After infection and a prolonged incubation period, the scrapie agent causes a degenerative disease of the central nervous system in sheep and goats. Six lines of evidence including sensitivity to proteases demonstrate that this agent contains a protein that is required for infectivity. Although the scrapie agent is irreversibly inactivated by alkali, five procedures with more specificity for modifying nucleic acids failed to cause inactivation. The agent shows heterogeneity with respect to size, apparently a result of its hydrophobicity; the smallest form may have a molecular weight of 50,000 or less. Because the novel properties of the scrapie agent distinguish it from viruses, plasmids, and viroids, a new term "prion" is proposed to denote a small proteinaceous infectious particle which is resistant to inactivation by most procedures that modify nucleic acids. Knowledge of the scrapie agent structure may have significance for understanding the causes of several degenerative diseases.

http://science.sciencemag.org/content/sci/216/4542/136.full.pdf

Novel proteinaceous infectious particles cause scrapie

Nobel Lecture: Prions

Prions cause transmissible and genetic neurodegenerative diseases, including scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Straussler-Scheinker diseases of humans. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein, which is encoded by a chromosomal gene. A posttranslational process, as yet unidentified, converts the cellular prion protein into an abnormal isoform. Scrapie incubation times, neuropathology, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neuro-degeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Understanding prion diseases may advance investigations of other neurodegenerative disorders and of the processes by which neurons differentiate, function for decades, and then grow senescent.

https://science.sciencemag.org/content/sci/252/5012/1515.full.pdf

Molecular biology of prion diseases

A cellular gene encodes scrapie PrP 27-30 protein

Purification of prions from scrapie-infected hamster brain yielded a protein that was not found in a similar fraction from uninfected brain. The protein migrated with an apparent molecular size of 27,000 to 30,000 daltons in sodium dodecyl sulfate polyacrylamide gels. The resistance of this protein to digestion by proteinase K distinguished it from proteins of similar molecular weight found in normal hamster brain. Initial results suggest that the amount of this protein correlates with the titer of the agent.

https://science.sciencemag.org/content/sci/218/4579/1309.full.pdf

Identification of a protein that purifies with the scrapie prion.

A better understanding of the infectious process in scrapie was sought by studying the temporal distribution of virus in naturally infected Suffolk sheep. Virus was detected (by mouse inoculation) first in lymphatic tissues and intestine of clinically normal lambs (age, 10-14 months). Titers were generally low. Infection of the central nervous system was first detected in a 25-month-old clinically normal sheep whose nonneural tissues had moderate amounts of virus. In sheep affected with scrapie, similar amounts in nonneural tissues accompanied high concentrations in the central nervous system, notably in sites of severest neurohistologic changes. No virus was found in clinically normal high-risk sheep 54 to 104 months old. The early appearance of virus in tonsil, retropharyngeal and mesenteric-portal lymph nodes, and intestine suggests that primary infection occurs by way of the alimentary tract, either prenatally from virus in amniotic fluid or postnatally from virus in a contaminated environment.

https://www.jstor.org/stable/pdfplus/30112442.pdf

Natural Infection of Suffolk Sheep with Scrapie Virus

Slow transmissible diseases of the nervous system

Virologic and neurohistologic findings in three dairy goats that became affected with scrapie while living with naturally infected Suffolk sheep were essentially like those in affected sheep. Virus, detected by mouse inoculation, was widespread in non-neural sites, particularly in lymphatic tissues and intestine. Im most sites, titers of virus ranged from 3.0 to 3.5 log10 mouse intracerebral LD50/30 mg of tissue. Virus was in nervous tissue in much higher titer. Ranging from 5.1 to 5.8 log10, the highest mean titers were in the diencephalon, midbrain, medulla oblongata and cerebellar cortex--sites of the most severe histologic changes. Although these changes were like those in naturally affected Suffolk sheep, they differed somewhat from those in goats affected with the experimental disease. Spongiform alteration of neuropil was minimal, and the more rostral parts of the brain, such as corpus striatum, globus pallidus and septal area, had few changes. Concentrations and distribution of virus in non-neural tissues were consistent with the conclusion that scrapie virus no doubt can be maintained by contagion in a herd of goats living apart from infected sheep.

https://journals.sagepub.com/doi/pdf/10.1177/030098588001700207

Virologic and neurohistologic findings in dairy goats affected with natural scrapie.

Information was sought on the comparative pathogenicity of four North American strains (isolates) of Aleutian disease virus for royal pastel (a non-Aleutian genotype) and sapphire (an Aleutian genotype) mink. The four strains (Utah-1, Ontario [Canada], Montana, and Pullman [Washington]), all of mink origin, were inoculated intraperitoneally and intranasally in serial 10-fold dilutions. As indicated by the appearance of specific antibody (counterimmunoelectrophoresis test), all strains readily infected both color phases of mink, and all strains were equally pathogenic for sapphire mink. Not all strains, however, regularly caused Aleutian disease in pastel mink. Infection of pastel mink with the Utah-1 strain invariably led to fatal disease. Infection with the Ontario strain caused fatal disease nearly as often. The Pullman strain, by contrast, almost never caused disease in infected pastel mink. The pathogenicity of the Montana strain for this color phase was between these extremes. These findings emphasize the need to distinguish between infection and disease when mink are exposed to Aleutian disease virus. The distinction has important implications for understanding the natural history of Aleutian disease virus infection in ranch mink.

Comparative pathogenicity of four strains of Aleutian disease virus for pastel and sapphire mink.

We used a naturally occurring, Chlamydia psittaci-caused eye disease in guinea pigs, guinea pig inclusion conjunctivitis, as an animal model to understand both the immune response and the pathogenesis of chlamydial eye infections. When instilled into the conjunctival sac of guinea pigs that had been previously infected and were immune, viable chlamydiae or a Triton X-100-soluble extract of them produced a short-lived (12-48 hr) eye disease indistinguishable clinically and histologically from that observed during primary chlamydial eye infection. The clinical and histologic findings were consistent with those of ocular delayed hypersensitivity. Ocular delayed hypersensitivity was induced by primary chlamydial infection at mucosal sites other than conjunctival, such as vaginal and intestinal. Preliminary characterization of the hypersensitivity allergen shows that it is heat sensitive and common to the genus Chlamydia. The allergen is apparently not surface-exposed on chlamydiae and requires viable but not replicating organisms for activity. Our observation should be useful in understanding pathogenetic mechanisms of Chlamydia trachomatis-caused infections in humans, in particular those that produce chronic inflammatory diseases, such as blinding trachoma and urogenital diseases.

https://www.pnas.org/content/pnas/83/19/7480.full.pdf

W. J. Hadlow

Papers

Natural Infection of Suffolk Sheep with Scrapie Virus

Slow transmissible diseases of the nervous system

Virologic and neurohistologic findings in dairy goats affected with natural scrapie.

Comparative pathogenicity of four strains of Aleutian disease virus for pastel and sapphire mink.

Ocular delayed hypersensitivity: a pathogenetic mechanism of chlamydial-conjunctivitis in guinea pigs.