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Researchers discover subclinical form of BSE

BMJ 2000; 321 doi: (Published 02 September 2000) Cite this as: BMJ 2000;321:530
  1. Roger Dobson
  1. Abergavenny

    Researchers have found new evidence for the existence of a subclinical form of bovine spongiform encephalopathy (BSE). Cattle, sheep, and other animals that are outwardly healthy may be harbouring the infection, with the risk that BSE could still be getting into the food chain.

    In a new study based on a mouse model, researchers at the Medical Research Council Prion Unit took a closer look at the species barrier, which limits the ability of prions to jump from one species to another (Proceedings of the National Academy of Sciences 2000;97:10248-53).

    In the study, scientists tried to infect laboratory mice with hamster prions and saw no apparent signs of disease. But when they looked more closely they found that the mice had high levels of prion in their brains.

    “Previously scientists have injected mice with the hamster disease, found no clinical signs of infection, and concluded it cannot jump the species barrier,” said Dr Andrew Hill of the University of Melbourne, one of the authors of the study.

    The team, led by Professor John Collinge, director of the MRC Prion Unit, also found that the new subinfection could be easily passed on when injected into healthy mice and hamsters.

    “These results have a number of important implications. They suggest that we should rethink how we measure species barriers in the laboratory and that we should not assume that just because one species appears resistant to a strain of prions they have been exposed to, that they do not silently carry the infection,” said Professor Collinge.

    He continued: “These new findings have important implications for those researching prion disease, those responsible for preventing infected material getting into the food chain, and those considering how best to safeguard health and reduce the risk that, theoretically, prion disease could be contracted through medical and surgical procedures.”

    Dr Hill said, “The bottom line is that healthy cattle may harbour infectivity and never show any signs of BSE. It has now become important to look at even healthy animals to investigate the levels of this subclinical disease.

    “It is entirely possible that in the same way, humans might be harbouring the disease at this subclinical level. As we don't know how many people may be incubating the disease, if people are carrying the infection at this level, then there are implications for medical and surgical procedures.”

    Both the Ministry of Agriculture, Fisheries and Food and the Department of Health said that safeguards were already in place to counter the possibility of subclinical forms of the disease.

    Members of the Spongiform Encephalopathy Advisory Committee (SEAC) are expected to consider the report when they meet later this month.

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