Prions could accumulate in organs previously thought to be safe, a study in mice suffering from chronic inflammation suggests. What are the implications for existing food safety programs?
A research group at the University Hospital of Zurich investigated the distribution of prions in the body of mice afflicted with inflammatory diseases of either the kidney, pancreas or liver concurrent with prion disease. In all cases, prions were found at significant levels at the site of inflammation. The authors suggest that B-cells that migrate into non-lymphoid organs under inflammatory circumstances induce up-regulation of the normal PrP protein expression on accessory immune cells called follicular dendritic cells. The presence of PrPC, high levels of which are also found in the brain and spinal cord, is crucial for prion replication.
So far the effect has only been seen in laboratory tests on mice. If the results hold true for cattle, they indicate that prions could accumulate in organs which are not included in the list of "specified risk materials" (SRM) and are therefore, not normally removed at slaughter. This means that meat, containing considerable amounts of prions, would land in the food chain.
Importance of the existing BSE testing regime
The U.K. Spongiform Encephalopathy Advisory Committee (SEAC) reviewed the new findings at its meeting on March 3rd 2005, and indicated that additional research in cattle and sheep should be undertaken to verify the impact of these findings on food safety. It also stressed the importance of inspections at abattoirs to identify those animals with severe inflammations, in order to remove them from the food chain.
The removal of SRM, such as brain and spinal cord, from all slaughtered cattle older than 12 months has been one of the public health measures imposed by the European Commission in 2001 as part of regulation 999/2001 and was thought to reduce the human exposure risk by approximately 90%. Remaining exposure risks originating from SRM contaminations of the carcass at slaughtering or from potential prion accumulations in peripheral tissues were addressed by implementing BSE-screening at slaughter of all cattle older than 30 months. The recent findings underline the importance of the existing BSE testing regime and could have implications for those countries that have opted for limited programs.
Potential additional BSE exposure risks originating from inflamed organs of BSE-infected slaughter animals are likely to be covered by the existing BSE testing and meat-inspection measures.
However, the situation is different for dairy products; it will be important to investigate whether or not mastitis leads to prion accumulation in udder tissue, and whether this could lead to prion contamination of milk.
