Overview of Scrapie
Classic scrapie, a natural disease of sheep and goats, is seen worldwide except in 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, including those with white faces.
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 still 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 136/154/171 AARRQQ genotype. However, sheep with codon 136V are at the highest risk of developing scrapie, even when they also carry one R at codon 171. Recently, it was shown that these cases are actually affected by another strain of the agent that at one time was referred to as the V-dependent strain. It was first described in Cheviots by Irish researchers and has been reported in Europe and the USA. The 171RR genotype is considered to be resistant to classic scrapie. Scrapie is also rare in sheep with the 136/171 AAQR genotype. However, at least 83 atypical cases of scrapie from more than 110,000 tested samples have been recorded by The Veterinary Laboratories Agency of Great Britain. These discordant cases, most classified as Nor 98 (first found in Norway in 1998) usually involve only a single sheep in a flock with no obvious contacts between affected flocks. Nor 98 has been seen in sheep with genotypes considered to be resistant, and many of them have one or two of the amino acid leucine (L) at codon 131. This has led to the belief that such cases represent a spontaneous prion disease, analogous to sporadic Creutzfeldt-Jakob disease in people, and that the prion is not transmitted by direct contact. It has been reported in almost every European country, the Falklands, Canada, and New Zealand, even though classic scrapie has not been reported from New Zealand. Eight such cases have been reported in the USA since 2003; the World Health Organization has classified this as a separate disease and declared that it should not affect trade.
The classic 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, although there is one report of PrPSc detected in semen of several 131/171 VVQQ and AVQQ V-dependent scrapie affected–rams. This occurred after apparent transmission of scrapie to some of their offspring from 131/171 VVQQ scrapie eyelid and rectal mucosa–negative ewes. However, because of the rarity of 136/171 VVQQ genotype in the USA at this time, it is unlikely that this should be of concern.
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. 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.
Classic scrapie, which results from ingestion of PrPSc by a genotypically susceptible sheep, is a longterm, progressive, and debilitating neurologic illness 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. The V-dependent strain has 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 seen. This strain, found in the University of Idaho research flock, made up of local flocks found to have scrapie, mainly infects 131/171 VVQQ and 131/171 AVQR sheep. It is probably present whenever scrapie and the 136AV and VV genetics are found together. Most veterinarians or producers would not recognize sheep with the V-dependent gene as having scrapie.
A complete necropsy should be performed on any sheep dying mysteriously, including submission of the brain for immunohistochemical (IHC) testing for scrapie. Differential diagnoses include caseous lymphadenitis, abomasal emptying disease, Johne 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 produce antibodies against self, antibodies are mostly monoclonal in origin or are made in rodents bred to 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) before 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; biopsy of the lymph follicles of the third eyelid is simpler and is validated in the USA 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 in the course of the disease when 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 initiated by the USDA/APHIS has been underway since 2003 in which brains and lymphoid tissue of aged black-face and black-face crosses and sheep sold for slaughter with signs of scrapie (ataxia, poor body condition scores, alopecia from rubbing) are tested for scrapie by IHC. Sheep 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 90%, from ~0.5% to~0.015% as of the end of 2013. The goal is to have the USA declared free of scrapie 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. So far there are no data to support that concern.
Rules prohibiting ruminant-to-ruminant protein feeding have been in place in many countries for >10 yr.