Bovine spongiform encephalopathy is a transmissible fatal neurogenerative disease caused by proteinaceous agents leading to neuronal loss, spongiform lesions, astrogliosis, and the disappearance of inflammatory reactions.
It is also known as ‘prion disease’ or ‘mad cow disease’ as it affects both humans and animals, especially domestic cattle like dairy cows.
- The proteinaceous infective agent that causes the infection is considered a normal host prion protein that has changed its conformation by undergoing a post-translational modification.
- The changed protein gets resistant to inactivation and enzyme degradation.
- Some commonly caused brain degenerative diseases are cerebral listeriosis, rabies, louping ill, and brain inflammation.
- The cases of BSE peaked at more than 300 cases per week in 1993 in the United Kingdom, which has become a great health concern since then.
- Other prion disease groups include Scrapie (host: sheep and goat), Chronic wasting disease (deer, elk, and moose), Transmissible mink encephalopathy (mink), Feline spongiform encephalopathy (Domestic cats and wild lions, pumas, cheetahs, and tigers), Exotic ungulate encephalopathy (antelopes).
- Creutzfeldt – Jakob disease, Gerstmann-Straussler-Scheinker disease, Fatal familiar insomania and Variant Creutzfeldt-Jakob disease infects human through oral route.
Sources and causes of contamination of Bovine Spongiform Encephalopathy (BSE)
- Bovine spongiform encephalopathy is caused by the prion, a misfolded protein that causes neurological disease and is resistant to high heat and pressure.
- The degenerated protein forms a beta-sheet from a normal alpha-helical structure and forms a small chain which causes cell death.
- The death of numerous cells causes lesions in the brain, which lead to various neurological diseases and eventually death.
- The disease is transferred when a healthy animal comes in contact with an infected food source, especially tainted meat.
- Many animals are believed to be infected by consuming contaminated meat and bone meal (MBM).
- Other cattle are infected by the scrapie-infected sheep products and feeding meat and bone meal to young calves.
- When humans get infected, variant Creutzfeldt-Jakob disease (vCJD) is believed to be caused which may be due to contaminated meat from slaughtered and dead livestock, mainly the brain, spinal cord, and digestive tract.
Pathogenic mechanism of Bovine Spongiform Encephalopathy (BSE)
- 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 enhances 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.
Signs and symptoms of Bovine Spongiform Encephalopathy (BSE)
- Early symptoms of the disease are nervousness and anxiety, similar to the individual’s cow habit suffering from herdsmen showing changes in behavior.
- Infection is neurological and slowly progressive, affecting mental status, changes in sensation and hyperesthesia to sound and touch.
- Abnormalation can be seen in movement and posture, low head carriage, tremors, hindlimb ataxia, loss of milk yield in mammals, cud-chewing reduction, slowing of pulse rate, and eventually terminal recumbency and death.
- The incubation period of the disease is seven days to more than a year, depending upon the occurrence and severity of the disease.
- The severity of the infection can be influenced by removing environmental stresses like increased stress during transport.
- BSE agent has not been detected on young cattle under 20 months old.
Epidemiology of Bovine Spongiform Encephalopathy (BSE)
- The United Kingdom has been the country with the highest reported cases, with a peak of 37,280 affected cattle.
- Then the spread of the disease to other European countries was from the breeding of cattle and meat-and-bone meals.
- Dairy cows are affected more than beef cattle due to different rearing systems. Therefore, dairy calves are separated after birth and are fed artificial milk.
- Other animals such as cats, eland, gemsbok, oryx, puma, cheetah, ocelot, and rhesus monkeys are also infected.
- The BSE cases declined by 40% per year after implementing the ban on food and feed.
- Japan is the country outside Europe experiencing 36 cases of BSE with non-imported cases, which established a governmental Food Safety Commission in 2003.
- Other countries like the United States, Canada, the Falkland, and the Sultanate of Oman slaughter and destroy, and the hazard is removed if the infection is detected and risk is not caused.
Diagnosis of Bovine Spongiform Encephalopathy (BSE)
- Clinical diagnosis is 85% accurate, and the confirmation of disease is done by microscopic examination of the brain.
- Bioassays in animals estimate the concentration of bovine spongiform encephalopathy prions.
- Enzyme-linked immunosorbent assay (ELISA) detects the prion protein (PrP) antigen by reacting with the antibody. It can also detect antibodies against the pathogen.
- Histological examination of tissue embedded with paraffin and stained with hematoxylin and eosin is used to examine pathological changes using light microscopy.
- Immunohistochemistry diagnoses the deposits of the prion protein. It uses secondary antibodies that target specific PrP epitopes to visualize the staining pattern of infection.
- They also pre-treat the affected tissues and differentiate strains of prions.
- The western immunoblotting technique detects the antigen by the application of antibodies attached to the prion proteins that separate according to their rate of migration.
- Three distinct bands can be visualized after gel electrophoresis with a high molecular weight band are diglycosylated, i.e., contain two sugar molecules, the middle one is monoglycosylated, and the lowest has no sugar molecules and is called unglycosylated.
- The unglycosylated band is used to differentiate isolates and strains. In contrast, di and mono-glycosylated bands are also used in isolate differentiation but complex interpretation depending on the laboratory methodologies.
Treatment and Vaccination
Currently, there are no treatments or vaccination for Bovine spongiform encephalopathy.
Prevention and control measures of Bovine Spongiform Encephalopathy (BSE)
- Bovine spongiform encephalopathy can be controlled by excluding all the meat and cattle-derived material in cattle feed products for farmed animals.
- It can also prevent disease transfer from one country to another by restricting the import and export of live cattle, beef, meat-and-bone meal, and other cattle products.
- Slaughterhouses in US and UK, the specified risk body materials such as brain, spinal cord, tonsil, intestines, eyes, and trigeminal ganglia are disposed of safely.
- Currently, international regulation of imports and export has implemented the BSE prion from entering the human food supply chain.
- When a BSE agent is suspected, the competent authority must be informed so they will dispose of the entire infected animal body by incineration.
- Laboratory workers working with infected material, sampling, and testing should follow laboratory safety rules and precautions.
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