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11.1 Infectious Agents

11.2 Epidemics

11.3 The Body's Response to Infection: The Immune System

11.4 Preventing the Spread of Prion Diseases

The brains of affected individuals look spongy.

In the late 1980s, many dairy cows in Great Britain started behaving in an uncharacteristic manner. Dairy farmers noticed that some of their cows were uncoordinated and seemed apprehensive. These cows would shake and tremble and rub parts of their bodies against walls or fences. Farmers watched with alarm as their cows staggered around, giving the impression that they were mad or irritated, leading to the name "mad cow disease." Several months after the onset of symptoms, the cows inevitably died. Autopsies revealed that the brains of the "mad" cows had holes in them characteristic of a class of diseases known as spongiform encephalopathies.

An encephalopathy is a pathology, or disease, of the brain. The diseased cows' brains resembled porous, natural sponges filled with holes. Mad cow disease is a type of spongiform encephalopathy that only affects cows, so it is also called bovine spongiform encephalopathy, or BSE.

Spongiform encephalopathies have been around for many years. They have been diagnosed in sheep, whose skin becomes so itchy that they are compelled to scrape off their wool (hair) on fences, leading the disease to be called scrapie. When the disease is present in elk and deer, it is called chronic wasting disease, due to the emaciated appearance of affected animals. In some regions

In deer, this disease is called chronic wasting disease.

This disease can also be spread to humans.

This disease can also be spread to humans.

of the United States, deer hunting and the consumption of deer meat has decreased dramatically due to concern about eating meat from animals with this disease.

Humans have also been affected by spongiform encephalopathies. An obscure disease called Kuru has long been known to affect natives of the eastern highlands of New Guinea. The disease occurs mostly in women and young children as a result of the tribal custom of honoring the dead by eating their brains. Affected individuals lose coordination and often become demented. They, too, inevitably die.

A long-recognized but previously very rare condition called CreutzfeldtJakob Disease (CJD) is another spongiform encephalopathy that affects humans. Like the diseased cows, affected humans became very agitated, dizzy, and short-tempered. They experienced short-term memory loss, lack of coordination, and slurred speech. As was the case with cows, this disease is lethal in all the affected humans. Typically, the disease affects only the elderly. However, a recent and alarming trend has been an increase in the rate of diagnosis of this disease in young British patients.

On average, these young patients lived just over a year after they were diagnosed, and when examined at autopsy, their brains looked more similar to those of BSE-infected cattle than CJD-infected humans. The increased number of people diagnosed with the disease, the structure of their brains after death, and the lower age of those infected led scientists to believe that BSE was, in some way, being transmitted from infected cows to these individuals. This new, transmissible form of the disease was named new-variant CJD (nvCJD).

Scientists and doctors, veterinarians, hunters, and patients and their families all over the world are concerned about the preventing the spread of these diseases. The first step toward understanding this disease and preventing its spread in cows and humans required that scientists determine what was causing it.

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