What Is Bovine Spongiform Encephalopathy?

Infectious spongiform encephalopathy is a group of subacute, progressive, lethal central nervous system degenerative diseases in humans and animals caused by prions, also known as prion diseases. Including sheep pruritus, bovine spongiform encephalopathy, mink infectious encephalopathy, and human Creutzfeldt-Jakob disease, Gurdle-Sjogren's syndrome, and Kuru disease.

Infectious spongiform encephalopathy

Infectious spongiform encephalopathy is a group of subacute, progressive, lethal central nervous system degenerative diseases in humans and animals caused by prions, also known as prion diseases. Including sheep pruritus, bovine spongiform encephalopathy, mink infectious encephalopathy, and human Creutzfeldt-Jakob disease, Gurdle-Sjogren's syndrome, and Kuru disease.

The common features of transmissible spongiform encephalopathy (TSE) are long incubation period (months to decades); no fever, no inflammation, and no specific immune response after infection: showing progressive ataxia, Nervous symptoms such as tremor, instability, hypersensitivity, dementia, and abnormal behavior, and the course of the disease develop slowly, but all end in death; histopathological changes are limited to the central nervous system, with neuronal vacuolation, and gray matter sponge lesions As a feature. There are nearly 10 types of human and animal infectious spongiform encephalopathy currently known.

(Scrapie) Infectious Spongiform Encephalopathy (Scrapie)

Itch disease, also known as "donkey run disease" and "pruritus", is a slowly developing infectious central nervous system disease that occasionally occurs in goats in adult sheep. It is characterized by a long incubation period, itching, muscle tremors, weakness, atrophy, progressive ataxia, and finally paralysis and death. The disease occurred in England as early as the middle of the 18th century, and then spread to many European countries, as well as North America and the rest of the world. In 1983, a suspected case was found in the Leicester sheep flock imported from Scotland in the United Kingdom. The disease was confirmed based on the symptoms of the diseased sheep and brain histopathological examination. Thanks to determined measures, the epidemic was put out in a timely manner.

Pathogen The pathogen of the disease belongs to prion in the subviral factor, and some people translate it as prion or prion protein.

Prion is a special infectious agent. It is different from general viruses and virus-like viruses. It has no nucleic acid but a special glycoprotein. Further research found that many kinds of normal animal and human brain cells and other cells also have this kind of prion protein, which is represented by PrP (Pr stands for Prion P for protein).

There are two types of PrP: one is normal cells, sensitive to proteases, easily digested and degraded by it, exists on the cell surface, non-infectious, and is represented by PrPc; when the structure is abnormal, it becomes the other type that is pathogenic PrP, which has a certain resistance to proteinase K, is represented by PrPsc. There is no difference between the two in mRNA and amino acid levels, but the biological characteristics and three-dimensional structure are different.

PrPsc (pathogenic prion) has a molecular size of 27 ~ 3OKD. Because it is combined with normal body cell membrane components, it is not easy to be recognized by the body's immune system, so it is difficult to detect using ordinary antigen-antibody reactions.

PrPsc is very resistant to heat, radiation, acids and alkalis and conventional disinfectants. The brain tissue homogenate of diseased animals is still infectious to experimental animals after being exposed to high temperature of 134 138 for 1h, so feed (meal and bone meal) made of tissue containing PrPsc or additives for human food or cosmetics, dry heat 180 There was still some infectivity at 1h. Diseased animal tissues are still infectious in 10% to 20% formalin for several months, and can tolerate 2mol / L sodium hydroxide for 2h.

PrPsc has no single invasiveness, and it has invasiveness only after combining three. About 1,000 PrPscs in the brain can form fibrous objects with a size of 100-200nm, that is, Scrapie associated fibrils (SAF). Can be checked out. SAF is found in the brain tissue of sheep with natural and artificial infections, and also in the brain tissue of bovine spongiform encephalopathy and human Creutzfeldt-Jakob disease. Because of its specificity, SAF can be used as a variety of spongiform encephalopathy. Pathological diagnostic indicators.

The content of PrPsc in various tissues of infected animals is different. According to the size of the infection titer, the brain is the highest, followed by the spinal cord. The infection titers of other lymph nodes, bones, lungs, heart, kidneys, and muscles are all very low.

PrPsc is known to cause a variety of infectious spongiform encephalopathy in humans and animals.

Epidemiology Sickness can occur in sheep of different sexes and breeds, but the susceptibility to breeds is obviously very different. The high incidence of certain infected lineages within the breed may be related to vertical transmission. It usually occurs in sheep 2-4 years old. Sheep and goats can spread by contact. The transmission route of pruritus is not completely clear, but it is known to spread vertically and horizontally. It has been confirmed that the placenta and placenta of infected ewes contain pruritus virus, which can vertically transmit fetal and lambs from the ewe. Taking blood and serum from sick sheep to vaccinate susceptible sheep can make them sick. The cerebral spinal suspension of sick sheep can be replicated in susceptible sheep by intracranial or subcutaneous injection. Parenteral injection (or oral) of infected brain tissue into mice, rats, hamsters, mink and monkeys can cause experimental pruritus.

Pathogenesis After infection with this disease in sheep, especially after experimental inoculation, the virus enters the blood and localizes in the spleen and lymph nodes. It can later be found in the salivary gland, thymus, lung, bone marrow, liver, kidney, bladder and intestine. In these organs, the virus multiplies slowly and persists throughout the course of the disease. A few months later, the virus entered the spinal cord and finally reached the brain. The virus multiplied in the target cells (nerve cells) and caused damage. The virus was injected parenterally to mice, and the following phenomena could occur in sequence: the virus titer in brain tissue increased, vacuole formation in neurons and glial cells began to hypertrophy after 12-14 weeks, DNase, - Gluconidase and N-acetyl--glucosidase activity increased, and clinical symptoms appeared after 19 weeks. May die due to damage to the functioning of the nervous system.

Symptoms and lesions The natural incubation period is 1 to 5 years or longer. Therefore, there are very few clinical symptoms in sheep under one and a half years old. After the incubation period, neurological symptoms gradually develop and worsen. At the beginning of the disease, the sick sheep showed depression and sensitivity, easily panic, and had epilepsy symptoms. These symptoms are more likely to occur when catching sick sheep, and they are severe. Or show excessive excitement, raise their heads, ears, and stare at the eyes, run in a characteristic high leg posture (like the stiff gait of a donkey), and often drive to fall repeatedly. As the disease progresses, ataxia becomes more severe during exercise. In particular, tremor occurs in the head and neck. Muscle tremor aggravates during excitement and decreases during rest. During the development period, sick sheep rubbed their backs, body sides, hips, and head against fences, trunks, and appliances. Some sick sheep also scratched the chest, ventral, and head with their hind limbs, and bite their body And hip skin. Due to the effect of friction, bald hair areas appear on a large area of the skin surface on the neck, body side, back and sacrum. There was no skin inflammation after mechanical abrasions. If you touch the itchy skin, you may irritate the affected sheep by stretching their necks, shaking their heads, biting their lips, and licking their tongues. When vision is lost, it can cause collisions with objects such as fences and appliances. In the late stage of the disease, the body is weak, drowsiness or coma, and cannot lie on the floor. Throughout the illness, body temperature did not rise, and although appetite was maintained, weight lost. The course of the disease ranges from weeks to months, or even more than a year, and all sick sheep eventually die.

Autopsy, with the exception of wool shedding caused by friction and biting, skin trauma and weight loss, the internal organs often have no visible lesions. Histopathological changes are only found in the brainstem and spinal cord. The characteristic lesions include vacuole degeneration and shrinkage of neurons, spongy looseness of gray matter, and astrocyte proliferation. Vesicle formation in neurons appears as single or multiple vacuoles appearing in the cytoplasm. The typical vacuole is round or oval, with clear boundaries, and it represents a liquefied cytoplasmic mass. Spongiformity is the cavitation of the neural matrix, which breaks down the matrix fibers and forms many small holes. No lesions of viral encephalitis were found.

The diagnosis is not difficult based on typical symptoms and histopathological changes. The incubation period is very long, and both parents have a history of pruritus, constant itching, reflexive lip biting, tongue licking, and motor ataxia, all of which are important symptoms. Histopathological examination revealed characteristic lesions in the thalamus and the medulla. However, in new epidemic areas or new disease groups, relevant laboratory diagnosis must be performed to confirm the diagnosis, such as animal infection tests, PrPsc immunological tests, and scab-related fibers (SAF,) tests.

1. Prion biological assays There is no cell culture system available for detecting prions. Only animals can be used for biological assays. Mice and hamsters are the most commonly used experimental animals. The test specimens (brain, spinal cord, spleen, etc.) are homogenized for 10-fold dilution (8-10 dilutions), and each dilution is usually inoculated with 6 mice (30 L each) or hamsters (50 L). The inoculated animals were examined every 3 days, and the brain was sacrificed for histological examination. The primary isolation observation period is one year or more for mice and 7-8 months for hamsters. Finally, the endpoint titers (ID50 and LD50) were calculated based on the number of animal morbidity and death. The endpoint titer of brain homogenate is generally 105 to 106 LD50 per gram of brain tissue.

2. Immunological detection of PrPsc PrPsc is only found in human and animal infectious spongiform encephalopathy. PrPs present in human and animal cells in health and other diseases can be digested and eliminated by proteases without interfering with the detection of PrPsc. Therefore, PrPsc is a specific diagnostic method for TSEs and has recently been confirmed by the WHO TSEs expert meeting. It can be used not only for histopathological examination after death, but also for biopsy (brain, tonsil) for prenatal diagnosis. Although PrPsc cannot produce specific antibodies to the infected host, rabbits as non-susceptible animals can produce antibodies to PrPsc of other animals. This antibody can be used for Southern blotting of the test material (brain tissue) to detect PrPsc.

3. Scritch-related fiber (SAF) examination SAF examination is also a specific diagnostic method for TSEs, which is particularly important when the test material is not suitable for histopathological examination. The frozen brain and spinal cord are usually used as the test materials. Take 1 10g of the test material, prepare a homogenate with 10% (W / V) N-Laruoulsarcosine solution, and then extract the extract in an ultracentrifuge 22000g / 10min, 540000g / 2min, 540,000g / 25min, and the final precipitate is treated with protease K (PK 10 & micro; g 37 ° C / h), then resuspended in 50 L of distilled water for negative staining for electron microscopy.

4. Monoclonal antibodies have been widely used in the diagnosis of prion disease. The use of various specific monoclonal antibodies can distinguish mouse, hamster and human PrPsc. Recently, specific monoclonal antibodies that only share sites with PrPsc of various animals and do not react with PrPsc have been prepared. When this monoclonal antibody is used, samples can be detected without proteinase K treatment.

Control Due to the particularity of the disease (long incubation period, slow development, no immune response), general control measures are not effective. In countries and regions where the disease has not occurred, if the disease is found after importing breeding sheep, it is necessary to completely eliminate it (incineration or deep burial). If any breeding sheep have been divided into other flocks, these flocks should be sealed off and isolated for three and a half years. If sick sheep are found during the observation period, the same treatment can be done. Sodium hypochlorite can be used for sterilization of sheep sheds, and the effective chlorine concentration should not be less than 5 × 10-4, and the action time should be no less than lh.

Sheep Sickness Sheep Bone Meal Feeding Animals such as cattle and mink can cause infectious spongiform encephalopathy. Scrub virus can also cause illness. Some people who have researched pruritus abroad have found a disease called multiple cerebrospinal sclerosis. Icelandic sheep were vaccinated from human brain tissue that had died from the disease, and a disease indistinguishable from pruritus occurred.

Bovine spongiform encephalopathy

Bovine spongiform encephalopathy (BSE)

It is also known as "mad cow disease", which is characterized by a long incubation period and progressively worsening conditions. It is mainly characterized by abnormal behavior, ataxia, paresis, weight loss, gray matter sponge edema, and neuronal vacuole formation. The sick cow eventually died.

The disease was first detected in Scotland (1985) and has since occurred in Ireland, the United States, Canada, Switzerland, Portugal, France and Germany. The BSE epidemic in the UK is the most serious, with a cumulative diagnosis of 168,578 cases in 1997, involving more than 33,000 cattle. It is preliminarily thought that the disease was caused by cattle eating the bone meal (high protein supplementary feed) that contaminates sheep scrapie or bovine spongiform encephalopathy. At the same time, some patients with human spongiform encephalopathy (new type of Creutzfeldt-Jakob disease) suspected to have been infected by eating sick beef milk products were also found, which caused a stir in the world. European Union countries and the United States, Asia, Africa and more than 30 countries including China have banned the import of British cattle and their products.

The pathogen is a prion that is similar to pruritus virus. It is generally believed that BSE is caused by "scrappy-like pathogens" crossing the "species barrier" and causing bovine infections. In 1986, Well isolated itch-related fibers (SAF,) from BSE diseased cow brain emulsion for the first time. After amino acid analysis of SAF in diseased cattle, it was confirmed to be identical to PrPsc derived from itch disease sheep. The distribution of BSE prion in diseased cattle is limited to the diseased brain, cervical spinal cord, spinal cord end, and retina.

Epidemiology has a wide range of hosts, both natural and experimental. BSE can be transmitted to cats and various wild animals, and it can also be transmitted to humans. Sheep, breeding cattle and infected cattle with pruritus are the source of the disease. Animals are mainly infected by the digestive tract due to ingestion of bone and meat powder processed from the carcasses of sick sheep or sick cattle.

The average incubation period of BSE is about aquarium, and the age of onset cattle is 3-11 years, but most of them are concentrated in young cattle 4-6 years old, and rarely occur in cattle under 2 years old and over 10 years old. Most beef cattle are slaughtered for consumption at the age of 2 to 3 years, so the actual number of cattle infected with BSE should be far more than the number of clinical cases. It is estimated that more than 700,000 latent BSE-infected cattle entered the human food chain from 1985 to 1995, which constituted a serious public health problem.

The duration of symptoms is usually 14 to 180 days, and the clinical symptoms are different. Most cases show symptoms of the central nervous system. Common disease cattle are irritable, behave abnormally, and are overly sensitive to sound and touch. It often shows aggressiveness due to fear and mania, ataxia, unstable gait, and often kicks and kicks so much that it falls. A few sick cows can see trembling and twitching of head and shoulder muscles. Ankylosing spasms occur in the later stages and lactation decreases. Ear symmetry is difficult to move, often one extends forward and the other backwards or remains normal. Sick cattle have normal appetite, hard stools, high body temperature, increased breathing frequency, and often die extremely thin.

The gross changes of the lesions were not obvious. The main pathological change of histological examination is the spongy appearance of brain tissue (vacuumization of brain tissue). Brain stem gray matter has bilateral symmetrical cavernous degeneration, and contains varying numbers of vacuoles in the neural fiber network and nerve cells. No inflammatory response.

Diagnosis Based on the characteristics of clinical symptoms and epidemiological characteristics, a preliminary diagnosis of BSE can be made. Because the disease has neither an inflammatory response nor an immune response, serological diagnosis has been difficult to date. Therefore, qualitative diagnosis is mainly based on histopathological examination of the brain. According to Well et al. (1989), the accuracy of diagnosing BSE was as high as 99.6% for the changes in vacuoles in the brainstem, especially in the nucleus pulposus and trigeminal spine. Vascularization of the brainstem neurons and neural fiber network is of symptomatic significance. The laboratory diagnosis needed to confirm the diagnosis, such as animal infection test, PrPsc immunological test and SAF test, etc. are described in detail in Pruritus.

Prevention In order to control the disease, it is required in the United Kingdom to kill and destroy the affected cattle; it is prohibited to add ruminant protein (meal and bone meal) in the feed; it is prohibited to eat the diseased cattle after slaughter, and it is prohibited to sell the diseased beef. In recent years, many countries (including China) have banned the import of cattle, bovine semen, embryos, and any meat and bone meal from the United Kingdom to prevent the disease from entering the country.

Mad cow disease has not been found in China, but it is still possible to pass it from overseas. To this end, we must strengthen port quarantine and postal inspections. It is strictly forbidden to bring and mail beef and its products into the country. A monitoring system for mad cow disease should also be established, and a mandatory quarantine and reporting system should be adopted for mad cow disease. Once a suspicious case is found, it should be slaughtered, and the brain tissues should be taken for neuropathological examination. If it meets the diagnostic criteria for mad cow disease, all contact with herds should be handled, and the corpse should be burned or buried below 3m.

Mink infectious encephalopathy

Mink infectious encephalopathy (TME)

Mink infectious encephalopathy is a subacute cerebral sponge-like degenerative disease with no inflammatory changes and a mortality rate of almost 100%. The characteristics of the causative agent are similar to pruritus pruritus. The symptoms and brain histological changes caused by inoculating mink with pruritus disease can not be distinguished from naturally occurring mink encephalopathy. Sick brain tissue emulsions can be used for subcutaneous or intraperitoneal inoculation, which can cause infections in mink, ferret, skunk, raccoon, hamster, goat, sheep and monkey. The incubation period of mink oral infection is about 1a, and the initial symptoms are not obvious. Generally manifested as loss of appetite, gradually thinning and weakening, fur thickened, dyskinesia began to occur from the hind limbs, and the tail bent up on the back. It then becomes lethargic and eventually fails to move and loses its ability to locate. He died after 4-6 weeks of failure. Diagnosis and control measures refer to pruritus.

Kuru

Kuru disease, also known as tremor, is a disease of the central nervous system that is characterized by cerebellar degeneration. The cause of Kuru disease, like the pruritus virus, can persist in infected brain tissue for a long time. Taking the brain tissue emulsion of the deceased through the brain or non-neurological route, it can cause the infection of many kinds of primates, mink and ferrets (with an incubation period of 10 months or more). the same.

After infection, the incubation period is about 4 to 20 years. The main symptoms are dyskinesia, tremor, gait instability, unclear speech, difficulty in pronunciation and swallowing, and death within 3 to 9 months. Nervous system lesions are the same as pruritus. The central nervous system neurons are vacuolated, astrocytes are increased, and the gray matter in the brain is sponge-shaped. The cerebellum, pontine, and striatum are the most obvious.

The disease only occurs in the Fore settlements in the eastern highlands of Papua New Guinea in Oceania. About 2600 cases have been recorded since 1956. Its occurrence and spread are caused by eating brain tissue of dead relatives. The original Kuru disease may have been caused by the brain tissue of patients with Creutzfeldt-Jakob disease. After removing this bad habit, the number of cases fell rapidly and is now almost eliminated.

- Creutzfeldt -Jakob disease

Creutzfeldt-Jacob disease (CJD)

Creutzfeldt-Jakob disease was first reported by Creutzfeldt in France in 1920, and the following year Jacob described it in detail, so the disease was named after the two men.

Creutzfeldt-Jakob disease is also a disease of the central nervous system caused by PrPsc. The clinical manifestations are acute progressive dementia, which occurs in elderly people over 50 years old. The incubation period is several years to 3Oa. Symptoms include blurred vision, slurred speech, muscle cramps, sitting and movement difficulties, and finally death due to dissolution of brain tissue. Histopathological examination showed neuronal defects, severe glial hyperplasia, spongy lesions and amyloid plaque formation in the brain parenchyma.

This disease is the most common human virus disease and is widely distributed in the world. It has been reported in many countries, with a general incidence of one in one million. China's first reported cases of this disease in 1989, a total of 28 cases were reported in 1992, the actual number of cases may be more than this number.

6. Variant Creutzfeldt-Jacob disease (vCJD)

vCJD is a new type of human prion disease, which was first discovered in the United Kingdom in 1994. By 1997, 22 cases had been reported, mainly in the United Kingdom. vCJD appeared about 10 years after the outbreak and epidemic of BSE, and was concentrated in the United Kingdom, consistent with BSE in time and space. In 1996, a British expert research report was published, suggesting that mad cow disease may infect people through food, causing people to suffer from a new variant of Creutzfeldt-Jakob disease, called human spongiform encephalopathy, which caused great panic in Europe. In 1997, British and American scientists experimentally proved that Mad Cow Disease PrPsc disease can indeed cause vCJD in humans.

vCJD is different from typical Creutzfeldt-Jakob disease. It mainly occurs in young people. People who eat beef hamburger often are most susceptible. The age of onset is 14 to 40 years, with an average of 26.3 years. The course of disease is 9 to 53 months, with an average of 14 Months. In most clinical cases, mental disorders are the main symptoms, including anxiety, depression, loneliness, malaise, and other behavioral abnormalities. Early in the course of the disease, he showed sensory disturbances in the limbs and face and progressive cerebellar syndrome. With the development of the disease, symptoms such as memory impairment, myoclonus, and dementia later appear. Histopathological examination showed that cavernous lesions were most pronounced in the basal ganglia and thalamus, existed in the form of foci in the brain and the cerebellum, and were more obvious in the fusion of cavernous lesions. This is different from that CJD cavernous lesions are more common in the cerebral cortex.

Control measures see bovine spongiform encephalopathy.