Mad cow disease is caused by a modified prion protein infecting the nervous system of cattle, and when this modified protein is transmitted to humans, it can cause human mad cow disease. The route of infection is primarily through ingestion of bodily fluids or meat from infected cattle, and there is currently no cure.

In 2008, during the beef trade negotiations related to the Korea-US FTA, full beef imports were negotiated, but the deal was eventually settled for limited imports due to the opposition of many Koreans, whose main concerns were that ‘cattle with mad cow disease could be imported into Korea’ and ‘if cattle with mad cow disease are consumed, it could be transmitted to humans’. In other words, there were fears that if beef was banned, people would eat beef and contract human mad cow disease. Amidst these fears, many unfounded rumors spread among people who were not familiar with mad cow disease, such as that it could be transmitted through the air. So what is the cause of mad cow disease and how does it spread?

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 the need for DNA and RNA, which is an important part of explaining how mad cow disease occurs. When a normal prion protein found in the body of a person or animal is modified and becomes an altered prion protein, a disease like mad cow disease occurs.
Proteins are made up of amino acids that combine to form a folded structure, which can be divided into a helical alpha-helix structure or a folding screen-shaped beta-sheet structure. In general, normal prion proteins have an alpha-helix structure, which is a stable and normally functioning state. However, when prion proteins are mutated, some of the alpha-helix structure is deformed and replaced with a beta-sheet structure. This modified structure is not easily broken down by proteolytic enzymes and accumulates, resulting in disruption of the normal function of nerve cells.

Amyloid and modified prion proteins

A substance called amyloid plays an important role in the accumulation of these modified prion proteins. Amyloid forms when abnormally folded beta-sheet structures bind to other proteins, a common phenomenon in a variety of neurodegenerative diseases. For example, amyloid is found in Alzheimer’s disease, Parkinson’s disease, and mad cow disease, and when it accumulates, it interferes with the normal function of nerve cells, eventually causing disease.
In mad cow disease, when the modified prion protein accumulates in the central nervous system of cows, the cow’s brain becomes filled with amyloid and loses normal brain function. This process eventually leads to mad cow disease, which causes cattle to exhibit abnormal behavior or die from nervous system problems.

How mad cow disease is transmitted

The exact mechanism by which the modified prion protein infects the normal prion protein is not fully understood, but the leading hypotheses are that the modified prion protein interacts directly with the normal prion protein and rearranges its structure, or that an unknown protein in the body modifies the normal prion protein. If the altered prion protein continues to modify the normal prion protein and accumulates in the cow’s body, mad cow disease develops.
The primary route of transmission of mad cow disease is through ingestion of bodily fluids or meat from infected cattle. Importantly, the modified prion protein is not destroyed by high temperatures, so eating meat from cattle with mad cow disease can lead to human mad cow disease. Once in the human body, the modified prion proteins alter normal prion proteins, which can cause a fatal neurological disease.

History of prion diseases and the spread of mad cow disease

Mad cow disease originated in the United Kingdom in the 1980s and was traced back to the use of infected sheep carcasses as feed for cattle. In an effort to increase the efficiency of the meat industry, sheep carcasses were used as feed for cattle, which resulted in the transfer of a prion disease called scrapie from the sheep to the cattle. Cattle that ate the carcasses of these scrapie-infected sheep became infected with mad cow disease, and the carcasses of these cattle were then fed to other cattle, causing the disease to spread rapidly.
As such, prion diseases can spread from one animal to another, and there have been reported cases of mad cow disease spreading from cattle to humans. This transmission is not just between cows and humans, but has raised important warnings about the way meat is consumed and feed is produced around the world.

Solutions and future research

To date, there is no cure for prion diseases, including mad cow disease. It is extremely difficult to develop a cure without a full understanding of how the modified prion protein modifies normal proteins and the process. However, scientists are continuing to study prion proteins, and it is possible that in the future, a way to prevent or treat them may be developed.
Given that the outbreak of mad cow disease originated in an industrial setting, humans are largely to blame, so it’s important to enforce stricter standards in the production and management of animal feed and to take preventative measures against prion diseases.