How could people get Mad Cow Disease? The theory I've heard is that people could get Mad Cow Disease from proteins that are misfolded. I thought in the digestive track proteins are split into amino acids before being absorbed into the bloodstream, so shouldn't the shape of the protein be irrelevent? Both classic and variant CJD belong to a broad group of human and animal diseases known as transmissible spongiform encephalopathies (TSEs). The name derives from the spongy holes, visible under a microscope, that develop in affected brain tissue.
TSEs in humans
One of the oldest known human TSEs is kuru. Found only in a remote New Guinea tribe, kuru reached epidemic proportions in the 1950s and 1960s. How the disease originated isn't clear, but scientists know that it spread when tribe members ritualistically consumed the brains of dead relatives.
Other human TSEs include Gerstmann-Straussler-Scheinker disease and fatal familial insomnia 鈥?hereditary diseases so rare they occur in just a few families.
TSEs in animals
The most widespread TSE in animals is scrapie, a disease that has affected sheep and goats since the mid-18th century. Infected animals become agitated, attempt to rub off their wool by scraping against trees and posts, and usually die a few months after symptoms appear. Many experts believe that British cattle developed mad cow disease after being fed the remains of scrapie-infected sheep.
Other animals affected by TSEs include domestic and exotic cats (feline spongiform encephalopathy); mule deer, white-tailed deer, black-tailed deer and elk (chronic wasting disease); and mink (transmissible mink encephalopathy).
The origin of TSEs
The cause of TSEs eluded scientists for centuries. But in the early 1980s, Stanley Prusiner, a researcher at the University of California at San Francisco, suggested that the infective agent in TSEs wasn't a virus or any other known pathogen. Instead, he blamed an infectious protein, which he called a prion 鈥?short for proteinaceous infectious particle. This was a daring leap because proteins don't contain RNA, the genetic material that allows viruses and bacteria to reproduce. But today, the link between prions and TSEs is widely accepted.
The nature of prions
Prions are proteins that occur naturally in the brains of animals and people. Normally, these proteins are harmless, but when they're misshapen they can cause devastating illnesses. The difference lies in the way the proteins are folded.
All proteins start out as loose strings of amino acids. But proteins can't perform their intended function until the amino acids fold into a specific three-dimensional shape. The shape a particular protein assumes is determined by the sequence of its amino acids. Most proteins fold spontaneously during or just after they're synthesized inside cells, the entire process lasting only a few tenths of a millisecond.
Protein folding isn't foolproof, however, and many proteins made by the body aren't usable. The rejects are sent to a kind of recycling center, where the amino acid chains are disassembled and the amino acids prepared for reuse. Normally, this system works well. But as people age, the recycling process may stop working efficiently. As a result, misfolded proteins begin to accumulate in deposits that can cause serious problems. This seems to be what happens in Alzheimer's disease, for example.
But prions go one step further. Not only do they misfold, they also enter brain cells and force normal proteins to misfold as well. When the infected cells die, prions are released into normal tissue and go on to infect more cells. Eventually, large clusters of cells die, leaving the brain riddled with holes.
Unlike any other known protein, prions are infectious. When misfolded proteins are transmitted to people or animals, they begin to transform healthy brain cells into abnormal ones, although signs and symptoms of disease may not appear for years.
Prions also seem capable of crossing species' barriers, jumping, for example, from sheep to cows to humans. Although infection may occur more readily when proteins have the same amino acid sequence and exist within a single species, evidence is strong that prions can move from one species to another.
To make matters worse, prions are nearly indestructible. They're impervious to radiation, washing, boiling and the intense heat of autoclaves used to sterilize surgical instruments. Unlike many harmful bacteria, prions aren't destroyed by cooking or by the strong juices produced in your stomach.
How CJD is transmitted
Most people with classic CJD develop the disease for no apparent reason. CJD that occurs without explanation is termed spontaneous or sporadic CJD and accounts for the great majority of all cases.
Researchers have been able to identify some causes of CJD, however, including:
* Heredity. In the United States, about 5 percent to 10 percent of people with CJD have a family history of the disease or test positive for a genetic mutation associated with CJD.
* Certain medical procedures. So far, classic CJD hasn't been linked to tainted beef. But a small number of people have developed the disease after being exposed to infected human tissue as a result of medical procedures (iatrogenic CJD), such as cornea and skin transplants and grafts of dura mater, the membrane that covers the brain. Some adults have developed CJD decades after receiving injections of human growth hormone (HGH). Traditionally, HGH has been used to treat children who fail to grow normally, but in recent years it also has become popular as an anti-aging therapy. Before the 1980s, when a genetically engineered version of the hormone became available, HGH was derived from human pituitary glands.
* Exposure to contaminated medical instruments. Because rogue prions aren't affected by standard sterilization methods including heat, radiation, alcohol, benzene and formaldehyde, instruments used in some types of brain surgery can harbor small bits of infected tissue, even after undergoing stringent cleaning procedures.
How vCJD is transmitted
To date, vCJD has been linked primarily to the consumption of beef infected with mad cow disease. The parts of meat most likely to harbor infection include:
* Bone marrow
* Brain
* Spinal cord
* Nerves attached to the brain, eyes, tonsils and spinal cord
* Small intestine
Hot dogs, ground meat and sausages are especially likely to contain contaminated tissues, whereas milk and other dairy products haven't been implicated in vCJD.
Studies have also shown that vCJD can spread through blood transfusions, even if the infected donor has no signs or symptoms of the disease.
TSEs in animals appear to spread through close contact; one study suggests infected prions may spread through animal urine. But current evidence doesn't suggest that either CJD or vCJD can be transmitted from one person to another through social or sexual contact. You get it from beef, cows have been fed brains that causes this disease and when eaten by humans it can be contracted. Interesting question!
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