Using an epidemiological framework and bovine spongiform encephalopathy investigation questionnaire to investigate suspect bovine spongiform encephalopathy cases: an example from a bovine spongiform encephalopathy case in Ireland in 2015

O’Connor, Jarlath T; Byrne, Justin P; More, Simon J; Blake, Martin; McGrath, Guy; Tratalos, Jamie A; Mcelroy, Maire C; Kiernan, Paul; Canty, Mary J; O’Brien-Lynch, Chris; Griffin, John M

The aetiology of BSE remains contentious. Early evidence suggested that BSE was caused by a single major strain, commonly referred to as classical BSE. 13 More recently, two sporadic atypical forms of BSE have been identified (namely high-type (H-BSE) and low-type (L-BSE)), which differ from classical BSE. 14 15 The unusually old age of all H-BSE and L-BSE cases, and their apparent low prevalence in the population, could suggest that these atypical BSE forms are arising spontaneously. 16 However, there is no comprehensive information available on the aetiology and pathogenesis of atypical BSE in cattle. Serial passage of H-BSE in other species can lead to the emergence of a classical BSE phenotype, raising the possibility that classical BSE emerged from H-BSE. [17] [18] [19] Such passage can also lead to the emergence of new strains of BSE, 20 which underlines the lack of aetiological clarity for BSE and the challenges in determining controls suitable to deal with it. For BARB cases, it is suggested that the majority of cases have arisen due to continued exposure to contaminated feed. 21 This view is also supported by a more recent UK study that hypothesises that the continued occurrence of classical BSE is due to an exogenous feedborne source, as a result of an over-reliance on imported feedstuffs into Great Britain and the later introduction of a ban on the use of mammalian MBM in other EU member states in January 2001. 12 Milk replacer, containing extracted animal fats, has been implicated as a source of BSE for cattle in a number of countries. [22] [23] [24] [25] [26] Other routes of transmission, such as maternal, environmental and iatrogenic, have also been proposed. In a review of the epidemiological features of cases of BSE born after in Great Britain, Wilesmith and others 12 concluded that there was no evidence of a maternally associated risk factor. However, these authors also state that the nature of the risk of maternal transmission remains uncertain. In the same paper, the authors considered two means of environmental contamination to be possible: first, that in the early years of the epidemic in the UK, a proportion of clinically infected animals were buried; and secondly, the potential excretion of the BSE agent in faeces. Nonetheless, the authors concluded that there was no evidence that a substantial number of BARB cases in the UK occurred as a result of environmental contamination (other than from feedstuffs). Also, of significance regarding environmental contamination is spatial information regarding the location of the index farm in relation to previously infected premises or other local features of potential interest, for example, knackeries. Iatrogenic transmission of Creutzfeldt-Jakob disease (CJD) has been demonstrated in human beings. This was linked to human growth hormone therapy, 27 donated tissues, 28 surgical instruments 29 and blood. 30 Iatrogenic transmission of scrapie through vaccines prepared from ovine material has also been documented. 31 32 Posterior pituitary extract was used in veterinary practices at the start of the BSE epidemic in the UK, but no association between its use and the occurrence of BSE was found. 2 A spontaneous origin has been proposed for a number of transmissible spongiform encephalopathies (TSEs), including atypical BSE as mentioned above, atypical scrapie 33 34 and sporadic CJD. 35 Because of the long interval between the implementation of the total feed bans and the time of o