It sounds like something from a nightmare: decades after eating a tainted hamburger you develop an incurable, fatal disease that literally eats holes in your brain.
Unfortunately, for some people this is a nightmare that is all too real. In the 1990s a small number of people in the UK developed a variant form of a disease called Creutzfeldt-Jakob disease (vCJD) after eating meat from cattle infected with bovine spongiform encephalopathy (BSE), or “mad cow disease”.
Every one of the documented vCJD cases in the UK has been in a person with two copies of the “M” variant of a gene called PRNP. People with just one or with no copies of this variant — MV and VV individuals, respectively — have been thought to be protected from developing vCJD. But the British media (here, here and here for examples) recently reported a case of apparent vCJD in someone with the MV genotype, raising concerns that more people in the UK could be at risk.
But in an email to the Blog, the UK’s Spongiform Encephalopathy Advisory Committee chair Christopher Higgins wrote, “There are no confirmed vCJD clinical cases in MV or VV. 100% of clinical cases are in MM. The latest reports concern someone who is definitely MV but a clinical diagnosis of vCJD has not yet been confirmed — and may not be for sometime.”
(23andMe customers can check their genotype for this variant in the PRNP gene at rs1799990 using the Browse Raw Data feature. An “A” corresponds to a copy of the M variant. A “G” corresponds to the other version, V.)
Higgins said that if MV and VV individuals were just as susceptible to infection with vCJD as MM individuals, the theoretical maximum number of additional cases the UK might see would be about 300.
But, he stressed, this is a big if.
“This is a maximum and I personally suspect the numbers will be very much less and could even be zero.”
Higgins said that mouse studies and research involving kuru, another type of spongiform encephalopathy, indicate that people with the MV and VV genotypes are less easily infected than MM individuals. Furthermore, these same studies have shown that if MV and VV individuals are infected, the incubation period between infection and development of the disease would be expected to be greatly extended.
“Thus many may die of other causes or simply ‘old age’ before clinical vCJD has a chance to emerge,” he said.
Higgins noted that in the UK, BSE has been effectively eradicated from the beef industry and precautions have been put in place to minimize the chance of person-to-person transmission of vCJD, meaning that anyone who was going to be infected would have been infected during the BSE crisis of the 1980s to mid-1990s.
“Younger people of any genotype should have little to worry about,” he said.
Finally, Higgins pointed out that the risk of vCJD should be kept in perspective.
“Compared with the 3000 people per year who die in road accidents, the total number of people who will ever die of BSE/vCJD is likely to be much less,” Higgins said. He estimates that no more than about 200 cases of vCJD in MM individuals will ever be seen in the UK, and said that around 160-170 cases of vCJD have already been diagnosed and the numbers diagnosed annually are now very low, indicating that the epidemic could be past its peak.
“There are far more important things in life to worry about.”