Animal Disease and Human Health Risk
How Many Humans Have Prion-related Illnesses?
Other than handfuls of cases in Europe and the relatively few individuals
diagnosed with CJD worldwide, are there other cases with prion-related conditions?
Is it possible that other individuals with dementia may have prion-induced illnesses?
There are concerns in some quarters that some cases of "Alzheimer's" may actually be
spongiform encephalopathy. Researchers at the University of Pittsburgh studied
patients that were thought to have died from Alzheimer's Disease. On closer
examination they found that some of them had actually died from the prion-related
Creutzfeldt-Jakob disease.39 What was interesting about this report is that these
individuals had a slower progression of disease than the typical CJD patient.
Such slow progression is reminiscent of one of the differences in the variant CJD that
has been diagnosed in Britain.
Neuropathologists with the Alzheimer's Disease Research Consortium occasionally do
an autopsy of a patient who has clear-cut CJD but whose paperwork suggested only
Alzheimer's.40 Despite carefully searching the medical literature, I have not come
across any systematic study of the number of CJD patients misclassified as having
Alzheimer's Disease. The experience of a couple of dementia research
neuropathologists left me with a reasonable guess that anywhere between 1 in 50 to 1
in 200 patients who are clinically diagnosed with Alzheimer's really have unequivocal
autopsy evidence of CJD. This may not sound like a very large percentage. However,
when you consider the total number with Alzheimer's, this small percentage of cases
accounts for a significant number of people. Currently, some four million Americans
have Alzheimer's disease.41 According to a Harvard medical report, the estimates are
that by 2050 the number will be over 10 million.42 If even one in 100 diagnosed
Alzheimer's patients had CJD, this would translate into 40,000 cases currently and
100,000 by the middle of the next century.
So far, we have only been talking about clear-cut autopsy cases of CJD. There are
concerns that not all prion-related conditions can be easily diagnosed even if an
autopsy is done. For example, a recent medical report observed that in some cases,
the distinction between Alzheimer's and CJD is "gray."43 Another report commented on
the brain changes in a 32-year-old man who died with a dementia. The authors concluded:
"This patient had features of both Creutzfeldt-Jakob disease and Alzheimer's disease,
providing additional support for the existence of an overlap between these disorders."44
The human prion-related disease called Gerstmann-Straussler syndrome provides additional
human evidence that these transmissible encephalopathies may strike and not leave their
classic footprints. An individual who was demonstrated to have this prion-related
condition did not show the sponge-like brain changes under the microscope.45 The
researchers concluded that spongiform encephalopathy in humans "cannot always be
excluded on neuropathologic grounds in an individual dying of a dementing condition,
and the true prevalence of these diseases is likely to be underestimated." In other
words, among people with dementias (conditions like Alzheimer's where there is severe
loss of mental faculties), we are probably missing cases of prion-related diseases
because not all such cases show the expected microscopic changes.
Perhaps an even more startling report found spongiform changes in the brains of 50 out
of 66 patients with Alzheimer's disease. Smith and colleagues said that these changes
were "virtually indistinguishable histologically [meaning, under the microscope] from
the spongiform change characteristic of Creutzfeldt-Jakob disease (CJD)."46 Although
the authors thought it unlikely that they were seeing evidence of CJD or another
prion-related condition, they did at least raise that possibility. From my perspective,
their work raises concerns that subtypes of Alzheimer's may exist. A vital question is:
could some of these subtypes be related at least in part to prion exposure? The
implications of such a possibility are far-reaching. The parallels between prion-related
diseases and Alzheimer's are recorded in Figure 10: Parallels Between Prion-Related
Diseases and Alzheimer's.
The sobering message from this collective body of research is: we need to seriously
question our assumption that CJD is a rare disease. If just a small percentage of
individuals who are diagnosed with Alzheimer's truly have CJD (in either typical or
atypical forms), the numbers with this prion-related disease would be staggering.
39 Lopez OL, Larumbe MR, et al, Difficulties in differential diagnosis of long-term Creutzfeldt-
Jakob disease. Neurologia 1995 Jan;10(1):37-40.
40 DJD personal communication with Dr. Ron Hamilton (412-647-6615) at the University of Pittsburgh
(had 1 case in 60 who had CJD and had no suggestion on very sketchy clinical data of a prion
disease being suspected) and Dr. Larry Hansen (619-534-6212) at UC San Diego who recalled 3 CJDs
in some 500 Alzheimer's autopsies. From Hamilton, if a patient is suspected of CJD they are not
supposed to go through the path eval that they are doing for Alzheimer's in their registry.
41 Taubes G. Misfolding the way to disease. Science 1996 Mar 15;271(5255):1493-1495.
42 Evans DA. Estimated prevalence of Alzheimer's disease in the United States. Milbank
43 Barcikowska M, Kwiecinski H, et al. Creutzfeldt-Jakob disease with Alzheimer-type
A beta-reactive amyloid plaques. Histopathology 1995 May;26(5):445-50.
44 Liberski PP, Papierz W, Alwasiak J. Creutzfeldt-Jakob disease with plaques and paired
helical filaments. Acta Neurol Scand 1987 Dec;76(6):428-432.
45 Collinge J, Owen F, et al. Prion dementia without characteristic pathology. Lancet 1990
46 Smith TW, Anwer U, et al. Vacuolar change in Alzheimer's disease. Arch Neurol 1987
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