A now-banned type of blood transfusion which was used globally until the 1980s may have given people Alzheimer's, a new study claims.

Between 1958 and 1985, abnormally short children in the US and the UK were given hormones harvested from cadavers to help spur their growth.

But in the early 1980s, there was a global outbreak of Creutzfeldt-Jakob disease (CJD), a fatal neurological disorder - and it was traced back to the blood transfusions.

The technique was banned in 1985, replaced with synthetic hormones, and researchers have been monitoring the survivors for other side effects ever since.

Now, a paper published today in the journal Springer presents new evidence for a popular theory that the technique created build-ups of amyloid protein in survivors' brains - paving the way to Alzheimer's.

The research team gave the hormones to mice, and the rodents developed signs of the disease within a year.

Alzheimer's has been linked to blood transfusions after scientists discovered the disease may have been transferred between patients in a now-banned procedure (stock)

The study, published in Nature, was carried out by University College London and led by Dr John Collinge, a professor of neurology at the UCL Institute of Prion Diseases.

Alzheimer's is the most common form of dementia and affects more than 520,000 people in the UK, according to the Alzheimer's Society. The disease has around 5.7 million patients in the US, Alzheimer's Association statistics show.

It occurs when connections between the brain's billions of nerve cells get lost due to a build-up of amyloid and tau proteins that form abnormal plaques and tangles.

Over time, different areas of the vital organ shrink, with the region responsible for memories usually being affected first. There is no cure with treatments focusing on slowing the disease's progression and managing symptoms.

The researchers drew one of their 2015 studies, which analysed the autopsied brains of eight people who died young from CJD.

The disease is thought to occur when high levels of prions - known as 'amyloid seeds' - cause irreversible damage to nerve cells. More than 200 people worldwide died of CJD as a result of hormone transfusions.

All of the patients had been treated as children in the 1980s with a human growth hormone taken from cadavers. This procedure was first carried out in the UK in 1958, and in the US in 1963.

Although none of them developed the 'full picture' of dementia, six of them had worrying amounts of the naturally-occurring amyloid proteins in their brains.

In an Alzheimer's patient's brain, abnormal levels of amyloids clump together to form plaques that collect between nerve cells and disrupt their function.

Six of the brains also had some degree of of cerebral amyloid angiopathy (CAA), which occurs when amyloids build in the brain's blood vessels and can cause bleeding. CAA is seen to some extent in nearly all Alzheimer's patients.

To build on this research, the scientists tracked down some of the batches of the human growth hormone that the deceased had been treated with.

Testing these samples revealed that some contained significant levels of both amyloid and tau proteins. Tau has been shown to form tangles in the area of the brain associated with memory loss in Alzheimer's sufferers.

The samples were then injected into mice who were genetically at-risk of developing the amyloid clumps seen in Alzheimer's. Other rodents were injected with brain tissue from known dementia patients.

The mice that were injected with the now-banned growth hormones showed clear signs of amyloid clumping along blood vessels in their brains and developed CAA within a year.

This occurred to a greater extent among the rodents who were injected with tissue from Alzheimer's patients.

The study therefore suggests that human-growth hormone batches that have been stored for decades can still cause worrying amyloid clumps in mice.

'We have now provided experimental evidence to support our hypothesis that amyloid pathology can be transmitted to people from contaminated materials,' Professor Collinge said.

'We cannot yet confirm whether medical or surgical procedures have ever caused Alzheimer’s disease itself in people, or how common it might be to acquire amyloid pathology in this way.'

'It will be important to review risks of transmission of amyloid pathology by other medical procedures still done today, including instruments used in brain surgery, drawing on other research and what we already know about accidental CJD transmission.'

Dr Rob Buckle, chief science officer at the Medical Research Council, which helped fund the study, stressed that the experiments were carried out on mice that were genetically at-risk of developing amyloid clumps.

Dr Tibor Hortobágyi, a reader in old age psychiatry at King’s College London, argued that normal mice do not have the capacity to develop human-like Alzheimer's and therefore genetically-modified rodents had to be used.

He did stress, however, that the hormones were injected directly into the animal's brains, which does not occur in existing medical procedures or the now-banned blood transfusion.

The researchers said the samples were injected into the mice's brains 'in order to optimise the chance of detecting seeds in this scarce material'.

Dr Hortobágyi added the study only provides evidence that amyloids can be transferred to mice - not tau or Alzheimer's disease itself.

'It is important to stress that there is no indication that Alzheimer’s disease is a contagious disease or is transmissible via blood transfusion,' he said.

'This study did not look at surgical instruments at all, so we can’t make any conclusions about that and any suggested implications about that are speculation beyond the scope of this work.'

The Royal College of Surgeons and the Society of British Neurological Surgeons welcomed the study but added more, larger trials are needed.

A spokesperson said: 'The risks of developing one of these degenerative diseases as a consequence of neurosurgery seems substantially smaller than the everyday risks of the procedures themselves and the diseases they are used to treat'.

Professor Bart De Strooper, director of the UK Dementia Research Institute at UCL, referenced a 2016 Scandinavian study that analysed almost 1.5million people's health records and found no evidence blood transfusions can cause Alzheimer's.

Dr James Pickett, head of research at Alzheimer's Society, concluded: 'There remains absolutely no evidence that Alzheimer’s disease is contagious.'