. Rev Sci Tech Off Int Epiz 15:827 852, 1996. 3. Hunter N: Scrapie. Mol Biotech 9:225 234, 1998. 4. Hunter N: PrP genetics in sheep and the implications for scrapie and BSE. Trends Microbiol 5(8):331 334, 1997. 5. Dawson M, Hoinville LJ, Hosie BD, Hunter N: Guidance on the use of PrP genotype as an aid to the control of clinical scrapie. Vet Rec 142(23):623 626, 1998. 6. Hunter N, Moore L, Hoise BD, et al: Association between natural scrapie and PrP genotype in a flock of Suffolk sheep in Scotland. Vet Rec 140:59 63, 1997. 7. Hunter N, Goldmann W, Foster JD, et al: Natural scrapie and PrP genotype: Case-control studies in British sheep. Vet Rec 141:137 140, 1997. 8. Linnabary RD, Hall RF, Wilson RB, Knowles AM: Scrapie in sheep. Compend Contin Educ Pract Vet 13(3):511 517, 1991. 9. Detwiler LA: Scrapie. Rev Sci Tech Off Int Epiz 11(2): 491 537, 1992. 10. Godon KAH, Honstead J: Transmissible spongiform encephalopathies in food animals. Vet Clin North Am Food Anim Pract 14(1):49 70, 1998. 11. Detwiler LA: Scrapie control in the United States: A review of the past with emphasis on the present flock certification program. Dev Biol Stand 80:109, 1993. 12. O Rourke KI, Holyoak GR, Clark WW, et al: PrP genotypes and experimental scrapie in orally inoculated Suffolk sheep in the United States. J Gen Virol 78:975 978, 1997. 13. Bossers A, Belt PBGM, Raymond GJ, et al: Scrapie susceptibility-linked polymorphisms modulate the in vitro conversion of sheep prion protein to protease-resistant forms. Proc Natl Acad Sci U S A 94:4931 4936, 1997. 14. Chaplin MJ, Aldrich AD, Stack MJ: Scrapie associated fibril detection from formaldehyde fixed brain tissue in natural cases of ovine scrapie. Res Vet Sci 64:41 44, 1998. 15. Dandoy-Dron F, Guillo F, Benoudjema L, et al: Gene expression in scrapie. J Biol Chem 273(13):7691 7697, 1998. 16. Stack MJ, Chaplin MJ, Aldrich AM, Davis LA: The distribution of scrapie-associated fibrils in neural and non-neural tissues of advanced clinical cases of natural scrapie in sheep. Res Vet Sci 64(2):141 145, 1998. 17. Schmerr MJ, Jenny A, Cutlip RC: Use of capillary sodium dodecyl sulfate gel electrophoresis to detect the prion protein extracted from scrapie-infected sheep. J Chromatogr B Biomed Sci Appl 697:223 229, 1997. 18. Come JH, Lansbury PT: Predisposition of prion protein homozygotes to Creutzfeldt-Jacob disease can be explained by a nucleation-dependent polymerization mechanism. J Am Chem Soc 116:4109 4110, 1994. 19. Gajdusek DC: Genetic control of nucleation and polymerization of host precursor to infectious amyloids in the transmissible amyloidosis of the brain. Br Med Bull 49:913 931, 1993. 20. Hsich G, Kenney K, Gibbs CJ, et al: The 14 3-3 brain protein in cerebrospinal fluid as a marker for transmissible spongiform encephalopathies. N Engl J Med 335:924 930, 1996. 21. Collinge J: New diagnostic tests for prion diseases. N Engl J Med 335:963 965, 1996.
S58 Food Animal Compendium April 2001 22. Schreuder BEC, van Keulen LJM, Vromans MEW, et al: Preclinical test for prion diseases. Nature 381:563, 1996. 23. Schreuder BEC, van Keulen LJM, Vromans MEW, et al: Tonsillar biopsy and PrP sc detection in the preclinical diagnosis of scrapie. Vet Rec 142:564 568, 1998. About the Author Dr. Machen is affiliated with the Department of Clinical Sciences, School of Veterinary Medicine, Tuskegee University, Alabama. ARTICLE #9 CE TEST The article you have read qualifies for 1.5 contact hours of Continuing Education Credit from the Auburn University College of Veterinary Medicine. Choose the one best answer to each of the following questions; then mark your answers on the test form inserted in Compendium. 1. With which group of clinical diseases would scrapie be classified? a. rabies, tetanus b. polioencephalitis, white muscle disease c. chronic wasting disease, kuru d. thromboembolic meningoencephalitis, listeriosis 2. Scrapie is not clinically characterized by a. neurodegenerative disease that is inevitably fatal. b. alterations in the affected animal s blood chemistries. c. occasional severe pruritus and fleece loss. d. 100% mortality. 3. Initial replication of the infectious agent causing scrapie begins in the a. nervous system during the silent phase. b. nervous system during the incubation period. c. lymphoreticular system during the incubation period. d. lymphoreticular system during the silent phase. 4. Which of the following statements about PrP C and PrP SC is true? a. PrP C is an intracellular protein in normal neurons of sheep. b. PrP SC acts as a catalyst to convert PrP C. c. PrP SC structurally has more α-helix formation than does PrP C. d. PrP C is converted to PrP SC during transcription. 5. Which two codons are associated with an increased susceptibility to scrapie? a. 136 and 171 c. 136 and 154 b. 154 and 171 d. 142 and 171 6. Susceptibility of different breeds of sheep to scrapie and the incubation period of disease are determined by a. the age at which sheep become infected. b. duration of exposure to the prion. c. genotype. d. amount of PrP C on the cell surface. 7. Which haplotype must valine breeds have to develop clinical scrapie? a. VQR c. ARR b. AHQ d. VRQ 8. Genetic testing can be done on all of the following samples except a. leukocytes. c. erythrocytes. b. buccal cells. d. tissue samples. 9. In order to attain scrapie-free status in the United States, a flock must be free of the disease for years and phases. a. 4; 3 c. 5; 4 b. 6; 4 d. 5; 3 10. Which of the following statements about PrP C and PrP SC is correct? a. PrP C is a glycoprotein on the cell surface of neurons. b. PrP SC is an intracellular protein in neurons. c. PrP SC can be directly converted from PrP C. d. all of the above