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Slow viruses.


Although most viral illnesses strike quickly, some act like tiny time bombs, ticking away for years before symptoms arise. When they attack the central nervous system (the brain and spinal cord), as they often do, they can be fatal.

Coined less than 40 years ago, the term "slow virus" refers to foreign substances that have an incubation period of months or years, are confined to a single organ system and progress slowly until (usually) death results. Diseases linked to slow viruses include mundane disorders such as colds, but are most likely those that deal the death blow, like AIDS and Alzheimer's disease. Because researchers are unsure about how to classify them, various names have been ascribed for them, such as slow virus, virino, viroid, virion and prion, or proteinaceous infectious particle. Because these agents neither provoke the normal antibody response, nor become fazed by disinfecting and sterilizing agents, they are considered unconventional, or, more succinctly, "slow."

Because of this, plus the fact that slow viruses can go undetected for years, some researchers believe slow viruses should be segregated from non-slow viruses. Slow viruses produce sharply reduced amounts of viral RNA molecules that enable the body to feel the effects of foreign substances when they enter the body. Thus the slow viruses are also "silent" and are thereby allowed to spread throughout the body undetected until, finally, they damage the host cells. The human immunodeficiency virus (HIV) causing AIDS is one such slow virus, often taking years from the time of infection before a disease state is present.

Likewise, Yale researchers have found that the starchlike protein deposits found in the brains of Alzheimer's patients have similar degenerative qualities to those of other slow viruses. Some researchers disagree, however, saying that because Alzheimer's brain plaques are so chemically dissimilar to those of, for example, scrapie, a fatal disease of sheep, and others, there can be no comparisons among these various disorders. Moreover, not enough research has been done for scientists to determine whether, indeed, a viral or subviral infective component to Alzheimer's disease could effectively turn the tide on this illness that primarily strikes elderly individuals.

The fact that scientists have segregated the so-called "unconventional" viruses from the more conventional ones may be a step toward finding antidotes to the crippling illnesses they cause. Nevertheless, like the slow viruses themselves, it may take researchers years of slow, grinding work before they can finally see the results, if any, of their labors.
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Publication:Medical Update
Date:Aug 1, 1989
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