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A study published online by The Lancet provides an estimate of the compared efficiency of oral transmission of bovine spongiform encephalopathy (BSE) to cattle and to man.
There has been uncertainty regarding the amount of BSE material that constitutes an oral infectious dose for humans. The study estimates that a person would have to eat at least 1.5kg of neural tissue from an infected animal that was just below the detection limit when tested negative at the slaughterhouse, to be at risk of developing variant Creutzfeldt-Jakob disease (vCJD).
Jean-Phillipe Deslys (Commissariat a lEnergie Atomique, France) and colleagues gave two adult primates a 5g oral dose of ground brain tissue from a BSE-affected cow. One primate developed a neurological disease closely resembling vCJD 5 years after exposure, whereas the other remained free of disease. On the basis of these findings and data from other studies the investigators estimated that the efficiency of infection from cow to primate could be 7 to 20 times lower than that of intraspecies infection for cattle.
The results of the present study also suggest that the incubation period for BSE transmission from cattle to human can be more than a third longer than that of human to human transmission.
Dr. Deslys states: The present data do not provide a definitive minimum infective dose for transmission of cattle BSE to primates, but they do give enough information for a preliminary assessment of the adequacy of existing measures to protect the human food chain.
Our results provide reassurance that BSE screening procedures combined with CNS removal are effective measures to protect the human food chain.
In an accompanying commentary James Ironside and Mark Head (National Creutzfeldt-Jakob Disease Surveillance Unit, University of Edinburgh, UK) write that the precise implications of this study for vCJD are difficult to assess since the number of animals used in the study was small and the minimum infectious dose of BSE-infected neural tissue for primates remains unknown.
Professor Ironside comments: There remain fundamental problems comparing likely human exposure to BSE with experiments of this type, even in primates that mimic vCJD pathology following oral BSE exposure. Multiple oral exposure events over a period of years seem likely in the UK, and vCJD occurs predominantly in young adults, raising the possibilities of age-related susceptibility or exposure. More data will be forthcoming on some of these points from larger studies, but it will be several years before they are likely to emerge.
Source: The Lancet