Bovine Spongiform Encephalopathy (BSE)- Mad Cow Disease
Bovine spongiform encephalopathy (BSE), also known as mad cow disease, is a fatal neurological disorder that affects cattle. It is one of the transmissible spongiform encephalopathies (TSEs), a group of diseases that cause sponge-like degeneration of the brain and spinal cord in various animals. BSE was first identified in the United Kingdom in 1986 and has since been reported in several other countries, mainly in Europe. BSE has a significant impact on animal health, public health, and trade.
BSE is not a contagious disease and therefore is not spread through casual contact between cattle or with other species. The primary route of spread of classical BSE infection in cattle is feed contaminated with the infectious agent. Scientific evidence shows that feed contamination results from incorporating ingredients (for example, meat-and-bone meal) that contain protein derived from rendered infected cattle and contaminated with BSE prions.
A cow gets BSE by eating feed contaminated with parts that came from another cow that was sick with BSE. The contaminated feed contains the abnormal prion, and a cow becomes infected with the abnormal prion when it eats the feed. If a cow gets BSE, it most likely ate the contaminated feed during its first year of life. Remember, if a cow becomes infected with the abnormal prion when it is one-year-old, it usually will not show signs of BSE until it is five-years-old or older. This is called the incubation period.
The incubation period for classical BSE from time of infection, thought to occur early in life, until the onset of clinical signs averages three to six years. During the incubation period, there is no way to tell that a cow has BSE by looking at it. Once a cow starts to show symptoms, it gets sicker and sicker until it dies, usually within two weeks to six months.
BSE belongs to a family of diseases known as transmissible spongiform encephalopathies (TSEs) that includes scrapie in sheep and goats, chronic wasting disease in deer, elk and moose, and in humans, classic and variant Creutzfeldt-Jakob disease (CJD) among other syndromes. The prion agent is resistant to enzymatic breakdown and most disinfection treatments.
BSE exists in two forms: classical (C-type) and atypical (L-type or H-type). Classical BSE is associated with consumption of contaminated feed, while atypical BSE occurs spontaneously in older cattle and is not linked to feed. Atypical BSE has a longer incubation period and different clinical signs than classical BSE.
BSE is caused by an abnormal form of a protein called prion, which can change the shape of normal prions and make them resistant to degradation by enzymes. Prions can accumulate in the brain and other tissues, causing damage and death to nerve cells. The exact mechanism of how prions cause disease is still unclear, but some steps have been proposed based on experimental evidence:
- Infection occurs via the oral or intracerebral route from consuming diseased meat and bone meal containing BSE agents.
- Once the BSE agent reaches the digestive tract, it crosses the mucosal barrier through the epithelium of the gut and tonsils by specialized macromolecule transporter M-cells. Protein like ferritin and direct uptake by dendritic cells also enhance the BSE agent to reach the digestive tract.
- Then it accumulates and replicates in the gut-associated lymphoid tissues (GALT), mainly in the ileum, jejunum, and tonsil.
- The BSE agent initially infects the neural tissue of the gut called the enteric nervous system (ENS).
- The ENS agent travels through the efferent neuronal pathways (parasympathetic and sympathetic) to the brain, cervical spinal cord, cerebral cortex, cerebellum, hippocampus, and basal nuclei.
- Then the infection spreads to the entire brain, at last causing brain degenerating disease.
- Cattle with bovine spongiform encephalopathy cause characteristic lesions in the brain and spinal cord, causing neuronal vacuolation, degeneration and loss, and astrocytic hypertrophy and hyperplasia.
The pathogenic mechanism of BSE is similar to other prion diseases that affect humans and animals, such as scrapie in sheep and goats, chronic wasting disease in deer and elk, and Creutzfeldt-Jakob disease in humans. However, there may be differences in the strains of prions, the routes of transmission, and the susceptibility of different species.
BSE is a progressive and fatal neurologic disease that affects the brain and spinal cord of cattle. The clinical signs of BSE usually appear when the cattle are between four and six years old, after a long incubation period of several years. The signs may vary depending on the stage and severity of the disease, but they generally include:
- Changes in behavior, such as nervousness, aggression, apprehension, or reduced interaction with other animals.
- Changes in movement and posture, such as incoordination, difficulty in rising, stumbling, trembling, high-stepping of the hind legs, or low head carriage.
- Changes in sensation and perception, such as hyperesthesia (increased sensitivity) or hypoaesthesia (reduced sensitivity) to touch or sound, or abnormal responses to visual stimuli.
- Changes in physiological functions, such as decreased milk production, weight loss, reduced appetite, or slowing of pulse rate.
The signs of BSE are progressive and irreversible, meaning that they get worse over time and cannot be cured. The affected cattle usually die within two weeks to six months after the onset of clinical signs. There is no treatment or vaccine for BSE. The only way to confirm the diagnosis of BSE is by examining the brain tissue of the dead animal under a microscope or by using test kits that can detect the abnormal prion protein.
Bovine spongiform encephalopathy (BSE) is a rare but fatal prion disease that affects the central nervous system of cattle. The first case of BSE was reported in the United Kingdom (UK) in 1986, and since then, more than 180,000 cases have been confirmed in the UK and other countries. The origin of the first case of BSE is unknown, but the epidemic was caused by the recycling of processed waste parts of cattle, some of which were infected with the BSE agent, to other cattle in feed. This practice was banned in the UK in 1988 and in the European Union (EU) in 1994.
The peak of the BSE epidemic in the UK was in 1992, when more than 37,000 cases were reported. The number of cases has declined steadily since then, due to the implementation of control measures such as feed bans, slaughter and disposal of infected animals, and surveillance and testing programs. According to the World Organization for Animal Health (OIE), as of September 2021, there have been only 14 cases of BSE reported worldwide in 2021.
BSE has also spread to other countries through the export of live cattle, beef products, and meat-and-bone meal (MBM) from the UK and other BSE-affected countries. The first case of BSE outside the UK was reported in Switzerland in 1990. Since then, BSE has been detected in more than 20 countries, including France, Germany, Spain, Italy, Japan, Canada, and the United States. The majority of these cases were linked to the import of MBM from the UK or other EU countries before the feed ban was enforced.
The risk of BSE transmission to humans is considered low, but not negligible. Humans can develop a variant form of Creutzfeldt-Jakob disease (vCJD), a fatal neurodegenerative disorder, by consuming contaminated beef products from BSE-infected cattle. The first case of vCJD was reported in the UK in 1996, and since then, more than 200 cases have been confirmed worldwide, mostly in the UK and France. The incubation period of vCJD is unknown, but it may range from several years to decades. There is no effective treatment or prevention for vCJD.
The OIE classifies countries into three categories based on their BSE risk status: negligible risk, controlled risk, and undetermined risk. The classification is based on various factors such as the history of BSE cases, the implementation of control measures, and the quality of surveillance and testing systems. As of September 2021, there are 67 countries with negligible risk status, 64 countries with controlled risk status, and 81 countries with undetermined risk status.
BSE is a serious animal and public health threat that requires continuous vigilance and cooperation among countries. The OIE provides guidelines and standards for the prevention and control of BSE and other transmissible spongiform encephalopathies (TSEs), as well as for the safe trade of animals and animal products. The OIE also supports countries in strengthening their capacities for surveillance and diagnosis of BSE and other TSEs.
BSE is a challenging disease to diagnose because there are no reliable tests for detecting the abnormal prion in live animals. The only definitive way to confirm BSE is by examining the brain tissue of a dead cow under a microscope or by using test kits that can detect the abnormal prion in the brain.
However, there are some clinical signs that can suggest BSE in cows, such as:
- Behavioral changes, such as nervousness, anxiety, aggression, or isolation.
- Abnormalities of posture and movement, such as low head carriage, tremors, hindlimb ataxia, or difficulty walking and getting up.
- Hyperesthesia, or increased sensitivity to sound and touch.
- Weight loss and decreased milk production.
These signs usually appear when the cow is five years old or older, and they progressively worsen until the cow dies, usually within two weeks to six months after the onset of symptoms. The incubation period of BSE, from the time of infection to the appearance of signs, can range from four to six years.
Because these signs are not specific to BSE and can be caused by other diseases or conditions, it is important to rule out other possible causes before suspecting BSE. Some differential diagnoses for BSE include:
- Lead poisoning
- Nervous ketosis
- Brain abscess
- Brain tumor
To rule out these diseases, veterinarians can perform various tests on blood, urine, cerebrospinal fluid, or other samples from the cow. However, these tests cannot confirm or exclude BSE with certainty.
Therefore, if a cow shows signs consistent with BSE and other causes have been ruled out, it should be reported to the competent authority for further investigation and possible euthanasia. The brain tissue of the cow should then be collected and sent to a laboratory for confirmation of BSE by histological examination or immunological detection of the abnormal prion.
Unfortunately, there is no treatment or vaccine to prevent BSE in cattle or vCJD in humans. Once an animal or a person is infected with the abnormal prion, there is no way to stop the disease from progressing and causing irreversible damage to the brain and nervous system. The only way to control the disease is to prevent exposure to the infectious agent through strict regulations on animal feed, slaughter practices, and meat products.
The U.S. Department of Agriculture (USDA) has implemented several measures to protect the health of cattle and consumers from BSE. These include:
- Banning the use of most mammalian protein in cattle feed since 1997
- Prohibiting the slaughter of non-ambulatory (downer) cattle for human food since 2004
- Removing specified risk materials (SRMs) such as brain, spinal cord, and certain nerves from cattle 30 months of age and older at slaughter since 2004
- Testing cattle that show signs of neurological disease or die unexpectedly for BSE
- Restricting the import of live cattle, beef, and beef products from countries where BSE is known to exist or at risk
These measures have been effective in reducing the incidence of BSE in the U.S. and preventing the spread of the disease to other countries. According to the USDA, only six cases of BSE have been detected in the U.S. since 2003, and none of them were linked to contaminated feed. The last case was reported in 2018 in Florida.
The Food and Drug Administration (FDA) also plays a role in preventing BSE and vCJD by regulating animal feed, human food, cosmetics, drugs, and medical devices that may contain animal-derived ingredients. The FDA has established rules to ensure that these products are safe and do not pose a risk of transmitting the abnormal prion.
In addition, the Centers for Disease Control and Prevention (CDC) monitors the occurrence of human prion diseases such as vCJD in the U.S. and collaborates with other countries and organizations to conduct surveillance and research on these diseases. The CDC also provides information and guidance to health care providers and the public on how to prevent and diagnose vCJD.
Despite these efforts, there is still no cure or vaccine for BSE or vCJD. Therefore, it is important to continue to follow the best practices and regulations to prevent exposure to the abnormal prion and protect the health of animals and humans.
Bovine spongiform encephalopathy (BSE) is a fatal neurological disease that affects cattle and can be transmitted to humans through consumption of contaminated meat. Therefore, it is essential to implement prevention and control measures to protect animal and public health. Some of the measures that have been taken or recommended by various authorities are:
- Restricting the importation of live ruminants and their products from countries with BSE cases. This measure aims to prevent the introduction of BSE agents into countries that are free or have a low incidence of the disease. For example, the United States has banned the importation of live ruminants and certain ruminant products from all European countries since 1997.
- Banning the use of ruminant-derived materials in animal feed. This measure aims to prevent the spread of BSE among cattle and other susceptible animals through ingestion of contaminated feed. For example, the United States and Canada have banned the use of mammalian protein in ruminant feed since 1997. Additionally, both countries have enhanced their feed bans by prohibiting the use of specified risk materials (SRMs), such as brain, spinal cord, and intestines, in all animal feed, pet food, and fertilizer since 2009.
- Removing specified risk materials from human food chain. This measure aims to prevent human exposure to BSE agents through consumption of potentially infected tissues. For example, the European Union has required all member states to remove SRMs from animal carcasses and human food since 2000. Similarly, the United States and Canada have mandated the removal of SRMs from cattle slaughtered for human consumption since 2004.
- Conducting surveillance and testing of cattle for BSE. This measure aims to detect and monitor the occurrence and prevalence of BSE in cattle populations and to trace and dispose of infected animals and their products. For example, the World Organisation for Animal Health (WOAH) recommends targeted surveillance of cattle with clinical neurological signs and transparency in reporting BSE findings. The United States and Canada have implemented active and passive surveillance programs that test thousands of cattle annually for BSE.
- Following laboratory safety rules and precautions when handling BSE-infected materials. This measure aims to prevent accidental exposure or transmission of BSE agents to laboratory workers or other animals. For example, laboratory workers should wear personal protective equipment, use appropriate disinfection methods, and dispose of waste properly when working with BSE-infected tissues or samples.
These prevention and control measures have been effective in reducing the incidence and spread of BSE in many countries. However, there is still no treatment or vaccine for BSE, so vigilance and compliance are necessary to ensure food safety and animal health.
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