Bovine Spongiform Encephalopathy (BSE), widely known as Mad Cow Disease, is a fatal neurodegenerative disorder affecting cattle. It causes progressive damage to the brain and spinal cord, leading to abnormal behavior, incoordination, and eventual death. The term “contagious” typically describes diseases spread through casual contact, air, or water. BSE does not transmit in this conventional manner, a distinction stemming from its unusual causative agent.
What Makes BSE Unique
The agent responsible for BSE is not a virus or a bacterium but an abnormal protein known as a prion. Prions are misfolded versions of a normal protein found on the surface of nerve cells in the brain. The term prion is short for “proteinaceous infectious particle,” highlighting that it is purely protein and lacks the genetic material (DNA or RNA) found in all other infectious agents.
This misfolded protein acts as a template, forcing normal prion proteins to change into the abnormal, disease-causing shape. This sets off a chain reaction where the misfolded proteins accumulate and form plaques in the brain tissue. The accumulation causes the brain to develop microscopic holes, giving it a sponge-like appearance, a condition called spongiform encephalopathy.
Because prions lack genetic material, the agent cannot replicate itself or be shed through respiratory droplets. The abnormal prion is also exceptionally resistant to standard sterilization methods like heat and common disinfectants. Transmission requires direct exposure to the infectious protein, primarily through ingestion of contaminated tissues.
How Transmission Occurs
Transmission of classical BSE occurs primarily through a specific route: the consumption of feed contaminated with the prion agent. Historically, the large-scale outbreak in the United Kingdom was linked to the practice of incorporating meat and bone meal (MBM) into cattle feed. This MBM was produced from rendered animal by-products, including the tissues of cattle that had developed BSE or sheep that had scrapie, another prion disease.
The infectious prions concentrate in the nervous tissue, such as the brain and spinal cord, of an infected animal. When these high-risk tissues were processed and recycled into cattle feed, the prions were passed from one animal to another through the digestive system. This mechanism of recycling rendered animal protein back into the feed supply was the main driver of the epidemic spread among cattle.
BSE is not transmitted through casual contact between animals, nor does it spread through the air, water, or soil. The infection is established when the animal ingests the abnormal prions, which then migrate from the digestive tract to the central nervous system over a long incubation period, typically two to eight years.
The Risk to Human Health
The human equivalent of BSE is variant Creutzfeldt-Jakob Disease (vCJD), which is caused by consuming beef products contaminated with the BSE prion. The risk to humans is tied to the consumption of tissues where the prions concentrate most heavily, such as the brain, spinal cord, and certain other nervous tissues. Solid muscle meat, such as steak, is considered to pose a very low risk of transmission.
Once the prions enter the human body, vCJD can have a very long incubation period, sometimes lasting a decade or more before symptoms appear. The disease is invariably fatal, with a typical survival time of about 13 to 14 months after the onset of symptoms. Early signs of vCJD often include psychiatric symptoms like depression and anxiety, followed by neurological issues such as poor coordination and dementia later in the illness.
Since its discovery in 1996, a total of 233 cases of vCJD have been reported globally, with the vast majority occurring in the United Kingdom. Global control measures implemented to prevent the infectious agent from entering the human food supply have been successful. The number of new cases has dropped significantly in recent years, making vCJD a very rare disease today.
Global Safety Measures and Mitigation
Regulatory bodies worldwide implemented strict measures to interrupt the transmission cycle and safeguard both animal and human health. A primary control measure was the implementation of strict feed bans, which prohibit the use of animal proteins, particularly those derived from ruminants like cattle and sheep, in cattle feed. This action directly addressed the main cause of the BSE epidemic in cattle.
Governments also mandated the removal of Specified Risk Materials (SRMs) from the entire food and feed supply chain. SRMs are parts of the animal carcass that are most likely to harbor the highest concentration of prions. These materials typically include:
- The brain, spinal cord, and parts of the small intestine from cattle of all ages.
- Additional tissues like the skull and eyes from older cattle.
SRMs are segregated at the slaughterhouse and disposed of through controlled methods, such as incineration, to prevent them from entering human food or animal feed. Comprehensive surveillance and testing programs are also in place globally to monitor the cattle population and quickly identify any new cases. These combined measures have dramatically reduced the incidence of BSE and the risk of human exposure to the agent.