Report on the epidemiological investigation of a BSE case in Scotland (RBSE24_00003) United Kingdom October 2024
Executive summary
On 9 May 2024, Scotland’s Chief Veterinary Officer (CVO) confirmed a case of classical bovine spongiform encephalopathy (BSE) in a 7.5-year-old cow on a beef suckler farm in Ayrshire, Scotland. This was the first case of classical BSE to be confirmed in the United Kingdom (UK) since 2021, and in Scotland since 2018. This report summarises the epidemiological investigations that have been carried out to describe and understand this single case of BSE.
The index case was a Simmental cross cow, born on 18 October 2016 in a holding in Dumfries and Galloway, Scotland. It was purchased and introduced into the incident herd on 27 June 2018, where it resided until its death.
The index case died on farm on 26 April 2024. The farmer did not suspect notifiable disease and the carcass was collected by the fallen stock company on the same day. The carcass was tested for BSE as per the UK’s statutory BSE surveillance procedures due to the cow’s age and because she was fallen stock.
A preliminary positive result was received on 1 May 2024. A final positive result was confirmed on 9 May 2024 by the Animal and Plant Health Agency (APHA) Weybridge. APHA Weybridge is the UK National Reference Laboratory (NRL) for transmissible spongiform encephalopathies (TSEs). It is also the World Organisation for Animal Health (WOAH) Reference Laboratory for BSE and scrapie.
Tracing investigations identified 2 offspring born in the 24 months prior to the clinical onset of disease and death of the index case (see also appendix 2, point (f)):
• The first one had a date of birth (DOB) of 13 May 2023. It was alive at the time of confirmation and placed under restrictions following BSE confirmation in the index case. It was transported alive to the NRL for TSEs in Weybridge for clinical observation. It was then euthanised and underwent a postmortem examination and BSE testing, with negative results.
• The second one had a DOB of 21 May 2022. It was already dead (slaughtered for human consumption and not eligible for BSE testing) when traced after the BSE case was confirmed.
Tracings investigations also identified 45 cohort animals born and/or reared with the index case during the relevant risk period (12 months either side of the date of birth of this case).
Of these, 43 were restricted and humanely culled on farm at their respective locations.
The carcasses were sampled for BSE testing and then disposed of as category 1 animal by products (ABP) at an approved ABP rendering facility. All the samples returned negative results for BSE.
The remaining 2 cohort animals were already dead when traced after the BSE case was confirmed. (They were slaughtered for human consumption and not tested for BSE as they were not eligible.)
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Epidemiological investigations were undertaken at both the holding of birth and the holding of death of the positive BSE case. Following these investigations, the most likely source of infection remains undetermined. Four potential risk pathways were identified and assessed as very low likelihood events, all with high uncertainty. These 4 potential risk pathways were:
• accidental exposure to contaminated feed (possibly feed delivered before the reinforced feed ban that had remained attached to the side walls of a feed silo decommissioned in 2017) (see also appendix 2, point (b))
• maternal transmission
• environmental source 1: exposure to previous potential presence of the BSE agent on the natal farm via birth products
• environmental source 2: exposure to previous potential presence of the BSE agent on the natal farm from on farm or local cattle burials (when it was still legal to do so before 1 May 2003) via contaminated groundwater or other pathways (see also appendix 2, point (c))
The likelihood of any other potential risk pathways has been assessed as negligible. Any identified sources of infection have been effectively controlled through the following measures:
• The positive animal died on farm and was not destined to enter the food chain. As fallen stock, the entire carcass was category 1 ABP and was appropriately disposed of.
• Rearing cohorts and offspring cohorts were traced, culled and disposed of. All those culled cohorts and offspring were tested for BSE with negative results.
• Surveillance and testing of at-risk animals and fallen stock (see appendix 2, point D).
• Elimination of animal proteins from cattle feed as primary route of transmission (reinforced feed ban in effect since August 1996, see appendix 2, point B).
• Effective disposal of specified risk material (SRM) as per legislative requirements (see appendix 2, point E).
• Ban on burying fallen stock (dead animals) on farms since 1 May 2003 (see appendix 2, point C).
• The old feed silo was decommissioned in 2017.
The implementation of these control measures ensures that the risk of BSE agents being recycled within the bovine population has remained negligible. There is no evidence or other cause for concern that statutory official BSE or feed controls have been breached at any point in relation to this case or its herd of origin.
The detection of this case is evidence that the UK surveillance system for detecting and containing BSE is robust and effective. There is no threat to food safety, to human health or to animal health as a consequence of this case.
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Introduction
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K – Spontaneous origin
According to EFSA opinion, ‘the classification of a case as spontaneous is circumstantial and may change over time subject to additional information. It does not infer that there is no external cause; just that it could not be ascertained. A case of disease is classified as spontaneous by a process of elimination, excluding all other definable possibilities.’ (Ricci and others, 2017.)
As not all other pathways have been excluded, the likelihood of spontaneous origin is assessed as negligible.
Medium uncertainty reflects that the highest likelihood of any other pathways has been assessed as ‘very low, with high uncertainty’.
Pathway assessment Negligible likelihood, medium uncertainty.
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Concluding remarks
Following an epidemiological investigation, 4 potential risk pathways have been identified as most likely source of infection. Each are assessed as a very low likelihood event, with high uncertainty.
1. Potential accidental exposure to contaminated feed concentrates at natal farm (old feed remnants of potentially contaminated feed – before the total feed ban in 1996– that might have remained in silo 1 and that could have accidentally been released in 2017).
2. Potential maternal transmission.
3. Environmental source
1: Potential exposure to previous potential presence of BSE on natal farm via birth products.
4. Environmental source
2: Potential exposure to previous potential presence of BSE on natal farm from on farm or local cattle burials via contaminated groundwater or other pathways.
The likelihood of any other potential risk pathways has been assessed as negligible.
The detection of this case is evidence that the surveillance system for detecting and containing BSE is solid and effective. There is no threat to food safety, to human health or to animal health as a consequence of this case. The implementation of control measures and continuous monitoring ensures that the risk of BSE agents being recycled within the bovine population has remained negligible. There is no evidence that any TSE regulations have been breached in this case. There is every reason to believe that current actions will contain any further potential exposure to cattle or the human food chain.
Acknowledgements
The views expressed in this report are those of the National Emergency Epidemiology Group (NEEG). We would like to express our thanks to the TSE experts within APHA, members of the One Health Team and the many other APHA colleagues who have assisted with this investigation. The NEEG is comprised of staff from APHA’s Veterinary, Operations and Science Directorates.
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Report on the epidemiological investigation of a BSE case in Scotland (RBSE24_00003)
