The infectious agent which causes BSE in cattle and a similar disease in humans could escape detection and occur in food products available for human consumption. The surprise discovery by researchers in Switzerland, Britain and the US may force new safety measures to protect against the disease.
BSE infection is usually detected only after an animal begins to show symptoms associated with damage to the brain or central nervous system. This new study, published this morning in the journal Science, indicates that other organs previously considered safe could also harbour the disease and pose a risk.
All human safety measures assume the prions which cause BSE only occur in the brains, spinal cords and spleens of infected cattle. These and other so-called "specified risk materials" (SRMs) are now removed from the food chain under Department of Agriculture regulations and destroyed.
The researchers found, however, that the liver, kidney and pancreas of infected mice could also sometimes carry BSE. They also report that BSE infection could exist in these organs long before the mice showed any signs of disease in the brain.
"In principle this is possible, and we are now examining this in farm animals," said Dr Mathias Heikenwalder, a member of the research group at the University of Zurich's Institute of Neuropathology, yesterday.
"It has been known for a long time that the immune system plays an important role in the progress of the disease before it gets into the brain," he added. "The question we investigated was whether an infected organ can accumulate prions."
They tested the theory in mice which had pre-existing infections in their liver, kidney or pancreas. They were infected with BSE and in all cases BSE prions began to accumulate in these organs. Immune cells always congregate at an infection site, but the immune cells apparently bring prions along with them into the organs.
It would not be uncommon for a bovine to have a pre-existing infection of one of these organs, Dr Heikenwalder said. He would not quote figures to back this up given he and colleagues were working on a new paper, but the level was "reasonably high".
This suggests that it may be necessary to reconsider the safety controls meant to prevent transfer of prions from food products to humans.
The current risk assessment held that it was enough to remove lymph and central nervous system tissues, "but there are cases where this is not correct", he said.
The authors wrote in Science: "Knowledge of the distribution of prions within infected hosts is fundamental to consumer protection and prevention of iatrogenic [caused by medical treatment] accidents.
"The risk to humans of contracting prion infection from other organs has been deemed small even in countries with endemic BSE." It might be "important" that this assumption was tested given the results of this study, they added.
Decisions on food safety related to BSE were now made via the European Food Safety Authority, which would consider these findings, according to a spokesman for the Department of Agriculture. "The SRMs are constantly under review," he said.
BSE testing was done on all animals older than 30 months and on fallen or injured "casualty" animals older than 24 months. "When there is any evidence that material should be removed, it is added to the list if it is considered appropriate. They err on the side of caution," the spokesman added.