In this blog post, we’ll take a detailed look at how prion proteins cause mad cow disease and how it spreads to humans.
How is mad cow disease transmitted?
In 2008, during trade negotiations on beef as part of the Korea-U.S. FTA, an agreement was reached to allow the full importation of beef; however, due to widespread public opposition, the negotiations ultimately resulted in a deal for limited imports. The main concerns among the public were that “cows infected with mad cow disease could be imported into Korea” and that “consuming meat from infected cows could lead to transmission to humans.” In other words, there was anxiety that if unrestricted beef imports were allowed, people could contract human mad cow disease by eating beef. Amid these concerns, various baseless rumors spread among those unfamiliar with mad cow disease, such as the claim that it could be transmitted through the air. So, what are the causes and transmission mechanisms of mad cow disease?
The Cause of Mad Cow Disease: Prion Proteins
The direct cause of mad cow disease is a proteinaceous infectious particle called a prion. Prion proteins are unique in that they can be transmitted without DNA or RNA, and this is a key factor in explaining the onset of mad cow disease. When normal prion proteins found in the bodies of humans or animals become misfolded and turn into misfolded prion proteins, diseases like mad cow disease occur.
Proteins are formed when amino acids bind together to create a folded structure, which can be classified into a helical alpha-helix structure or a fan-like beta-sheet structure. Generally, normal prion proteins have an alpha-helix structure, which is a stable and functional state. However, in misfolded prion proteins, part of the alpha-helix structure is altered, transforming it into a beta-sheet structure. This misfolded structure is not easily broken down by proteases and accumulates, ultimately disrupting the normal function of nerve cells.
Amyloid and Misfolded Prion Proteins
A substance called amyloid plays a crucial role in the process by which these misfolded prion proteins accumulate. Amyloid forms when abnormally folded beta-sheet structures bind to other proteins, a phenomenon common to various neurodegenerative diseases. For example, amyloid is found in Alzheimer’s disease, Parkinson’s disease, and mad cow disease; when this substance accumulates, it disrupts the normal function of nerve cells and ultimately causes disease.
In mad cow disease, when misfolded prion proteins accumulate in the central nervous system of cattle, the brain becomes filled with amyloid, leading to the loss of normal brain function. This process ultimately triggers mad cow disease, causing the cattle to exhibit abnormal behavior or die from neurological problems.
Transmission Routes of Mad Cow Disease
The exact mechanism by which misfolded prion proteins infect normal prion proteins has not yet been fully elucidated. However, leading hypotheses suggest that misfolded prion proteins interact directly with normal prion proteins to rearrange their structure, or that unknown proteins present in the body cause normal prion proteins to misfold. When misfolded prion proteins continue to convert normal prion proteins and accumulate within a cow’s body, BSE develops.
BSE is primarily transmitted through the consumption of infected cattle’s bodily fluids or meat. It is important to note that since misfolded prion proteins are not destroyed by high temperatures, consuming meat from a cow infected with BSE can lead to variant Creutzfeldt-Jakob disease (vCJD) in humans.
Once the misfolded prion protein enters the human body, it alters normal prion proteins, which can cause fatal neurological diseases.
The History of Prion Diseases and the Spread of Mad Cow Disease
Mad cow disease occurred on a large scale in the United Kingdom in the 1980s, and the underlying cause was the use of infected sheep carcasses as cattle feed. At the time, sheep carcasses were used as cattle feed to increase the efficiency of the meat industry, and during this process, a prion disease called scrapie, which had occurred in sheep, was transmitted to cattle. Cattle that consumed the carcasses of sheep infected with scrapie contracted BSE, and as the carcasses of these cattle were then fed to other cattle, BSE spread rapidly.
As such, prion diseases can spread from one animal species to another; in fact, there have been reported cases of BSE transmitted from cattle to humans. This transmission is not merely an issue between cattle and humans; it has raised significant global concerns regarding meat consumption and feed production methods.
Solutions and Future Research
To date, there is no known cure for prion diseases, including mad cow disease. Developing a treatment is extremely difficult without a full understanding of how the misfolded prion protein alters normal proteins. However, scientists are continuing their research on prion proteins, and it is possible that methods to prevent or treat these diseases may be developed in the future.
Since the outbreak of mad cow disease originated in an industrial setting, human responsibility is significant. Therefore, applying stricter standards to the production and management of animal feed and establishing preventive measures against prion diseases remain critical tasks.
