All scientific papers are important, but some are more important than others. Aside from its scientific importance, this paper is particularly important to me in purely personal terms. It comes from my own department. Ben Maddison was a PhD student in my laboratory many years ago and now heads up his own research group within the department. It’s also one of the final papers to come from Gary Whitelam, my former head of department, who died tragically last year. And as if all that wasn’t enough, as the UK starts to forget about how close we came to disaster with BSE, we’re still not completely sure that it’s all over.
Using the cutting-edge research technique of serial protein misfolding cyclic amplification (sPMCA), my colleagues show that prions are secreted in the milk from scrapie-exposed sheep. The sPMCA method involves incubating a small amount of abnormal prion with an excess of normal prion protein, so that some conversion takes place. The growing chain of misfolded protein is then blasted with ultrasound, breaking it down into smaller chains and so rapidly increasing the amount of abnormal protein available to cause conversions. By repeating the cycle, the mass of normal protein is rapidly changed into misfolded prion.
Since scrapie is not transmissible to humans, these findings do not indicate the likely introduction of zoonotic prions from sheep into the human food chain. Nevertheless, the data do indicate caution in the risk assessment associated with such foods. Although it is unknown if analogous shedding of prions into milk occurs with bovine BSE, evidence from previous epidemiological and bioassay studies suggests that such a scenario seems unlikely to cause clinical disease. However, the present report strongly suggests that given the importance of cow’s milk in the human diet the potential presence of low levels of prions within milk warrants further investigation. Analyzing milk samples by sPMCA offers a methodology with clear potential for the identification of clinically sick animals and those with preclinical/subclinical prion disease. Such a non-invasive, live animal assay has the potential to contribute to the epidemiological study, management and control of prion diseases within farmed animals.
Prions are secreted in milk from clinically normal scrapie-exposed sheep. J Virol. Jun 3 2009. doi:10.1128/JVI.00051-09
The potential spread of prion infectivity in secreta is a crucial concern for prion disease transmission. Here, serial protein misfolding cyclic amplification (sPMCA) allowed the detection of prions in milk from clinically-affected animals as well as scrapie-exposed sheep at least 20 months before clinical onset, irrespective of the immunohistochemical detection of protease-resistant PrP(Sc) within lymphoreticular and CNS tissues. These data indicate the secretion of prions within milk during the early stages of disease progression and a role for milk in prion transmission. Furthermore, the application of sPMCA to milk samples offers a non-invasive methodology to detect scrapie during preclinical/subclinical disease.