Viroids And Prions

figure 24-10

This cow, which is unable to stand and walk, is showing signs of mad cow disease, a disease caused by prions.

figure 24-10

This cow, which is unable to stand and walk, is showing signs of mad cow disease, a disease caused by prions.

Even simpler than viruses is a group of disease-causing agents called viroids. Viroids (VIE-roydz) are the smallest known particles that are able to replicate. A viroid is made up of a short, circular, single strand of RNA that does not have a capsid. Viroids infect plants. These naked RNA molecules can disrupt plant-cell metabolism and damage entire crops. Economically important plants affected by viroids include coconuts, potatoes, and oranges.

In late December 2003, the U.S. Department of Agriculture diagnosed a single cow in Washington State with bovine spongiform encephalopathy (BSE), commonly known as mad cow disease. BSE is a degenerative brain disease that results in muscle paralysis, wasting, and death. A cow with BSE is shown in Figure 24-10. Research suggests that mad cow disease is caused by prions (PRIE-AHNZ). Prions are infectious protein particles that do not have a genome. They are abnormal forms of a natural brain protein that appear to convert normal brain proteins into prion particles. Scientists hypothesize that the prions then clump together inside cells and cause cell death.

Prions appear to cause a number of other degenerative brain diseases, including scrapie in sheep and Creutzfeldt-Jakob (KROYTZ-felt-YAH-kohb) disease (CJD) and kuru in humans. The BSE epidemic in the United Kingdom in the early 1990s was most likely caused by the feeding of scrapie-contaminated meat and bone meal to cattle. Scientific evidence indicates that BSE has been transmitted to humans, causing a variant form of CJD (vCJD). This evidence comes from studies that followed the unusually high numbers of young people in the United Kingdom that developed this rare disease in the late 1990s. Eating BSE-contaminated beef and beef products was the probable cause of vCJD in these cases. Measures to protect the food supply are the best safeguards. However, the overall risk to human health from BSE is very low.

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