Classical scrapie, a natural disease of sheep and goats, is seen worldwide with the exception of Australia and New Zealand. It is one of the transmissible spongiform encephalopathies (TSE), related to bovine spongiform encephalopathy and chronic wasting disease of deer and elk, all of which are thought to result from the accumulation of an abnormal form of a cellular protein in the brain. Natural transmission of scrapie to other species has not been shown. In the USA, scrapie primarily affects black-faced sheep breeds (eg, Suffolk, Hampshire, and their crosses), accounting for ~96% of cases. In other countries, the disease is commonly seen in other breeds.
Etiology, Transmission, and Pathogenesis
An abnormal protein designated as PrPSC, found in all TSE, appears to convert a normal cellular protein called prion protein (PrPC) to its abnormal form (PrPSC) in susceptible animals. In sheep, the susceptibility is controlled genetically; however, genetic susceptibility has not been established in goats. As it accumulates within a cell, PrPSC is deposited as an amyloid plaque in lymphoreticular and nervous tissue, where its accumulation is hypothesized to cause the nervous signs associated with the disease. Although some researchers do not believe that PrPSC itself is the disease agent, its presence is a reliable diagnostic test for prion disease.
When PrPC is induced to change to PrPSC, the conformation of the protein's structure is reconfigured. Although the chemical composition of the molecule is the same, its chemical properties change. The normal protein is soluble in denaturing detergents and is digested by cellular proteases such as proteinase K. However, PrPSC (as well as the infectivity) is not destroyed by detergents and is resistant to breakdown by rendering processes presently used, heat sterilization temperatures, ultraviolet light, ionizing radiation, and most disinfecting agents. It is only partially inactivated by protease K.
The genetics of scrapie in sheep is located on chromosome 13, and three codons (136, 154, and 171) seem to control most of the susceptibility to scrapie. Codon 136 codes for either valine (V), alanine (A), or threonine (T); 154 codes for arginine (R) or histidine (H); and codon 171 codes for glutamine (Q), lysine (K), H, or R. Resistance to scrapie is correlated with 136A, 154H, and 171R. These small changes in the amino acid components of PrPC apparently enable it to resist reconfiguration. Scrapie is usually related to polymorphisms at codon 171. Among sheep in the USA, 91% of brain samples testing positive for scrapie originated from sheep with the AARRQQ genotype. However, sheep with codon 136V are at the highest risk for developing scrapie even when they also carry one R at codon 171. The 171RR genotype is considered to be resistant to classical scrapie. Scrapie is also relatively rare in sheep with the 136,171 AAQR genotype. At least 83 atypical cases of scrapie from 110,000 tested samples have been recorded by The Veterinary Laboratories Agency of Great Britain. These discordant cases usually involve only a single sheep in a flock with no obvious contacts between affected flocks, leading to the belief that such cases represent spontaneous prion disease, analogous to sporadic Creutzfeldt-Jakob disease in humans, and that the prion might not be transmitted by direct contact.
The classical disease is naturally transmitted during lambing from infected dams via ingestion of infected placenta or allantoic fluids by flock mates and newborn lambs. Infected males are not believed to transmit the disease. The embryo or fetus is not exposed to scrapie while in utero in a scrapie-infected dam because there is physical separation from PrPSC-containing allantoic fluid and chorioallantois by the amnion, which remains free of PrPSC even when the other placental tissues are infected. Lambs delivered via cesarean section from infected dams, kept separate from the allantoic fluid, and isolated from infected sheep remain disease-free.
Despite the wide distribution of normal prion protein in reproductive, placental, and fetal fluids, PrPSC has been detected only in the caruncular portion of the endometrium and cotyledonary chorioallantois (the fetal-maternal interface) of pregnant scrapie-infected ewes—but only if both the dam and fetus are of a susceptible genotype. Although tissues of the maternal side of the placenta carry a susceptible prion protein, it requires susceptible prion-containing cells from the fetal side of the placenta for conversion to PrPSC. Thus, an infected ewe introduced into a clean flock and bred to a ram with a resistant genotype is unlikely to transmit scrapie. However, there can be partial or incomplete anastomosis between fetal blood supply to the cotyledons among fetuses of different genotypes on the same side of the uterine horn, which in rare cases can result in PrPSC accumulation in cotyledons with resistant genotypes.
Previous contamination of premises is believed to be another source of scrapie infection. Anecdotal accounts abound of flock depopulation and premises decontamination that are followed by recurrence of disease in repopulated infection-free but susceptible sheep.
Classical scrapie, which results from ingestion of PrPSC by a genotypically susceptible sheep is a longterm, progressive, and debilitating neurologic illness that is believed to always be fatal. Clinical signs may be noticed 18 mo to 5 yr after exposure, and include progressive weight loss with no concurrent loss in appetite, progressive ataxia, fine head tremors (most apparent in the ears), and cutaneous hypersensitivity. Pruritus develops in ~70% of cases. Sheep may assume a vacant, fixed stare or, less often, become suddenly aggressive. Signs of hypersensitivity are often elicited by rubbing or scratching the sheep's back, which induces the sheep to throw its head back, make chewing motions and lick at the air, or compulsively nibble at the limbs below the carpus.
Ataxia is first detected when sheep are running. The hindlimbs appear to be uncoordinated with the forelimbs, and affected animals adopt a bunny-hopping gait. Sheep often have a high-stepping gait in the forelimbs, resembling a prancing horse. As signs worsen, the hindquarters sway while standing.
Clinical signs last from 1 to >3 mo; sheep generally become recumbent because of weakness and incoordination. If helped up, an affected sheep may be able to remain standing for hours, but cannot rise unassisted if it falls or lies down. Death follows 1–2 wk after a sheep can no longer right itself. Blindness, resembling that seen with polioencephalomalacia, occasionally develops. The clinical signs of scrapie can vary, depending on the sheep's genotype and the strain of scrapie. There is a shorter incubation strain of scrapie that tends to affect sheep with the amino acid valine at codon 136 of chromosome 13, with a substantially more rapid clinical course of 3–30 days and death in 2–3 days after recumbency. In these cases, weight loss and signs of pruritus are often not observed. Most veterinarians or producers would not recognize sheep with these signs as having scrapie.
A complete necropsy should be performed on any sheep dying mysteriously, including an evaluation and submission of the brain for immunohistochemical (IHC) testing for scrapie. Differential diagnoses include caseous lymphadenitis, abomasal emptying disease, Johne's disease, ovine progressive pneumonia (visna), dentition problems, and meningitis.
Currently, all diagnostic tests for TSE require infected tissue and the use of antibody reactions. Because animals generally do not make antibodies against self, antibodies are mostly monoclonal in origin or are made in rodents that have no prion protein. IHC and ELISA are used for routine testing. In the EU, ELISA tests are used for slaughter surveillance, but they are not licensed for scrapie testing in the USA. However, several test kits are used for diagnosis of chronic wasting in wild elk and deer. The IHC test is used as a confirmatory test and is considered to be the gold standard worldwide.
Detection of PrPSC in reticuloendothelial organs (spleen, lymph nodes) prior to appearance of clinical signs or PrPSC in the brain is possible. Detection of PrPSC has been reported in 76% of tonsils examined and 57% of lymphoid tissue specimens collected from the third eyelid of infected sheep. A small percentage of sheep in which the brain contains PrPSC do not have detectable PrPSC in the lymph nodes, which may be influenced by genotype or scrapie strain. The atypical strains are not often found in lymphoid tissues.
The palatine tonsil has been used for biopsy and diagnosis, but biopsy of the lymph follicles of the third eyelid is simpler and is validated as a diagnostic test. It yields a high percentage of unreadable samples due to lack of follicles in the sample in as many as 40–60% of adult sheep. Biopsy of the lymphoid ring in the rectal mucosa has also been validated and yields positive diagnoses in ~55–65% of positive sheep. Use of the mandibular lymph nodes has not been validated; however, biopsy of that site may be useful diagnostically, because examination of several tissues improves the chances of a positive diagnosis. It has not been established when in the course of the disease the agent will consistently appear in these tissues, but it may be as early as 14 mo after exposure. The interval likely depends on the age of the sheep at exposure and genotype or strain of scrapie.
The pathologic changes associated with scrapie are confined to the CNS and include vacuolation, neuronal loss, astrocytosis, and accumulation of amyloid plaques. However, because histologic changes are often lacking, diagnosis is made on the basis of IHC staining of the obex, other parts of the brain, and/or lymphoid tissue for PrPSC.
For disinfection, instruments should be soaked in 2.5N NaOH or a disinfectant shown to be effective against abnormal prions. Incineration or digestion by sodium hydroxide are considered appropriate to adequately inactivate infected carcasses.
Individual and premises identification are required for all breeding sheep leaving their original premises as part of the USA mandatory Scrapie Eradication Program. A scrapie slaughter surveillance program has been underway for several years in which brains and lymphoid tissue of aged ewes and rams sold for slaughter are tested for scrapie by IHC. Animals found to be positive are traced back to the herd of origin, which is quarantined, and all animals are tested. All positive sheep and those with the 171QQ genotype are euthanized. Animals sold from the flock are also traced and tested. Since the program was initiated, the prevalence of scrapie has decreased from ~0.5% to ~0.1% as of 2008. The goal is to eliminate scrapie and have the USA declared scrapie-free by 2017.
Voluntary efforts to control and eliminate the disease have been undertaken by producers in the USA by selecting sheep with at least one 171R codon and culling sheep with susceptible genotypes; the expectation is that the remaining sheep will be resistant to scrapie infection. However, some concern has been expressed that reliance on genetics for elimination of the disease may inadvertently select for the atypical strain of scrapie that infects the 171RR genotype.
Rules prohibiting ruminant-to-ruminant protein feeding have been in place in many countries for >10 yr.
Last full review/revision July 2011 by Marie S. Bulgin, DVM, MBA, DACVM