Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative disease of cattle caused by foodborne exposure to prions. First described in 1986, this novel disorder was clinically characterized by altered behavior, sensory changes, and locomotor signs. For almost two decades, BSE, now named classical BSE (C-type BSE), has been regarded as the only and exclusive prion disorder of cattle. The introduction of an active surveillance system for BSE in 2001 allowed the identification of two additional atypical forms of BSE, named H-type and L-type BSEs, because of distinct conformations of the pathological prion protein, or PrPSc, with higher (H-type) or lower (L-type) electrophoretic mobility of the unglycosylated protease-resistant PrPSc fragment. Up to 2019, a total of 69 L-type BSE and 62 H-type BSEs have been detected in European Union (EU) reporting countries, in addition to 2 H-type BSE in Norway and Switzerland, as opposed to 190,469 C-BSE cases. The clinical phenotypes of atypical BSE forms are only partially known in field animals, although indirect information has been obtained from intraspecies transmission studies. Transmission studies to mice show that C-type, H-type, and L-type BSE forms display distinct molecular properties, consistent with the occurrence of three different prion strains. Intriguingly, upon serial passages, H-type and L-type BSEs may acquire C-type properties, hence suggesting their possible role in the origin of BSE epidemics, and, in addition, are transmissible to mammalians, including non-human primates, issues that raise public health concerns.
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