The Generative Brain

BSE (mad cow disease): research

Plain explanation

Classical BSE is the only prion disease of animals shown to be zoonotic. In transgenic mice overexpressing human prion protein, L-type atypical BSE transmitted more readily than classical BSE while H-type did not transmit at all, and valine at codon 129 was strongly protective against L-type even in heterozygotes.

Research on BSE has moved from the epidemic itself to the atypical forms, and the central question has changed with it. Classical BSE is the one animal prion disease with demonstrated zoonotic transmission. Whether the H-type and L-type agents share that property, and what happens when they pass through an intermediate species, is what current transmission work addresses. For the veterinary and food-control picture see food chain and the plain-language BSE page.

How do the atypical strains behave in human prion protein mice?

Marin-Moreno and colleagues ran the systematic comparison. Atypical BSE isolates from several European countries were inoculated into three transgenic lines overexpressing human prion protein: TgMet129, TgMet/Val129 and TgVal129.

L-type BSE showed higher zoonotic potential in TgMet129 mice than classical BSE did. The Val129 variant acted as a strong molecular protector against L-type prions, including in heterozygous animals. H-type BSE did not transmit to any of the lines.

The second half of the experiment is the part with the most consequence for surveillance. One H-type and one L-type isolate were first adapted to sheep-PrP transgenic mice, then inoculated into human-PrP mice. After that passage the atypical agents infected both TgMet129 and TgVal129 mice, and the authors report that the resulting agents bore features making them indistinguishable from sporadic CJD prions.

Two readings follow, and they should be kept apart. The experiment demonstrates that an agent’s species barrier is a property of the agent’s current host history, not a fixed attribute, so a negative result in a direct challenge does not settle zoonotic potential. It does not show that this sequence of events occurs in the field, and it does not establish that any human sporadic case arose this way. Transgenic lines overexpressing human PrP also exaggerate susceptibility relative to people.

What is the E211K variant and why does it matter?

Richt and Hall reported an H-type BSE case in a cow heterozygous at Prnp nucleotides 631 to 633, a non-synonymous change designated E211K. The homologous human position is codon 200, where E200K is the most common cause of genetic CJD. An epidemiological survey they cite found no K211 allele in 6062 cattle from commercial beef processing plants and 42 breeds, which places the prevalence below roughly 1 in 2000.

Tahir and colleagues reported the second such animal in 2025: the Canadian H-type case found in 2021, in which the codon 211 change was confirmed as a germline mutation, the first Canadian BSE case with this predisposing variant. Their tissue analysis found PrPSc largely restricted to the central nervous system, with no or occasional weak involvement of peripheral tissues.

This reframes some atypical cases as a bovine counterpart to genetic prion disease rather than as feed-borne infection, which is consistent with the WOAH position that atypical BSE is believed to occur spontaneously in all bovine populations. It also means the two H-type cases with the clearest genetic explanation cannot be used to argue that all atypical cases are spontaneous: most atypical cases have no reported PRNP variant.

What do current national counts show?

The Canadian Food Inspection Agency’s table, current as of 30 September 2026, lists three animals since 2011: an Alberta beef cow confirmed on 17 December 2021 with H-type atypical BSE at 104 months of age, an Alberta beef cow confirmed on 11 February 2015 at 70 months, and an Alberta dairy cow confirmed on 18 February 2011 at 77 months. Cases before 2011 are held in the National BSE Surveillance Program record rather than on that page.

WOAH describes the incidence of both classical and atypical forms as negligible and approaching zero cases per million bovines, with classical BSE the only form that is WOAH-listed and subject to official risk status recognition. Atypical cases continue to be found in old cattle in countries with negligible risk status, which is what a spontaneous origin predicts and what active surveillance of fallen stock is designed to detect.

What is unresolved?

  • Whether L-type BSE has ever infected a person. The mouse data establish capability under overexpression, not occurrence.
  • Whether any sporadic CJD cases are misclassified transmissions from an adapted ruminant strain. The strain indistinguishability reported after sheep passage makes this hard to exclude by typing alone.
  • How frequently PRNP variants underlie atypical cases, given that only two cases with a reported predisposing variant exist.

Related research pages: chronic wasting disease, scrapie, variant CJD and biosafety.

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