Recent studies show that chronic inflammations can cause an accumulation of prions, the infectious agent of TSEs, in unexpected parts of the body. These findings challenge the list of Specified Risk Materials (SRM), which have to be removed before meat from cattle or small ruminants enters the food- and feed chain.

For many years it has been generally accepted that prions typically colonize in nervous and lymphoid tissues. Accordingly, the protease-resistant form of the prion protein (PrP^Sc) is often detectable not only in brain and spinal cord tissue, but also in spleen, tonsils and lymph nodes of infected animals. Recently the research group of Adriano Aguzzi found evidence that chronic inflammations involving the lymphoreticular system can cause prion accumulation in unexpected parts of the body. Aguzzi and colleagues reported the detection of prion deposits in scrapie-infected mice which also suffered from an inflammation of the liver, kidney or pancreas. TSE-infectivity can even be excreted into urine in scrapie-infected mice that also suffered from an inflammation of the kidney. In prion-infected mice without inflammations, the peripheral organs remained free of prions. Liver, kidney and pancreas are currently not on the SRM list.

Farm animals also affected
A follow up study has now shown that in farm animals similar phenomena can be observed. In a sheep herd in Italy, seven out of 261 animals showed clinical symptoms of scrapie. Four of these sheep coincidently suffered from mastitis, an inflammation of the udder. The investigators detected PrP^Sc in mammary glands of all four scrapie-sick sheep with mastitis, but not in the mammary glands of sheep that suffered only from scrapie or in healthy animals.
Whether the combination of a prion infection and mastitis leads to prion secretion into the milk remains unclear at the moment.

SRM list for cattle still appropriate
What are the implications of these findings for food safety and consumer protection? Until now scientists have found no involvement of the lymphoreticular system in mad cow disease. Thus, the presence of considerable amounts of infectivity in tissues that are not included in the list of cattle SRM seems unlikely at the moment. From an epidemiological view, the reported inflammation-associated excretion to urine and - possibly - milk is interesting since it may explain TSE transmission routes among sheep.