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Do we need to explain the occurrence of atypical scrapie?
  1. Giuseppe Ru, DVM, PhD, MSE, Head of BEAR
  1. BEAR - Biostatistica Epidemiologia e Analisi del Rischio, Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d'Aosta, Via Bologna 220, 10154 Torino, Italy; e-mail:

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WHY do diseases occur? A recent paper by Tomasetti and colleagues (2017) tries to provide new evidence in support of their hypothesis that most cancers may be accounted for by mutations due to unpredictable, random mistakes made during normal DNA replication in the division of normal stem cells (somatic mutations) (Tomasetti and Vogelstein 2015). They suggest that other mutations that arise due to environmental or heredity factors may only play a minor role. The debate on this issue is still open while the media translated the hypothesis into the concept of ‘bad luck’.

As highlighted in a Centers for Disease Control and Prevention (CDC) reference manual, ‘epidemiologists assume that illness does not occur randomly in a population, but happens only when the right accumulation of risk factors or determinants exists in an individual’ (CDC 2011). The same assumption should also hold when applied to transmissible spongiform encephalopathies (TSEs), whose origin and causes are not obvious. That is the case for sporadic Creutzfeldt Jakob disease (CJD) in humans or for livestock TSEs other than classical bovine spongiform encephalopathy (BSE) or scrapie. Let's take a step back.

During the 1990s, at the peak of the BSE epidemic in the UK, the possibility that the disease might have spread into the small ruminant population led to renewed attention to surveillance for scrapie. Although scrapie had been known since the 18th century, the real distribution of the disease was not clear. Therefore, throughout Europe, passive surveillance targeting sheep …

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