Experimental transmission of bovine spongiform encephalopathy to sheep has prompted the implementation of a surveillance plan of scrapie in small ruminants by the European Union in all member states. Since its start over 30 000 animals have been tested, and the first seven cases of sheep with detectable PrP res deposition in the central nervous system have been identified in Portugal. Notably, the pattern of PrP res distribution in the brainstem was different from that previously described for scrapie and consistent in all seven animals. Moreover, the profile of the electrophoretic mobility of PrP res after proteinase K treatment was equivalent in all cases analysed but distinct from that observed for scrapie. Notably, four animals had genotypes rarely associated with scrapie, including one animal homozygous for A 136 R 154 R 171. There were no cases found to exhibit vacuolation, a pattern of PrP res distribution or PrP res electrophoretic mobility corresponding to scrapie. These data reveal a putative atypical scrapie strain in Portugal not linked to specific Prnp genotypes. Scrapie affects sheep and goats and is the most common natural form of a group of diseases designated transmissible spongiform encephalopathies, which include Creutzfeldt-Jakob disease (CJD) of humans and bovine spongiform encephalopathy (BSE). Scrapie is endemic in several European Union (EU) countries and has been recognized, as a disease identity, for over 250 years (McGowan, 1922). To date, there are no apparent clinical or epidemiological data linking scrapie to human disease. However, since it has been shown that sheep could be experimentally infected with BSE (Foster et al., 1993), the possibility has been raised that BSE could have been accidentally introduced in this species. This would constitute a grave problem in terms of public health, since, in contrast to scrapie, BSE has been directly linked with a new form of CJD (Bruce et al., 1997; Collinge et al., 1996). Furthermore, it has been proposed that like scrapie, BSE in sheep could be naturally transmitted and thus become endemic (Foster et al., 2001). Clinically both diseases are indistinguishable (Foster et al., 2001). These possibilities prompted the EU to implement a surveillance scheme for scrapie in small ruminants.

Transmissible Spongiform Encephalopathies (TSEs) or prion diseases are a fatal group of infectious, inherited and spontaneous neurodegenerative diseases affecting human and animals. They are caused by the conversion of cellular prion protein (PrPC) into a misfolded pathological isoform (PrPSc or prion- proteinaceous infectious particle) that self-propagates by conformational conversion of PrPC. Yet by an unknown mechanism, PrPC can fold into different PrPSc conformers that may result in different prion strains that display specific disease phenotype (incubation time, clinical signs and lesion profile). Although the pathways for neurodegeneration as well as the involvement of brain inflammation in these diseases are not well understood, the spongiform changes, neuronal loss, gliosis and accumulation of PrPSc are the characteristic neuropathological lesions. Scrapie affecting small ruminants was the first identified TSE and has been considered the archetype of prion diseases, though atypical and new animal prion diseases continue to emerge highlighting the importance to investigate the lesion profile in naturally affected animals. In this report, we review the neuropathology and the neuroinflammation of animal prion diseases in natural hosts from scrapie, going through the zoonotic bovine spongiform encephalopathy (BSE), the chronic wasting disease (CWD) to the newly identified camel prion disease (CPD).

Active transmissible spongiform encephalopathy (TSE) surveillance in small ruminants across Europe was implemented in 2002 following the epizootic of bovine spongiform encephalopathy. Here, we report the potential emergence of classical scrapie in Portugal, in a background of enzootic atypical scrapie. Between 2003 and 2008, 375 459 small ruminants were screened in total, with 328 animals confirmed positive for NOR98 atypical scrapie. During this period, the prevalence rate of atypical scrapie for all years combined was 0.0874 % across the country. In this scenario, classical scrapie emerged as a single outbreak in 2008, with 12 identified cases. In contrast to other European countries, where classical scrapie has been enzootic for decades, these data indicate that, in Portugal, atypical scrapie is the predominant form of TSE. The findings reported here will have implications for the control of classical scrapie in Portugal, namely in terms of keeping the country free of enzootic classical scrapie.

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