FAQ - scrapie
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can any one help me out in knowing about the scrapie disease in sheep caused by virions?
Scrapie is a fatal, degenerative disease affecting the central nervous system of sheep and goats. It is among a number of diseases classified as transmissible spongiform encephalopathies (TSE). Infected flocks that contain a high percentage of susceptible animals can experience significant production losses. Over a period of several years the number of infected animals increases, and the age at onset of clinical signs decreases making these flocks economically unviable. Female animals sold from infected flocks spread scrapie to other flocks. The presence of scrapie in the United States also prevents the export of breeding stock, semen, and embryos to many other countries. TSEs are the subject of increased attention and concern because of the discovery of bovine spongiform encephalopathy (BSE) in cattle, the link between BSE and variant Creutzfeldt-Jakob disease (vCJD) in people, and feline spongiform encephalopathy (FSE) in cats in Europe. This increased concern has led to the following:
• Packers and producers have had difficulty finding options for disposal of sheep offal and dead sheep causing packers and producers to incur significant increases in disposal costs,
• Other countries have expressed concerns and have indicated that they may prohibit or restrict certain ruminant products because the United States has scrapie, and
• Domestic and international markets for U.S. sheep–derived meat and bone meal have been adversely affected.
The combination of all of these factors has led to the decision to develop a strong scrapie eradication program in the United States.
Epidemiology and Transmission
The agent responsible for scrapie and other TSEs is smaller than the smallest known virus and has not been completely characterized. There are three main theories on the nature of the scrapie agent: (1) the agent is a prion, which is an abnormal form of a normal cellular protein, 2)the agent is a virus with unusual characteristics, and (3) the agent is a virino, a very small piece of DNA that acts like a virus. The scrapie agent is extremely resistant to heat and to normal sterilization processes. It does not evoke any detectable immune response or inflammatory reaction in sheep and goats.
The scrapie agent is thought to be spread most commonly from the ewe to her offspring and to other lambs through contact with the placenta and placental fluids. Signs or effects of the disease usually appear 2 to 5 years after the animal is infected but may not appear until much later. Sheep may live 1 to 6 months or longer after the onset of clinical signs, but death is inevitable. The genetics of the sheep affects their susceptibility to scrapie.
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.
Signs of scrapie vary widely among individual animals and develop very slowly. Due to damage to nerve cells, affected animals usually show behavioral changes, tremor (especially of head and neck), rubbing, and locomotor incoordination that progresses to recumbency and death.
Early signs include subtle changes in behavior or temperament. These changes may be followed by scratching and rubbing against fixed objects, apparently to relieve itching. Other signs are loss of coordination, weakness, weight loss despite retention of appetite, biting of feet and limbs, lip smacking, and gait abnormalities, including high–stepping of the forelegs, hopping like a rabbit, and swaying of the back end.
An infected animal may appear normal if left undisturbed at rest. However, when stimulated by a sudden noise, excessive movement, or the stress of handling, the animal may tremble or fall down in a convulsive–like state.
Several other problems can cause clinical signs similar to scrapie in sheep, including the diseases ovine progressive pneumonia, listeriosis, and rabies; the presence of external parasites (lice and mites); pregnancy toxemia; and toxins.
On the farm, veterinarians diagnose scrapie based on the appearance of its signs combined with knowledge of the animal's history. Scrapie can be diagnosed in the live animal by biopsy of the lymphoid tissues on the inside of the third eyelid. This test is used by the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) to determine whether exposed flocks are infected. Scrapie is most often diagnosed by microscopic examinations of brain tissue at necropsy or by procedures that detect the presence of the abnormal prion protein in brain tissue.
Scrapie research efforts are current (+ info
How can prion (abnormally fold protein) can cause CNS damage? Why the disease is most known in sheep? ?
Prion disease in animals is sheep scrapie. The disease form the sponge like aappearancein the brain. If the scrapie infected sheep is fed to cows, the mad cow disease develop. If a person eats poorly cooked beef, we can also be transmitted with mad cow disease.
This is a very complicated question. And there are some clarifications that should be made to statements in the text which follows the question.
First - prion disease in animals takes different forms depending on the species of origin (if the disease was transmitted) and the species which has contracted the disease. Prion disease in cows is called Bovine Spongiform Encephalopathy (otherwise known as Mad Cow Disease). Other animals like goats, deer, elk, moose, mink, some felines, and others can get prion disease. In humans there are several variants of prion diseases . Humans can become ill with Creutzfeldt-Jakob Disease (CJD), variant CJD (from eating 'mad cow beef'), Kuru, Fatal Familial Insomnia (FFI), and Gerstmann-Sträussler-Scheinker (GSS) disease.
Prion disease in sheep is indeed called Scrapie, and is partially characterized by microscopic sponge-like appearance in the brain.
Second - the transmission of Scrapie to cows has been documented experimentally, however it is unknown whether feeding cows sheep meat caused the Mad Cow epidemic of the UK, or whether this would happen in "real life" or non experimental conditions.
Third - eating poorly cooked beef does not conclusively increase your chances of contracting mad cow disease. Although prions are in very low concentration in muscle tissue or meat, they are very hard to inactivate. So cooking may not affect infectivity. The bottom line: in the entire world, just over 200 cases of humans becoming ill from this type of infection (vCJD) have been documented.
Finally - to answer the two questions:
A. How a protein can cause CNS damage
- The prion protein, when misfolded, is not in its normal cellular state. Also, the newly misfolded proteins aggregate and form into very stable 'clumps' called amyloid plaques. One could say that the human body does not know how to deal with the protein in this state, and normal cellular processes cannot break down the clumps. Cells in the immediate area die because normal processing cannot occur. There is a broad range of neural degeneration events that follow. Autophagy is a type of programmed cell death which is characterized by the formation of large vacuoles that eat away and break down cell parts. This process, when caused by the excess of prion clumps (amyloid plaques) occurring throughout the brain and CNS is as close as I can get to answering the question of how a protein can cause CNS damage.
B. Why the disease is most known in sheep
- I think this is because Scrapie is the most prevalent Transmissible Spongiform Encephalopathy. Many sheep accross the world (with the exception of New Zealand and Austrailia) die of Scrapie every year, whereas, the other diseases are quite rare. Also, Scrapie was the first TSE disease humans were really aware of. It was documented by farmers/vets in the 18th century. (+ info
Are there any theories suggesting the outbreak of BSE?
besides the scrapie-infected animal feed hypothesis, (the one suggest that the use of contaminated cattle feed)
Its not contaminated feed, per se, it is the viruses and prions in the CATTLE that are ground up to feed live stock. Once these prions are ingested they seek out nerve tissue and thrive causing spongiform encephalitis. The 'scrapie-theory' you mention holds water. Sorry to burst your bubble. (+ info
Which do you think is the most interesting disease from this list that people would know the most about?
Creutzfeldt-Jakob Disease (CJD)
Dengue fever encephalitis
Puerperal fever (infection after childbirth)
Typhus yellow fever
West Nile Fever
Its the source of the worlds first vaccine and actually kinda fascinating to study. Not to mention the oozing sores make it all the more fun. (+ info
government livestock disease control?
How does the government handle reported cases of scrapie? Are there any sort of penalties? Will they pay for testing to confirm the presence of scrapies?
I don't think they will pay for tests. I do think that the owner of said sheep is looking at depopulation of the herd. (+ info
Corneal implants have been implicated in a few cases of?
c) Creutzfeld-Jakob disease
e) rabies AND Creutzfeld-Jakob disease
I am not entirely sure if this is what you are looking for. In the first article, there were 6 reported cases of human to human rabies transmission via corneal implants. In the second article, was a case report of a patient acquiring CJD which was attributed to an earlier corneal transplant from a CJD patient. (+ info
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