There is no evidence that any form of CJD is contagious through casual contact with a CJD patient.1 Age is a major risk factor for sCJD, but by definition there is no known environmental source of infection for this category of CJD. The mean age of onset, duration and clinical presentation vary for the different subtypes of sCJD (see Figure 5). The incidence of sCJD does not seem to be significantly higher in countries where BSE and scrapie are common than it is in countries that are free of these diseases. Based on present evidence, a link between animal prion diseases and sCJD seems unlikely.4

Hereditary or familial CJD is caused by inheritance of a mutated gene which causes a change in the amino acid sequence of the normal prion protein. This change is believed to facilitate the mutated prion protein’s taking on of the abnormal prion protein conformation.8 More than a dozen different mutations in the gene that encodes the prion protein are known. All of the mutations are inherited in an autosomal dominant pattern. Therefore, if one parent carries the mutation, there is a 50-50 chance for each child to inherit the mutation and the presence of a mutation in only one allele is sufficient to cause the disease.4 However, not every mutation carrier becomes symptomatic. The chances that the mutation carriers develop the disease, called mutation penetrance, roughly varies between 40 percent and 90 percent from mutation to mutation.9

Iatrogenic CJD is a form of acquired CJD. Both laboratory and clinical research has determined that human-to-human transmission of CJD can occur as the result of tissue implant, use of contaminated neurosurgical instruments or administration of pituitary-derived hormones extracted from human cadavers.8 In a 2006 review, Paul Brown, M.D., reported that outbreaks of the disease due to contaminated human growth hormone (hGH) and dura mater grafts have largely subsided and, apart from variant CJD transfusion-associated disease, the new cases that occur each year are the result of longer and longer incubation periods following infections acquired during the 1980s.10

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