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.