BSE in Sheep: Oral Transmission & Emerging Risks (2025)

A new study from the University of Toulouse and collaborating institutions reveals a concerning wrinkle in our understanding of prion disease transmission: the genetic resistance conferred by the ARR allele in sheep isn’t as absolute as previously believed. While breeding for this allele has been a cornerstone of European Union policy to combat scrapie and prevent bovine spongiform encephalopathy (BSE) spread, this research demonstrates that ARR/ARR sheep *can* be infected with BSE, albeit with a slower progression and potentially reduced zoonotic risk. This finding doesn’t invalidate past efforts, but necessitates a recalibration of risk assessment and continued vigilance in prion disease surveillance.

Prion diseases, like BSE (commonly known as mad cow disease) and its sheep equivalent, scrapie, are devastating neurodegenerative disorders. The core problem is a misfolded protein (PrP^Sc) that induces normal proteins to misfold, creating a cascade of damage. The 1990s BSE crisis in the UK, linked to cattle feed contaminated with infected animal remains, triggered widespread public health concerns and led to stringent regulations. The ARR allele in sheep was identified as conferring resistance, and breeding programs were implemented to increase its prevalence, aiming to minimize the risk of BSE jumping from cattle to sheep and potentially back to humans (through variant Creutzfeldt-Jakob disease, vCJD).

This new research, however, throws a subtle complication into that narrative. Researchers orally challenged both ARQ/ARQ (susceptible) and ARR/ARR (resistant) lambs with BSE, finding that while ARR/ARR sheep exhibited a delayed onset and lower prion levels, they *were* ultimately infected. Crucially, the lambs were inoculated very early in life – within 24 hours of birth and again at 14 days – a route that appears to bypass some of the protective effects of the ARR allele. Previous studies showing resistance used older animals and a different route of exposure (intracerebral or through contaminated feed), highlighting the importance of inoculation timing and method.

The study also investigated whether passage of the BSE agent through ARR/ARR sheep altered its characteristics. Surprisingly, the modified BSE strain retained its zoonotic potential, still capable of infecting mice engineered to express human prion proteins, though with a slightly reduced efficiency. This suggests that while the ARR genotype might offer a limited barrier to transmission, it doesn’t eliminate the risk entirely.

The Forward Look

The implications of these findings are significant, though not alarming. The EU’s breeding-for-resistance program has demonstrably reduced scrapie incidence, and that benefit remains. However, this research underscores the need for a nuanced understanding of prion disease transmission. Here’s what to watch for:

• Re-evaluation of SRM Measures: Given the potential for BSE to infect ARR/ARR sheep, a review of current Specific Risk Material (SRM) regulations for small ruminants may be warranted. While SRM measures are already in place, the study suggests that the distribution of prions in lymphoid tissues warrants continued scrutiny.
• Enhanced Surveillance: Continued, robust surveillance of prion diseases in both cattle and sheep populations is paramount. Early detection is critical for containing any potential outbreaks.
• Further Research into Transmission Dynamics: More research is needed to fully understand the factors influencing BSE transmission to ARR/ARR sheep, including the role of maternal transmission, inoculum dose, and the precise mechanisms by which the ARR allele confers partial resistance.
• Modeling the Impact of Genotype Shift: Epidemiological modeling should incorporate these new findings to assess the long-term impact of the ARR allele selection on the overall risk of BSE emergence and spread.

Ultimately, this study serves as a reminder that prion diseases are complex and require ongoing research and vigilance. While the ARR allele remains a valuable tool in reducing scrapie prevalence, it’s not a silver bullet against BSE. A multi-faceted approach, combining genetic resistance, stringent surveillance, and robust risk mitigation measures, is essential to protect both animal and public health.

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