Around 1 in every 2,000 people in the UK may carry variant Creutzfeldt-Jakob disease (vCJD) proteins, more commonly known as "mad cow" disease or bovine spongiform encephalopathy (BSE). This is according to a study published in the BMJ.
Variant CJD is a fatal degenerative brain disease. According to the Centers for Disease Control and Prevention (CDC), the median duration of illness from vCJD is 14 months, while the median age at death from the disease is 28-years-old.
The illness was first described in the UK in 1996 and is thought to have resulted from transmission of infection from BSE in cattle to humans through meat products.
According to the team of UK researchers, there have been 177 clinical cases of vCJD to date in the UK.
According to the World Health Organization (WHO), there have been only three cases of vCJD reported in the US between 1996 and March 2011. However, previous studies have estimated that around 1 in 4,000 people may carry vCJD prions.
There is uncertainty surrounding the number of people who will eventually develop the disease, and it is unclear whether carriers risk transmitting the disease through blood transfusion or surgery.
Because of these cloudy areas, the researchers wanted to conduct a survey in order to more accurately determine how many people in the UK could be carriers of vCJD, and to identify the genotype of the carriers.
The research team analyzed more than 32,000 appendix samples over 41 hospitals from people who had their appendix removed between 2000 and 2012.
It was found that 16 samples were positive for abnormal prion proteins. This is the equivalent of an overall prevalence of 493 per million population. From this, the researchers estimated that 1 in 2,000 people are likely to be carriers of vCJD.
Furthermore, it was found that the presence of the vCJD protein was similar in both men and women, and did not differ significantly in people born between 1941 and 1960 and those born between 1961 and 1985.
VV genotype 'may be more susceptible to vCJD'
The researchers then carried out genetic testing of the 16 vCJD-positive appendix samples.
Compared with the general UK population, the samples showed a higher proportion of valine homozygous (VV) genotype on codon 129 of the gene encoding the prion protein (PRNP). The study authors note that to date, all 177 patients with vCJD have been of methionine homozygous (MM) genotype.
From this, the researchers say they are concerned that those with the VV genotype could be more susceptible to vCJD over prolonged incubation periods, or they may show no clinical signs of the disease.
Unanswered questions 'warrant further studies'
Although the study authors recognize the number of patients with clinical vCJD is significantly below the prevalence suggested in their research, they note further studies are needed in order to develop tests able to detect abnormal protein in the blood.
Additionally, the researchers say that further studies are needed to analyze tissue prior to the 1970s, before BSE first appeared.
In an editorial linked to the study, Roland Salmon, a retired consultant epidemiologist, notes that although there has been increased knowledge of the disease, there are still many unanswered questions:"What is the disease phenotype and natural course of variant CJD in genotypes other than MM? What other animal prion diseases may be zoonotic?"
"The replication mechanisms first seen in prion proteins have now been identified in other proteins involved in other common neurodegenerative diseases, including Aβ, amyloid-β in Alzheimer's disease, a-synuclein in Parkinson's disease, and tau in several different conditions. How often, if ever, are any of these transmissible?"
He concludes that the UK's prion research capacity with expertise in human and veterinary disease surveillance and pathology are well placed to answer these questions and that "further disinvestment would be premature."