A man’s death from CJD - the human variant of 'mad cow disease' BSE - has raised concerns for a section of the population previously thought to be immune to the disease.

BSE is a brain disease caused by misfolded proteins - or prions - which was at the centre of a health scare stemming from an epidemic of the disease in cattle that saw nearly four and a half million cows killed in an eradication programme in the 1990s. The epidemic was caused by feeding normally herbivorous cows the remains of other cattle in mat and bone meal. The human variant of the disease, thought to be most easily transmitted by eating meat from infected cattle, has killed 178 people in the UK and around 50 in Western Europe, where infected beef from the UK was sold in the late 1980s and 90s.

Little is known about the disease, which is made all the more obscure by the amount of time it takes for symptoms to manifest.

Until now, all human victims of the disease were thought to have a particular genotype called MM, leading some experts to suspect only people with this genetic variant were susceptible, but a paper published in the New England Journal of Medicine (NEJM) last week revealed that a man with another genotype - MV - had died from CJD last year. Cases of CJD in people with the MM genotype were on the wane by the mid-2000s, but doctors reporting on the recent case warned that their findings could indicate that the disease has a longer incubation period in people with the more common MV genotype, not that they are immune.

The man, who was 36 when he was referred to the UK’s National Prion Clinic in August 2015, died in February 2016. The case wasn’t typical of human variant CJD (at first he was thought to be suffering from sporadic CJD, a disease unrelated to BSE), but an autopsy confirmed he had contracted human variant CJD. The NEJM paper’s authors, from University College London (UCL) and Gloucester NHS Trust, concluded, “It remains uncertain whether this case marks the start of a second wave of variant Creutzfeldt–Jakob disease in persons with the MV genotype at PRNP codon 129 (the most common genotype in the United Kingdom).”

They said that, if this were the case, it would mirror “Long incubation periods seen in persons with the MV genotype who have other acquired prion diseases, notably kuru,” a disease similar to CJD and associated with cannibalism.

Commenting on the report, Professor Giovanna Mallucci, from Cambridge’s Department of Clinical Neurosciences, said, “This is not unexpected given experience with other transmitted human prion diseases, particularly kuru, where some individuals had extremely delayed onset of disease, decades after exposure, due to the genetic makeup of individuals: being less susceptible and hence having much longer incubation times.”

Prof Mallucci said, “As the almost same proportion of people in the UK have the less susceptible MV genotype as the susceptible MM genotype, it is possible we might expect similar numbers of cases with the MV genotype to what we have already seen with the MM genotype, but spread over a longer time period, as the authors comment.”

Also commenting, Dr Graham Jackson, from the Prion Unit at UCL’s Institute of Neurology, said the fact that the patient’s clinical features initially pointed to sporadic CJD means BSE related prion disease could be missed in people with the MV genotype.

Dr Jackson said previous studies have suggested that one in 2,000 UK residents could be infected with human variant CJD, and warned that, if vCJD carries different signs in people with the MV genotype, “then this figure may be too low as such cases will not have been detected.” The Doctor said, “Whilst the incidence of clinical variant CJD has fallen from a peak in 2000 it interesting that the incidence of sporadic CJD appears to have been steadily rising over the same period.

“The case reported satisfied all the criteria for classification as sporadic CJD and would have been reported as such if the unusually young age of the patient had not prompted detailed investigations following autopsy. There is now a possibility that BSE related prion disease can present cryptically as sporadic CJD in addition to the typical variant CJD.”