Wednesday, March 14, 2007

Prion Diseases Rare But Deadly

By: Scott P. Edwards

Mad cow and other prion diseases, although fatal, are also very rare and have caused what some scientists says is irrational fear.

Mad cow and other prion diseases—so named by Nobel Prize winner Stanley Prusiner, M.D., for the tiny protein molecules believed to cause these degenerative brain diseases—have been around for years. These diseases, although fatal, are also very rare and have caused what some scientists says is irrational fear. 

Animal Prion Diseases

A number of common animal prion diseases have been identified. Mad cow disease, formally known as bovine spongiform encephalopathy (BSE), has a long incubation period (up to four or five years), but is usually fatal to cattle within weeks or months of onset of symptoms. BSE is characterized by sponge-like changes in the brains and spinal cords of infected cattle that alter the animals’ temperament, sometimes causing aggressiveness, swaying gait, and difficulty in rising. Scientists traced the cause of the disease to cattle feed prepared with diseased cow brains and spinal cord tissue. Though rare, BSE is transmissible to humans through the consumption of contaminated beef.

Scrapie (so-called because infected animals were observed scraping against fences) is a slow, progressive disease that causes central nervous system degeneration in sheep and goats. The first symptom of the disease is a slight change in animal behavior, followed by nervousness, aggressiveness, and separation from the flock. Animals with scrapie appear normal until they are startled, which causes them to go into convulsions. Over time, infected animals become uncoordinated, especially in their hind limbs. Once symptoms appear, the disease is generally fatal within six months. Scrapie is not believed to be transmissible from animals to humans.

Chronic wasting disease (CWD) is common in deer and Rocky Mountain elk in the tri-corner area of Wyoming, Colorado, and Nebraska. The disease, first recognized in 1981, is characterized by weight loss, behavioral changes, excessive salivation, difficulty swallowing, excessive thirst, and frequent urination, as well as a lack of muscle coordination and head tremors in some animals. Most animals die within several months of onset. No cases of human prion disease have been associated with CWD.

Other forms of animal prion diseases include transmissible mink encephalopathy, feline spongiform encephalopathy (a neurological disease in household cats and feline species in zoos that has been linked to BSE), and a form of the disease in nonhuman primates such as macaque monkeys and lemurs. Most of these diseases are extremely rare.  

“Insidious” Human Diseases

In humans, the most common forms of prion diseases are Creutzfeldt-Jakob disease (CJD), variant CJD, fatal familial insomnia, Gerstmann-Straussler-Scheinker disease, and kuru.

Like animal prion diseases, CJD is characterized by sponge-like holes in the infected brain. The disease is rare, affecting one person per million worldwide, with about 250 to 300 new cases in the United States each year. CJD is hard to diagnose because in its early stages the symptoms are similar to those of other neurodegenerative diseases such as Alzheimer’s, Huntington’s, and Parkinson’s diseases. Early symptoms of CJD include mood swings, depression, memory difficulties, social withdrawal, and anxiety, as well as visual disturbances, fatigue, and bizarre behavior. As the disease progresses, CJD patients suffer from dementia, muscle paralysis, slurred speech, difficulty swallowing, and eventual blindness. Some patients fall into a coma. All die. Most cases of CJD are considered “sporadic” because the disease occurs for no known reason.

Variant CJD (vCJD) is a relatively new disease, first discovered in 1996. This form of CJD, the human form of mad cow disease, typically strikes younger people, with an average age of onset of 29 years (compared to 65 for other forms of CJD). Early in their illness, people with vCJD experience psychiatric symptoms, including depression and schizophrenic-like psychosis. Nearly half of the people diagnosed with vCJD have reported unusual sensory symptoms like “stickiness” of the skin. The disease is also characterized by unsteadiness, difficulty in walking, and involuntary movements. These symptoms develop as the illness progresses, and toward the end stage of the disease many people are completely immobile and unable to speak. Few cases of vCJD, which is caused by an inherited mutation of the prion protein gene, have been reported in the United States.

Fatal familial insomnia is a rare genetic disorder that affects the thalamus, the region of the brain that controls sleep and wakefulness. Symptoms are directly related to a malfunction of the thalamus’s sleep mechanism. Over the first few months, patients suffer from psychiatric problems, including panic attacks and bizarre phobias. The second stage includes hallucinations, panic, and agitation. The third stage is characterized by near-total insomnia and significant weight loss. During the final stage of the disease, which lasts about six months, patients suffer from dementia, total insomnia, and sudden death. The time from onset of symptoms to death is about 18 months.

An extremely rare disease found in only a few families around the world, Gerstmann-Straussler-Scheinker disease (GSS) is almost always inherited. Early symptoms include lack of muscle coordination, or ataxia, and difficulty in walking. The disease progresses to more pronounced ataxia and dementia. Other symptoms include speech difficulties, rigid muscle tone, and vision and hearing loss. Some people with GSS also have Parkinson's-like features. The disease causes extreme disability and eventual death, often after a patient develops a secondary infection or slips into a coma.

Kuru has now largely been eradicated, but epidemic levels occurred in the 1950s and 1960s among indigenous peoples of the New Guinea highlands because of ritualistic cannibalism practices in which the natives consumed tissues, including the brains, of their deceased relatives. The disease affected the brain’s cerebellum, which is responsible for muscle coordination. People infected with kuru were unable to stand or eat, and they died in a comatose state.

While all of these human prion diseases are rare, Johns Hopkins University neurologist Richard Johnson, M.D., says they are particularly insidious. “You get them, you die,” he says. “There’s no way around it.”

 



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Carolyn Asbury, Ph.D., consultant

Scientific Advisory Board
Joseph T. Coyle, M.D., Harvard Medical School
Kay Redfield Jamison, Ph.D., The Johns Hopkins University School of Medicine
Pierre J. Magistretti, M.D., Ph.D., University of Lausanne Medical School and Hospital
Robert Malenka, M.D., Ph.D., Stanford University School of Medicine
Bruce S. McEwen, Ph.D., The Rockefeller University
Donald Price, M.D., The Johns Hopkins University School of Medicine

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