Bovine spongiform encephalopathy (BSE) commonly known as "mad cow disease" is a disease which affects the central nervous system and is characterized by the development of cavities within brain nerve cells, giving the brain a sponge-like appearance.
BSE is a transmissible, inoculable disease but it is not contagious. It affects adult bovines following a long incubation period. The disease leads inevitably to death.
The disease was declared to the OIE by Canada in 2004 and the USA in 2003.
• Domestic and wild bovidae: bovines, bisons, etc.
• Domestic and wild felines: cats, tigers, pumas, ocelots, etc.
• Man: the BSE causes a rare form of the Creutzfeld Jakob (CJD) disease in man, the new variant CJD.
• Possibility of experimental transmission to ovines, caprines, pigs, mice, etc.
The "prion" hypothesis:
BSE is caused by an infectious protein particle, or pathogenic prion, which is currently the only infectivity-related macromolecule that we are able to detect. This non-conventional transmissible agent (NCTA) is strictly similar to the agent responsible for scrapie in small ruminants.
The pathogenic prion protein is a partially protease-resistant isoform from a host cell prion protein, expressed normally in the central nervous system. Compared to the cell prion protein, the pathogenic prion protein can not be physiologically eliminated by the proteinase K enzyme and instead accumulates in brain cell cytoplasms causing their degeneration.
The normal prion protein becomes pathogenic through conformation modification. The pathogenic prion protein, abnormally conformed, also has the ability to irreversibly modify normal protein conformation, causing it to become pathogenic.
Non-Conventional Transmissible Agent characteristics:
Can be distinguished from other pathogenic agents (bacteria, viruses, etc.) by the following properties:
• Absence of detectable nucleic acid.
• Absence of any detectable immunological reaction.
• High resistance to principal physical and chemical agents.
• Preserved through refrigeration and freezing.
• Inactivation (but not sterilisation):
- Through heat: 136°C, at 3 bars for 20min.
- In 2N sodium hydroxide solutions for 1h at 20°C.
NB: decontamination measures reduce pathogenic agent titres but their efficacy is often limited. The infectious agent survives especially in cadaveric tissue even after thorough quartering processes.
Incubation period: very long, on average 3 to 5 years.
Great individual variability as to the expression of symptoms. Central nervous signs dominate clinical presentation.
• Behaviour disorders: nervousness, fear, aggressiveness, stereotypical behaviour (licking, pruritis), depression. Affected animals graze apart from the rest of the herd.
• Sensory modifications: hyperaesthesia originating from overreaction to noise, light and touch (trembling, panic, falls).
• Locomotor disorders: unsteady gait, falls, difficulty in rising and in changing direction. Hypermetria and ataxia of the hind limbs may occur as motor lack of coordination.
• Neurovegetative disorders: suspended rumination, bradycardia, cardiac arrhythmia.
• Other symptoms: drop in milk production, occasionally pruritis.
• Terminal phase: general deterioration of health, recumbence. Progression towards death in several weeks.
Comments:
- Absence of fever. Absence of any detectable immune response.
- Cows presenting BSE symptoms at the end of the gestation period will fully express the disease after farrowing, which is considered a "stress" that reveals the disease.
No effective cure is known either for man or for animals.
Sanitary prophylaxis
Disease-free countries:
• Targeted monitoring: detecting symptoms evocative of BSE
• Importation-related protection measures
• Ban on the use of meat and bone meal in animal feed
Infected countries:
• Slaughter rewarded by financial compensation for confirmation of cases by the laboratory before complete carcass destruction.
• Cohort slaughter (ancestors and offspring of the infected animal, animals of the same age having received the same feed).
• Ban on the use of meat and bone meal in animal feed.
• Animal protein recycling controls.
• Cattle identification and traceability.
• Withdrawal and systematic destruction of specified risk material (head, spinal cord, ileum, spleen, thymus, intestines, etc.) in carcasses for consumption.
No human or animal vaccine exists.