Prions are unique infectious agents composed solely of misfolded proteins, distinct from viruses or bacteria because they lack genetic material like DNA or RNA. These abnormal proteins can induce normally folded proteins in the body to also misfold, leading to cellular damage and ultimately death. One notable prion disease is Bovine Spongiform Encephalopathy (BSE), commonly known as Mad Cow Disease, which affects cattle.
Understanding Prions and Bovine Spongiform Encephalopathy
Prion diseases involve a normal cellular protein (PrPᶜ) misfolding into an abnormal form (PrPˢᶜ). This PrPˢᶜ is resistant to degradation in the body and induces other healthy PrPᶜ proteins to misfold, leading to an accumulation of these abnormal proteins, particularly in the brain.
Bovine Spongiform Encephalopathy (BSE) is a fatal neurodegenerative disease that affects the central nervous system of cattle. It is characterized by the accumulation of these misfolded prion proteins in the brain, giving the tissue a spongy appearance. The incubation period for BSE in cattle can range from 2 to 8 years before symptoms become apparent. Once clinical signs emerge, the disease progresses over weeks to months and is invariably fatal.
Cattle affected by BSE display neurological symptoms as their nervous system degenerates. Signs include changes in temperament (e.g., nervousness or aggression), abnormal posture, incoordination, difficulty rising, and a high-stepping gait. Other symptoms can include weight loss, decreased milk production, tremors, and an exaggerated reaction to touch or sound.
Human Variant Creutzfeldt-Jakob Disease
Variant Creutzfeldt-Jakob Disease (vCJD) is the human form of BSE, directly linked to the consumption of products from cattle infected with the BSE prion. This rare, fatal brain disease was first reported in the United Kingdom in 1996. Approximately 233 cases of vCJD have been reported globally since its discovery, mostly in the UK during the late 1990s and early 2000s.
The incubation period for vCJD in humans can be quite long, often taking years or decades to manifest. Once symptoms begin, the disease progresses rapidly and is always fatal, with patients typically surviving 13 to 14 months. Initial symptoms often include prominent psychiatric issues, such as depression, anxiety, and withdrawal, along with painful sensations like tingling or burning.
As the disease advances, individuals with vCJD develop neurological signs, including poor coordination, involuntary movements, and progressive dementia. Differentiating vCJD from classic CJD can be challenging, but vCJD tends to affect younger individuals, with an average age of onset around 28 years.
Transmission and Risk Factors
The primary route of BSE transmission within cattle populations was through contaminated feed. This involved cattle feed containing meat-and-bone meal (MBM) from rendered, infected cattle. This practice allowed the infectious prion to spread through the cattle population, particularly in the United Kingdom where the epidemic originated in 1986.
The transmission of BSE from cattle to humans occurred primarily through the consumption of specific bovine tissues from infected animals. Tissues with the highest infectivity include the brain, spinal cord, and retina. While muscle meat and milk are considered safe, the consumption of products containing these specified risk materials (SRMs) was the direct link to human illness.
The BSE epidemic in cattle peaked in the UK around 1993, with nearly 1,000 new cases per week. This widespread animal infection subsequently led to the emergence of vCJD in humans about a decade later, consistent with the long incubation period of prion diseases.
Prevention and Public Health Measures
Stringent public health measures have been implemented globally to control BSE and prevent vCJD in humans. A primary and effective safeguard is the ban on specified risk materials (SRMs) from entering the food chain. These materials, which include the brain and spinal cord from cattle, are removed at slaughter because they are known to harbor the highest concentration of the BSE prion.
Another significant measure is the implementation of feed bans, which prohibit the use of mammalian protein in ruminant feed. This prevents recycling contaminated animal by-products into cattle feed, breaking the chain of infection. These bans, such as the one imposed by the U.S. Food and Drug Administration in 1997, are considered a key measure to prevent BSE transmission.
Surveillance programs are also in place to monitor for BSE in cattle populations. These programs involve targeted testing of cattle exhibiting neurological signs or other symptoms consistent with BSE, as well as older animals. Combined with feed bans and SRM removal, surveillance has drastically reduced BSE incidence, making the food supply very safe.