The discovery that prions may be an effect of scrapie rather than a cause could turn the conventional wisdom regarding diseases such as BSE and variant Creutzfeld-Jakob on its head, according to a report in this week’s New Scientist.
Prions are thought to trigger disease by getting into the body through food or cuts in the skin and triggering a chain reaction that transforms native prions into harmful ones. These accumulate in the brain with fatal consequences.
Prions eaten in contaminated food were thought to pass undigested through the gut wall into specialised lymphoid tissue called Peyer’s patches, where they multiplied up before spreading to the central nervous system.
Now, Martin Jeffrey of the UK Veterinary Laboratories Agency in Penicuik, Midlothian, has examined fifty sheep to see what happens to the prions thought to cause scrapie, the ovine equivalent of BSE, and found the opposite.
They injected enormous half gram doses of liquefied brain tissue from sheep that had died of scrapie directly into the gut lining of healthy sheep. They then monitored what happened to the millions of abnormal prions. Surprisingly, these prions didn’t migrate to Peyer’s patches as expected, but were digested or vanished into lymph nodes.
Separate experiments also revealed that abnormal prions can easily be digested by sheep stomach juices, so even if an animal ingested large quantities of infected feed, hardly any abnormal prions would survive.
The results don’t challenge the assumption that infected cattle feed and beef can cause disease. In the three sheep that developed scrapie after being injected with the diseased tissue, abnormal prions began accumulating in the Peyer’s patches 30 days later, even though all the prions from the original gut injections had long gone. Further investigations revealed that the prions were being formed afresh by cells in the Peyer’s patches.
Jeffrey suggests that another ingredient of the liquefied sheep brain may be responsible for triggering the cells to produce the abnormal prions – a highly controversial view (Journal of Pathology, vol 209, p 4). “It would be over-interpretation to say we have evidence for another infectious agent,” says Jeffrey. “But the prion hypothesis is less than satisfactory.”
Adapted from a New Scientist Press Release