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THE DISEASE Need to know!
1.At the outset on scrapie...
There is the neurodegenerative disease of central nervous system, scrapie. It is known for a long time, the first remarks have been appeared on some 1730-1750. Scrapie affects sheep and goats naturally, without human intervention and seems to be not related to human activity. In spite of the great age of scrapie and progress in basic research, medical and veterinary sciences, the origin of this unusual disorder remains mysterious to now. During last two decades, a few additional remarks of scrapie have appeared. Genetics turned out to be a necessary condition for affecting sheep with scrapie. It means that not all animals in succession but only selected ones are affected in the nature. Scrapie is an extremely wide-spread disease throughout the world.
2.Other diseases similar to scrapie
An neurodegenerative disease similar to scrapie in sheep and goats affects human. It has another name, the Creutzfeldt-Jakob disease (CJD). It is known since twenthies and seems to be an extremely rare event occurring at the frequency of one per million persons per year (the annual incidence of CJD throughout the world).There are several types of diseases like scrapie in sheep and CJD in human. These are three in man (Gerstmann-Straussler-Scheinker disease, fatal familial insomnia, and kuru) and three in animals (transmissible mink encephalopathy of farm reared mink, feline spongiform encephalopathy, and chronic wasting disease in captive deer and elks). Chronic wasting disease was revealed only in North America. Kuru only in the early 20 century and in fifthies has affected the Fore aboriginals in New Guinea via ritualistic cannibalism by eating brains of died relatives. All the diseases arise suddenly, are fatal paralyses with rapidly progressive dementia, disturbances in behaviour and locomotion, for which there are no methods of certain diagnosis and no treatment. Presently, human diagnostics can be made only by autopsy, posthumously. The diseases above are now established as the group of diseases known as the sub-acute transmissible, or infectious, spongiform encephalopathies. In spongiform encephalopathy, pathological changes occur in the central nervous system. These were studied well on the microscopic, cellular level. In tissue slices, the gray matter of the brain has spongious appearance due to multiplying intraneuronal vacuoles giving the name of all the disease class. In each brain region and in each nervous tissue, neuronal loss is observed. For example, the number of neurons in affected animal brains was reduced by 50%, and autopsy in humans died from CJD showed that brain mass was about 1,000 g as compared to 1,400 g in nonaffected women and 1,700 g in men.
The incubation period is typically 4-5 years, but can be as small as 2 years and as long as 40 years.
3.Mad cow disease and prions
A new disease of domestic animals appeared in Great Britain in 1986 and was documented as “a mad cow disease”. The disease was recognized to be the epidemic one and to spread with some new agent. As affected cattle showed the very aggressive behavior, this agent was believed to be rabies virus. However, some clinical signs forced veterinarians and scientists to change such the opinion and to reject the term “mad cow disease”. A Kurd proverb says, the mad dog lives forty days. People and animals perish of mad cow disease and other spongiform encephalopathies in 2-14 months after the onset of clinical illness. Hence, one sign of hydrophobia differs from that of mad cow disease. Another inconsistent sign is impossibility to protect animals from mad cow disease immunologically; there is no vaccine to now.Cattle affected with mad cow disease becames unusually obstinate, exessively aggressive, capable of falling on man and even biting, does not perceive usual and simple commands of fermers. The clinical signs of BSE include abnormal behaviour, posture, gait and an increased sensitivity to visual and aural stimuli. There is loss of condition and milk yield.
Two years following recognition, the cow disease get a new name, bovine spongiform encephalopathy (BSE). Its popular name was remained the same, the mad cow disease. American S. Prusiner, at present the most authoritative scientist in spongiform encephalopaties, the Nobel Prize winner in medicine of 1997, has introduced the term “prion” to distinguish infectious agents of spongiform encephalopathies from viruses, viroids, and other agents. Therefore the new terminology was accepted in international society: mad cow disease was caused with prions, the agents of new type on principle. All spongiform encephalopathies were attributed to the class of prion diseases.
Shortly after a reason emerged to believe that prion diseases were both infectious and genetic simultaneously. It came to light that the prion was commonly a component of normal, non-affected neuron. A mutation in the protein-coding region of the prion gene caused the chain of slowly-proceeding biochemical events leading to converting the cellular prion protein into an abnormal infectious isoform. Neuropathology, incubation times of prion diseases, and prion synthesis were controlled by the prion protein gene. Point mutations in the prion protein gene of animals and humans were genetically linked to development of neurodegeneration. Transgenic mice expressing mutant prion proteins spontaneously developed neurologic dysfunction and spongiform neuropathology.Purification of this agent yielded an unique sialoglycoprotein, associated with the neuronal cell membrane. The prion protein was unusually resistant to phisical and chemical inactivation procedures that disrupt other known proteins, nucleic acids, viruses, and so on. Prion infectivity was reduced only slightly if any after exposure to doses of ultraviolet and ionising radiations, to desinfectants, formaldehyde, DNAase, RNAase, propionolactone and proteinases. The latter is of special significance as pronases occuring in gastro-intestinal tract are not able to render harmless prions of meal. The prion protein is resistant to cooking, too.
The prion protein was found only in the brain, spinal cord, spleen, thymus, pancreas, and lymph nodes of animals infected with BSE. The main “pleasant” feature of the prion diseases is damages to only nervous tissues. Muscles, heart, liver and kidneys seem to remain non-affected.
4.Danger of prion diseases. Prions and public health.
Statistic analysis showed that BSE is the disorder of world-wide fame and of great economic loss. To 1990, the incidence of BSE throughout the world (mainly in Great Britain} was about 14,000 cases, to 1992, of 48,000 cases, and to 1996, more than 165,000 cases.Prion diseases were either found or roused in numerous domestic and wild animal species such as pig, cat, hamster, chimpanzee, marmoset, squirrel monkey, mouse, farmed mink, elk, mule deer, antelope, puma, cheetah, tiger, greater kudu, gemsbok, eland, arabian oryx, kanna, nyala, ostrich. They are all transmissible among individuals of the same species and some of different species. The main danger of prion diseases consists in possible transmissibility of prion from animals to human. Moreover, a new variety of the CJD has been accepted, the so-called variant CJD (vCJD). The vCJD differs from CJD in its origin; bovine prions from "mad cows" were thought to pass to humans through the consumption of tainted beef products. It seems to be not fantasy or suggestion as 30 persons in Great Britain and France have died from vCJD since 1995. About 100 young adults have developed CJD (and died!) after treatment with cadaveric human growth hormone, and more than 60 persons did during various neurosurgical procedures.
How much deathes may we expect? Taking into consideration the incubation times of prion diseases, in several years we can see the epidemic of the human illnesses of unprecendented scale. By English scientists’ calculations, at present more than 5 million people can be infected with BSE. And such the epidemic can be ...
This page was last updated on 06/12/99.
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