Incurable bovine spongiform encephalopathy is characterized by irreversible changes in the brain tissue under the influence of prions, special proteins of animal origin. In the process of development of the pathology, the central nervous system of the animal is affected, the final is death in 100% of cases. The incubation period of the disease in cows lasts from 2.5 to 8 years.
History of the spread of the disease
According to some reports, the disease was discovered in 1965. However, it was officially documented in the UK only in November 1986. The disease of the "mad" cow, as it was christened at that time, infected about 200 thousand head of cattle.
Spongy (or spongiform) encephalopathy, the disease was named in 1987 by a group of researchers under the supervision of Wells, since one of the obvious symptoms of the disease was the sponginess of the structure of neurons and the gray matter of the brain. Since 1990, the spread of the disease began in France, Switzerland and other European countries. As a result of the importation of infected livestock, cases of the disease were also recorded in Oman, Israel, Canada, Japan and Australia.
At present, it is believed that the development of bovine spongiform encephalopathy is caused by overcoming the interspecific barrier by the sheep scrapie pathogen. He gets into the feed for cows with meat and bone meal containing it.
Mistakes in the pursuit of profit
Despite the fact that now spongiform encephalopathy has spread around the world, the causes of the disease of cows may be different. For example, in the UK, for example, this happened because of the increase in production during the processing of infected sheep heads for meat and bone meal.
Another reason for the widespread spread of the disease was the change in the way raw materials were processed during sterilization. More precisely, the heat treatment of materials of animal origin was replaced by ordinary drying using organic solvents. In the second half of the 20th century, many utilization plants began to use this method.
In addition, an earlier weaning of calves from the udder of the cow began to be applied in practice. This was done to increase the production of milk for sale. As a result, the kids were intensively fed with meat and bone meal contaminated with a dangerous infection.
European countries suffered enormous financial damage associated with the slaughter and destruction of carcasses of more than 4 million head of sick cattle. In particular, Great Britain, where the disease encephalopathy was first recorded, suffered economic damage amounting to more than 7 billion pounds sterling. About 3 million heads of cattle in the incubation period of the disease were slaughtered and sent to meat processing plants.
Thus, the meat got into the food of people. According to forecasts, because of this, about 7,000 people in the world can become ill with Creutzfeldt-Jakob disease with similar effects on the body. Since the discovery of the disease has already died about 200 people.
Economic damage is exacerbated by the 100% mortality of cows from spongiform encephalopathy.
Conditions and causes of development
Spongiform encephalopathy is caused by prions, protein infectious particles that have microscopic dimensions and high resistance to physical and chemical effects. Infectiousness of the infected brain is maintained during heat treatment, X-ray irradiation, soaking in enzyme solutions, and many other procedures that usually lead to the elimination of the infection.
The infectious nature of prions was confirmed by the American biochemist Prusiner in 1982. As a result of his research, it was found that prions, being modified protein mutants, do not contain nucleic acids, which radically distinguished them from all previously studied viruses. Spongiform encephalopathy in cattle results from a change in the shape of protein molecules, with prions entering the body, as a rule, when eating infected feed.
About infection and the development of the disease
It is believed that prions, once in the body of a cow, are first found in lymphatic organs, such as the spleen. Then enter the brain. Prions alter the shape of ordinary protein molecules that are inserted into the cell membrane of brain neurons, leading to the synthesis of pathogenic prions. They are not absorbed by the body and lead to the destruction of neurons.
When this happens vacuolation of the tissues of the cerebellum and brainstem, called encephalopathy. Vacuoles have a round or ovate, less often irregular shape. Brain tissues become like a sponge, which caused such a name of the disease. At the same time, there is no brain inflammation in case of encephalopathy in a cow.
The highest concentration of prions is observed in the brain and spinal cord, adjacent vertebrae, spleen, lymph nodes, eyeballs. The concentration is minimal in the meat, skin, bones of the body and limbs of a cow.
Thus, by eating meat of cattle infected with spongiform encephalopathy, the risk of disease is practically eliminated, due to the negligible content of prions in it.
Traffic disruption and constant fright
The incubation period of the disease lasts on average from 2.5 to 8 years. As a rule, animals aged 2 years and older are ill. In sick cows, appetite does not decrease, the temperature does not rise, but the milk yield decreases. With the disease, a complete and irreversible damage to the central nervous system occurs, lasting from several weeks to a year or more.
The following symptoms of spongiform encephalopathy are observed:
- the constant fear of the animal (the cow is afraid to pass through the gate, it cannot overcome the simplest obstacle);
- nervousness and anxiety, turning into aggression towards a person or fellow tribesmen;
- gnashing of teeth;
- twitching of individual limbs or the whole body;
- active ears, face licking, scratching the head on various objects;
- motor impairment (trotting movements, strayed legs, falling during sharp turns, raising the tail), the overall weakening of the back of the animal;
- increased sensitivity with noise, touch.
Spongiform encephalopathy is an incurable disease. Only isolated cases of remission and a single case of recovery are known.
Blame dangerous feed
Spongiform encephalopathy of cattle has spread due to the processing of the heads of infected scrape sheep for meat and bone meal. It was abundantly added to feed for cows and bulls, which allowed scrapie to overcome the species barrier due to large amounts of cows entering the body. Infection often occurred in dairy farms, as calves were striving to take away from their mothers and translate into compound feed earlier. The Holstein-Friesian dairy population suffered the most from the disease, mostly cows, not bulls.
There have been cases of infection of antelopes, oryx in zoos, eating scrapi-infected meat and bone meal, and even domestic cats that eat canned meat from processed sheep heads. Under laboratory conditions, encephalopathy of pigs, goats, marmosettes, and other animals was successfully infected.
Diagnosis and pathological signs of the disease
During the life of an individual, it is still not possible to diagnose spongiform encephalopathy. The diagnosis is made only after an autopsy of the animal. Due to the impossibility of producing antibodies upon infection, cows have not developed methods for analyzing blood or other physiological fluids.
The pathologist makes a conclusion based on the results of microscopy of preparations of the brain and lympho-containing organs. At research by means of a microscope spongy formation or vacuolization of cytoplasm of neurons is found. Also, histological methods, electron microscopy and deliberate infection of laboratory mice with brain particles of a dead animal give results.
In the diagnosis of spongiform encephalopathy, such diseases as encephalitis, rabies, tetanus, lack of magnesia, calcium, botulism, poisoning with poisons, arsenic, mercury and other substances are being sought to rule out.
Prevention and precautions
In countries where no outbreaks of spongy encephalopathy were detected, preventive measures are a ban on the use of meat and bone meal from sheep for the production of animal feed. Importation of livestock and products of processing of cow carcasses, for example, leather, bones, fat, is restricted and strictly controlled. Laboratory testing of seed material is carried out, brain samples of dead cows over 3 years old are being taken to investigate the presence of spongy formations.
In countries undergoing spongiform encephalopathy, it is prohibited to use animal supplements in feed for cattle. Beef offal can not be eaten. A deep diagnosis of sick animals is also carried out, and infected carcasses of cows must be burned.
For disinfection of instruments used to open or dissect tissue, prolonged pressure digestion in autoclaves is used. Soaking in sodium or bleach solutions or burning if the material of the tools is combustible helps. Disposal of carcasses of cows carried out by burning. It may be buried on cattle cemeteries in concrete tanks with pouring disinfectant mixture.
To eliminate the risk of food contamination should not eat beef by-products, such as the brain, kidneys, intestines.
About harm to humans
If the infection still happened, the person becomes at risk of developing spastic pseudosclerosis or Creutzfeldt-Jakob disease. This type of prion disease causes people to die in 85% of cases when they are mild. If the form is severe, death is guaranteed.
The disease is able to hit a person of any race, gender and age. It is more common in Polynesia because of the ritual custom of eating the brain of a deceased tribesman. In this case, prions in large quantities enter the body of cannibal.
The mechanism of disease development is similar to that in cattle. Prions modify the shape of protein molecules, block metabolic processes, activate the excess production of active substances, and the cell destroys itself. Once in the brain, prions vacuolate the cytoplasm of neurons. With the disease, only symptomatic treatment is possible, the mortality rate of patients is very high.
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