Spongiform Encephalopathies
Transmissible Spongiform Encephalopathies (TSEs), viral types of organisms often called "slow viruses", cause progressive and fatal organic brain disease. Thus far, several types have been identified. Given the name due to the spongy appearance of the brain when examined microscopically, TSEs affect humans and/or animals, appear to have a species barrier (are most likely to affect members of the same species), and are primarily transferred through ingestion of--or direct exposure to--body tissue contaminated with the disease. In humans, symptoms resemble senile dementia/Alzheimer's disease with a much faster onset and progression. Just what causes the disease, for which there is no cure, remains uncertain.
Scrapie, found primarily in sheep and reported in many countries, was first recorded in British flocks in 1732. Although it occurs naturally in sheep and goats, it can be transmitted to certain other animals. Bovine spongiform encephalopathy (BSE), or "Mad Cow Disease," was discovered in the United Kingdom in 1986 and quickly reached epidemic proportions. To protect public health, the British Government banned from sale for human and pet consumption bovine tissue which harbors BSE--the brain, spinal cord, spleen, tonsils, intestines and thymus. The wide-spread practice of supplementing cattle feed with mammalian protein was also banned; authorities believe such food contaminated with BSE caused the epidemic.
Other animal forms of TSEs are Transmissible mink encephalopathy (TSE), which occurs primarily in farms where mink are fed animal tissue or food containing processed animal protein; Chronic wasting disease (CWD), which affects elk, mule deer, black tailed deer and mule-white tailed deer primarily in captivity but cases have also been reported in wild animals indicating this form may not be transmitted through feed; and Feline spongiform encephalopathy (FSE), first discovered in Britain in 1990 with only one case reported since the ban on the use of BSE- specific beef tissue in pet food that same year.
In humans, Creutzfeldt-Jakob Disease(CJD), a rare disorder affecting two out of one million people world-wide, was first recorded in Germany in 1920. Traditionally developing between the ages of 50 and 70, 14% of cases appear to be hereditary and 1% transmitted by contact with contaminated medical or surgical equipment. The few reported adolescent cases were caused through contaminated growth hormones extracted from the pituitary gland cadavers. The long history of humans eating sheep and the fact that it has infected lifelong vegetarians and people in Australia--a country free of scrapie and BSE--reduces the likelihood that this disease is caused by animal TSEs. (Gerstmann-Stäussler-Scheinker is an even rarer form of this disease.) However, a previously unrecognized strain of CJD calledNew Variant (nvCJD), appeared in recent years in people under 50, which may result from exposure to BSE before the 1989 ban. Also, its incidence is greater in North Africa where sheep brains and eyes are regularly consumed. Kuru, which once flourished in "head-hunter" tribes in New Guinea and probably spread through handling and ingesting human brains contaminated with the disease, has decreased drastically since 1956 when the practices ceased. The incubation period for this disease is 30 years or more.
While the causative agent in TSE is not clearly understood, three theories are under investigation: one involves an infectious pathogen called virino; another, and perhaps the most popular, concerns a modified form of aprion protein called "PrP;" and a third involves a filamentous virus.
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