NHFT Libraries Health News: 'Mad cow protein' carried by 1 in 2,000 people

Thursday, 17 October 2013

'Mad cow protein' carried by 1 in 2,000 people

'Mad cow protein' carried by 1 in 2,000 people "Twice as many Britons as previously thought could be carrying the human form of "mad cow" disease, variant CJD," BBC News warns.

However, the number remains low – 1 in 2,000 – and it is uncertain whether people who carry the vCJD protein will develop any associated symptoms.

This headline was prompted by a new study of more than 32,000 human appendix samples from 41 hospitals in the UK – presumably removed from people who had appendicitis.

Researchers found abnormal proteins (prions) indicating carrier status for a prion disease in 16 of the 32,000 samples, evidence they were carrying the so-called “Mad Cow Protein” – an abnormal protein that can cause vCJD in humans. They estimated that this would mean one person in every 2,000 was affected, which is double the previous estimate.

Prion diseases can be inherited, occur by chance, be transmitted from infected tissue or due to exposure to bovine spongiform encephalopathy (BSE), causing variant Creutzfeldt-Jacob disease (vCJD).

Carrier status means that although the prions are present, the person does not have any symptoms of prion disease. The research does not show how many of these cases were of the vCJD type, and it is not known how many people with the prions will develop symptoms of prion disease.

There have only been 177 recorded cases of vCJD so far, which is much lower than the estimated number of people who have carrier status for a prion disease.

It might be that the abnormal prions are not related to BSE, and ongoing research is being conducted to see what the prevalence was of the proteins in appendixes removed during the 1970s before the BSE outbreak.

Also further research is required to understand if the carrier status of prions can be transmitted by blood transfusions and to develop a test that would be able to reject any such samples if required.

Where did the story come from?

The study was carried out by researchers from Public Health England – formerly the Health Protection Agency, and the University College London Institute of Neurology and the Animal Health and Veterinary Laboratories Agency. It was funded by the Policy Research Programme of the UK Department of Health.

The study was published in the peer-reviewed medical journal BMJ.

In general the UK media reported the findings of the study in an accurate and responsible manner. They highlighted the fact that only a few people have developed symptomatic vCJD. They also made clear there are limitations to the clinical knowledge about whether vCJD can be transmitted through blood donations from people without symptoms, but with detectable prions.

What kind of research was this?

This was a cross-sectional survey of human appendix samples from 41 hospitals in the UK. It aimed to see how many people had prions (abnormal proteins) indicating carrier status for a prion disease.

As this was a cross-sectional survey it can only give us an estimate of the number of people who carry the prions, it cannot say how many people will develop vCJD or when.

What did the research involve?

The researchers collected tissue samples from 40,022 operations removing the appendix. The operations occurred between 2000 and 2012 in 41 hospitals covering most regions of England.

They made sure the samples were anonymous so the results could not be linked back to the patients.

They only kept details of the sex, hospital region and year of birth within five year cohorts from 1941-60 and 1961-85.

They detected the prions (PrP) using laboratory techniques. Positive and suspect samples were re-examined by two independent experts and they had three meetings to arrive at a consensus of opinion.

The results were analysed, looking for any links between sex, hospital region and year of birth and having prions in the appendix.

What were the basic results?

Of the 40,022 samples, around 7,500 were rejected as the samples did not show the follicles adequately (the appendix contains lymphoid follicles which are part of the immune system). Sixteen out of the remaining 32,441 samples were positive for prions. None of the samples were from known cases of vCJD.

This gave an overall estimated prevalence of abnormal prions of 493 per million population (95% confidence interval (CI) 282 to 801).

There were no statistically significant differences between the 16 samples in terms of age, sex or broad geographical area.

How did the researchers interpret the results?

This “second appendix survey has provided the most robust measure of abnormal prion prevalence to date, and has shown a wider birth cohort and all genotypes to be affected.” The researchers discussed potential reasons why “the number of patients with clinically manifest vCJD (177 cases at June 2013) is well below the number suggested by the prevalence of abnormal prion”. This included whether different genes could have an effect on whether a person becomes a carrier of vCJD or develops the disease. They concluded that while “the risks of transmission of vCJD by blood transfusions is likely to be low… it is essential to continue research into tests to detect abnormal PrP in blood”. Further research is being conducted, looking at appendixes removed in the mid to late 1970s before BSE was detected in people’s diets.

Conclusion

This study provided a more accurate estimate of the prevalence of prions (abnormal proteins) which indicate carrier status of a prion disease in England. Prions were found in 16 of 32,441 removed appendixes. This indicates a prevalence of a prion disease carrier status of 1 in 2,000 people, which is double the previous estimate of 1 in 4,000. Prion diseases include vCJD, but it is not clear if any of the 16 cases had this type of carrier status (none of the known cases of vCJD were involved in this study).

This is important information to gather, as it indicates that if prion diseases can be spread by a blood transfusion, the risk of this occurring would be double that previously thought. However, due its relative rarity, there are a lot of facts about prion diseases that are unknown, including:

how prion diseases are transmitted and whether this could occur through blood transfusions
how to detect prions in blood
the likelihood of developing vCJD if prions are detectable in the body and how long after infection the symptoms could occur
what protects most people from developing vCJD

Overall this is valuable research which uses a large sample of appendix samples from England and reliable methods to indicate the prevalence of abnormal prion proteins.

As the researchers suggest, study of appendixes removed prior to the emergence of BSE among cattle, and therefore prior to potential introduction into the human food chain, would be valuable to compare prevalence figures (prevalence is the amount of people affected by a specific condition).

Much remains unknown about the transmission of prion diseases and development of symptomatic disease. So it is not possible to say what implications the findings of the study have on the risk from blood transfusions or from contact with blood products or other tissues of the body.

As vCJD is so rare, the benefits of receiving a blood transfusion, often used to treat life-threatening conditions, far outweigh any potential risk.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.