"I think it's reassuring to people who had transplants, blood transfusions, morticians and researchers who work on these diseases," said the study's senior author, Dr. John Trojanowski, from the University of Pennsylvania Perelman School of Medicine in Philadelphia.
Reviewing the records of children who were injected as long as 50 years ago with human growth hormones derived from cadavers showed that the recipients did not go on to develop Alzheimer's or Parkinson's disease with any greater frequency than the rest of the population - which suggests the two conditions are not contagious.
Trojanowski and his colleagues, who published their findings in JAMA Neurology, in fact found no cases of Alzheimer's or Parkinson's disease in about 800 individuals who were injected with growth hormones that came from the bodies of people who may have had one of those two illnesses.
Doctors switched to synthetic hormones in the 1980s when they realized some children had received contaminated cadaver growth hormones that later led to their developing Creutzfeldt-Jakob disease (CJD), the human version of "mad cow" disease, which causes a fast decline in mental functioning and generally death within a year. (See Reuters Health story of August 19, 2011 here:).
Because CJD is caused by a rogue protein called a prion, which causes other proteins in the brain to fold in strange ways and lose their function, there was concern that abnormal proteins involved in Parkinson's and Alzheimer's diseases could transmit those illnesses in the same way.
"There's been concern recently because a lot of the proteins involved in neurogenic diseases are similar to these prions," said Karen Duff, who studies Alzheimer's and Parkinson's at Columbia University Medical Center in New York.
Based animal studies, some researchers feared that receiving cornea or organ transplants, blood transfusions, or working with the brains of people who had Alzheimer's and Parkinson's may put people at an increased risk for those diseases, according to Trojanowski.
For the new study, his team looked at data on 6,190 people who had a growth problem as children and were injected with human growth hormones taken from cadavers between 1963 and 1985.
The researchers also determined how likely it was that the hormones injected into the children came from cadavers with any of a variety of brain diseases.
Examining brain tissue from 34 cadaver donors, the researchers found 10 had no diseases, 9 had Alzheimer's, 3 had Parkinson's, 4 had both and the rest had other types of brain diseases.
Turning to the group of growth hormone recipients, the researchers then examined the death certificates of the 796 who died before 2008.
Overall, none of the hormone recipients' death records mentioned Parkinson's or Alzheimer's, even though the researchers determined that it's very likely they were injected with hormones from people who had those diseases.
Duff, who was not involved in the new study, agreed that the results allay fears about the diseases being contagious, especially for people involved in simply handling brain tissues and fluids.
She added that while this study can't confirm there's no risk, any chance of "catching" Alzheimer's or Parkinson's disease from even a contaminated piece of medical equipment would be much less than from having the hormones directly injected, so the study should be reassuring.