SCRAPIECommon names: Scrapie, paraplexia enzootica ovium, transmissible spongiform encephalopathies

Etiology: Scrapie is a member of the transmissible spongiform encephalopathies (TSEs), a group of neurodegenerative disorders caused by unconventional disease agents. These agents are resistant to the treatments that ordinarily destroy bacteria, spores, viruses and fungi. They are generally thought to be prions, although a minority opinion suggests that TSEs may be caused by virinos or retroviruses. Prions are infectious proteins that seem to replicate by converting a normal cellular protein into copies of the prion. This cellular protein, which is called PrPc, is found on the surface of neurons. The scrapie prion, which is a pathogenic isoform of PrPc, is designated PrPSc.

Transmission: 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. In the laboratory, the Scrapie agent has been transmitted to hamsters, mice, rats, voles, gerbils, mink, cattle, and some species of monkeys by inoculation. There is no scientific evidence to indicate that Scrapie poses a risk to human health. There is no epidemiologic evidence that Scrapie of sheep and goats is transmitted to humans, such as through contact on the farm, at slaughter plants, or butcher shops, or through the consumption of sheep or goat meat/milk products.

Clinical Signs: Clinical signs may be noticed 18 months to 5 years 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 usually last from 1 to 3 months; 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 weeks 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. For example, the V-dependent strain found in the University of Idaho’s research flock 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.

Predisposing Conditions: Animals may be genetically predisposed to Scrapie. 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.

Diagnostic Test: A complete necropsy should be performed on any sheep dying mysteriously, including submission of the brain for immunohistochemical (IHC) testing for Scrapie. In Europe, ELISA tests are used for slaughter surveillance, but they are not licensed for Scrapie testing in the USA. The IHC test is used as a confirmatory test and is considered to be the gold standard worldwide. The pathologic changes associated with Scrapie are confined to the CNS and include vacuolation, neuronal loss, astrocytosis, and accumulation of amyloid plaques. Since 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

Differential diagnoses: caseous lymphadenitis, abomasal emptying disease, Johne disease, ovine progressive pneumonia (visna), dentition problems, and meningitis

Treatment of Choice: THERE IS NO TREATMENT

Prevention: 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. Packers and producers have had difficulty in disposing of sheep offal and dead sheep causing them to incur significant increases in disposal costsIncineration or digestion by sodium hydroxide are considered appropriate to adequately inactivate infected carcasses The Scrapie agent is extremely resistant to heat and to normal sterilization processes. To disinfect instruments, they should be soaked in 2.5N NaOH or other disinfectants proven to be effective against prions

Owner Education: The selection of genetically resistant sheep is a highly effective method of minimizing the risk of Scrapie infection. However, maintaining a closed ewe flock is still the most effective preventative measure. Owners should be informed of the classical clinical signs associated with Scrapie and asked to notify their veterinarian if any members of their flock present with neurologic signs or severe pruritis.

Sources:

"Overview of Scrapie." : Scrapie: Merck Veterinary Manual. N.p., n.d. Web. 03 July 2014.

Adlam, G.h. "Scrapie." New Zealand Veterinary Journal 25.11 (1977): 356.Scrapie. The Center for Food Security and Public Health, 4 June 1977. Web.

"Scrapie." The Lancet 299.7747 (1972): 418. Scrapie. USDA APHIS, May-June 2013. Web.

"Veterinary Division - Animal Health Programs." NCDA&CS. North Carolina Department of Agriculture, n.d. Web. 03 July 2014.

“The Epidemiology of Scrapie." National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 03 July 2014.