Reader (the second) wrote;
Flounder just posted a worrisome article on CJD family site which I assume he'll post here shortly. I met with one of the activist family members of the vCJD families while in Scotland, who has become a friend of mine. They are quite concerned about secondary transmission of vCJD via iatrogenic means such as dentistry/surgery/tissue and blood donation now that it has been shown in the past months that MV and VV individuals are in fact susceptible but that their incubation periods are longer or that they remain asymptomatic throughout their lifespans. ....
HERE they are...............terry
SEAC
Position Statement
--------------------------------------------------------------------------------
Position statement vCJD and Endodontic dentistry
Issue
1. The Department of Health (DH) asked SEAC to advise on the findings and implications of a preliminary risk assessment of potential vCJD transmission via endodontic procedures (dental procedures involved in the maintenance of dental pulp and the treatment of the pulp cavity) 1. This is particularly pertinent because of the large number of endodontic procedures undertaken in the UK.
Background
2. There are no reported definite or suspected cases of vCJD transmission arising from dental procedures. However, prions are more resistant than other types of infectious agent to the conventional cleaning and sterilisation practices used to decontaminate dental instruments 2. Therefore, should dental instruments become contaminated from tissues in the oral cavity of infected individuals, there is a risk of transmission to subsequent patients.
3. A quantitative DH risk assessment 3, accepted by SEAC in 2003, considered two possible mechanisms for the transfer of vCJD infectivity via dental instruments: (i) accidental abrasion of the lingual tonsil, known to carry infectivity in vCJD cases; and (ii) contact with dental pulp that evidence from animal studies suggested may be infective. On the basis of the information available, the DH analysis suggested that the risk of transmission to individual patients via accidental abrasion of the lingual tonsil is very low. Furthermore, should dental pulp be infective, the risk of transmission via endodontic procedures, although higher, is also low. Although a very large number of dental procedures are conducted, the relative risk to public health from potential transmission via dental, compared with hospital, surgery was considered to be relatively low.
4. In 2006, SEAC considered a new preliminary risk assessment by DH of the risks of vCJD transmission via endodontic procedures, taking into account new information on decontamination of dental instruments, the potential infectivity of dental pulp, and the possible existence of subclinical vCJD carrier cases.
Endodontic instruments
5. Evidence suggests that the files and reamers used in endodontic procedures are reused and are difficult to reliably decontaminate 4. Appreciable quantities of residual material remain adherent to the surface after normal cleaning and sterilisation 5. Thus, there is potential for transfer of dental pulp between patients undergoing endodontic procedures.
vCJD infectivity in dental tissues
6. There are no data on vCJD infectivity in dental pulp. Although no abnormal prions were found in a study of dental tissues, including dental pulp, from vCJD cases 6, dental pulp includes blood and peripheral nerve tissue known to carry vCJD infectivity 7,8. In addition, appreciable infectivity has been found in the dental pulp of hamsters with hamster scrapie 9. Although it is possible that the peripheral nerve may only become infective close to, or after, the onset of clinical vCJD, inflammation may promote the propagation of prions 10. Thus, although the data are limited and indirect, it is reasonable to assume that the dental pulp of individuals subclinically-infected with vCJD may be infectious although the level of infectivity is unknown. Studies underway will provide direct data on the infectivity in dental tissues from vCJD cases.
Subclinical carrier state
7. A study of humanised mice showed that vCJD infections may not always progress to clinical disease within the normal lifespan of the animals 11. Another study suggested that prion infections in mice that remain at a subclinical level can be transmitted to other mice, resulting in clinical disease 12. Thus, there is evidence to suggest that individuals infected with the BSE / vCJD agent may remain in a subclinical infection carrier state instead of developing vCJD. A discrepancy between prevalence estimates based on a survey of abnormal prion protein in appendix and tonsil tissue and data on vCJD cases supports this hypothesis 13. As no diagnostic test exists to identify such individuals, they could over the course of their lives be potential sources of numerous secondary infections arising from invasive medical or dental procedures.
8. The prevalence of subclinical infection in the UK population is uncertain. A recent estimate suggests the number of subclinical carriers may be of the order of several thousand 14. SEAC has strongly recommended that further studies to ascertain better the prevalence of vCJD infection be urgently considered 15.
Transmission risks
9. The new DH analysis suggests that, on the basis that residual dental pulp on endodontic files and reamers is transferred relatively efficiently to patients on reuse, dental pulp is as infective as peripheral nerve tissue and a subclinical carrier population for vCJD exists, a self-sustaining vCJD epidemic arising from endodontic surgery is plausible. There are uncertainties about the efficiency of vCJD transmission via endodontic procedures, the vCJD infectivity of dental pulp and the existence of a subclinical infection carrier state. However, even if a self-sustaining epidemic were not possible, clusters of vCJD infections could arise from the use of instruments contaminated with the vCJD agent from endodontic procedures on infected patients. Interactions between this and other routes of secondary transmission, such as blood transfusion and hospital surgery, would make a self-sustaining epidemic more likely.
Potential risk reduction measures
10. Endodontic files and reamers have a limited lifespan, restricting the number of possible secondary transmissions. Improving the effectiveness of procedures used to decontaminate dental instruments would reduce the risk of transmission. Restricting endodontic files and reamers to single use would prevent potential secondary transmission via these instruments.
Conclusions
11. A preliminary risk assessment produced by DH suggests that vCJD transmission via endodontic dentistry may, under certain hypothetical but plausible scenarios, be sufficient to sustain a secondary vCJD epidemic. However, there are uncertainties around the data and assumptions underpinning the assessment. Research underway will address some of these uncertainties and allow the risk assessment to be refined. Once the research is complete and / or other data become available, the risks should be reassessed. A watching brief should be maintained.
12. It is unclear whether or not vCJD infectivity can be transmitted via endodontic files and reamers. However, given the plausibility of such a scenario and the large number of procedures undertaken annually, it would be prudent to consider restricting these instruments to single use as a precautionary measure. Since sufficiently rigorous decontamination of these instruments is difficult, single use of these instruments would eliminate this risk, should it exist.
SEAC
May 2006
--------------------------------------------------------------------------------
1. Department of Health. Dentistry and vCJD: the implications of a “carrier state” for a self-sustaining epidemic due to endodontic dentistry. A Preliminary Risk Assessment. Unpublished.
2. Smith et al. (2003) Prions and the oral cavity. J. Dent. Res. 82, 769-775.
3. Department of Health. (2003) Risk assessment for vCJD and dentistry.
4. Letters et al. (2005) A study of visual and blood contamination on reprocessed endodontic files from general dental practice. Br. Dent. J. 199, 522-525.
5. Smith et al. (2005) Residual protein levels on reprocessed dental instruments. J. Hosp. Infect. 61, 237-241.
6. Head et al. (2003) Investigation of PrPres in dental tissues in variant CJD. Br. Dent. J. 195, 339-343.
7. SEAC 91 minutes paragraph 9. www.seac.gov.uk/papers/papers.htm
8. Department of Health (2005) Assessing the risk of vCJD transmission via surgery: an interim view. Unpublished.
9. Ingrosso et al. (1999) Transmission of the 263K scrapie strain by the dental route. J. Gen. Virol. 80, 3043-3047.
10. Heikenwalder et al. (2005) Chronic lymphocytic inflammation specifies the organ tropism of prions. Science. 307, 1107-1110.
11. Bishop et al. (2006) Predicting susceptibility and incubation time of human-to-human transmission of vCJD. Lancet Neurology.
12. Hill et al. (2000) Species-barrier-independent prion replication in apparently resistant species. Proc. Natl. Acad. Sci. USA. 97, 10248-10253.
13. SEAC Epidemiology Subgroup (2005) Position statement on the vCJD epidemic. www.seac.gov.uk/statements/state260106subgroup.htm
14. Clarke & Ghani. (2005) Projections of future course of the primary vCJD epidemic in the UK: inclusion of subclinical infection and the possibility of wider genetic susceptibility. R. J. Soc. Interface.
15. SEAC (2005) SEAC response to the SEAC Epidemiology Subgroup statement on the vCJD epidemic. www.seac.gov.uk/statements/state260106.htm
Page updated: 8th May 2006
http://www.seac.gov.uk/statements/statement0506.htm
Subject: Predicting susceptibility and incubation time of human-to-human transmission of vCJD
Date: March 27, 2006 at 6:24 am PST
Lancet Neurology DOI:10.1016/S1474-4422(06)70413-6
Predicting susceptibility and incubation time of human-to-human transmission of vCJD
MT Bishop a, P Hart b, L Aitchison b, HN Baybutt b, C Plinston b, V Thomson b, NL Tuzi b, MW Head a, JW Ironside a, RG Will a and JC Manson b
Summary
Background
Identification of possible transmission of variant Creutzfeldt-Jakob disease (vCJD) via blood transfusion has caused concern over spread of the disease within the human population. We aimed to model iatrogenic spread to enable a comparison of transmission efficiencies of vCJD and bovine spongiform encephalopathy (BSE) and an assessment of the effect of the codon-129 polymorphism on human susceptibility.
Methods
Mice were produced to express human or bovine prion protein (PrP) by direct replacement of the mouse PrP gene. Since the human PrP gene has variation at codon 129, with MM, VV, and MV genotypes, three inbred lines with an identical genetic background were produced to express human PrP with the codon-129 MM, MV, and VV genotypes. Mice were inoculated with BSE or vCJD and assessed for clinical and pathological signs of disease.
Findings
BSE was transmitted to the bovine line but did not transmit to the human lines. By contrast, vCJD was transmitted to all three human lines with different pathological characteristics for each genotype and a gradation of transmission efficiency from MM to MV to VV.
Interpretation
Transmission of BSE to human beings is probably restricted by the presence of a significant species barrier. However, there seems to be a substantially reduced barrier for human-to-human transmission of vCJD. Moreover, all individuals, irrespective of codon-129 genotype, could be susceptible to secondary transmission of vCJD through routes such as blood transfusion. A lengthy preclinical disease is predicted by these models, which may represent a risk for further disease transmission and thus a significant public-health issue.
Affiliations
a. National CJD Surveillance Unit, Bryan Matthews Building, Western General Hospital, Edinburgh, UK
b. Institute for Animal Health, Neuropathogenesis Unit, King's Buildings, Edinburgh, UK
Correspondence to: Prof J C Manson, Institute for Animal Health, Neuropathogenesis Unit, Ogston Building, King's Buildings, West Mains Road, Edinburgh EH9 3JF, UK
SNIP...
further into this study;
SNIP...
Discussion
Although the cattle BSE epidemic in the UK has
amounted to more than 180 000 cases since the 1980s,
the extent of the human vCJD epidemic has so far
remained limited with the total number of cases
worldwide currently at 190. One explanation for this
apparent discrepancy is that there exists a significant
species barrier between cattle and human beings, which
limits the susceptibility of the human population to
BSE. The data shown here suggest that this could
indeed be the case since BSE was readily transmissible
to the bovine transgenic mice but not to the human
transgenic mice. However, once BSE has passed
through human beings in the form of vCJD, the
transmissibility of this TSE strain is altered for the
human population.
All the human transgenic lines inoculated with BSE
were negative for TSE transmission, which suggests that
either the human transgenic lines are relatively resistant
to transmission of BSE or the incubation time is longer
than the length of the experiment (approximately
700 days). BSE transmission previously observed by
others, in human transgenic lines overexpressing the
human prion protein, could be due to overexpression of
the PrP gene and may not therefore give a true reflection
of the species barrier between BSE and human
beings.15,25,26 This apparent resistance of human transgenic
mice to BSE could be explained by a large species barrier
and this in turn could explain the low number of vCJD
cases in the human population.
vCJD was transmitted to all three human lines with
different pathological characteristics for each genotype,
and a gradation of transmission efficiency from MM to
MV to VV. The greater transmission efficiency in HuMM
mice suggests that homozygosity for methionine at
codon 129 leads to earlier onset of TSE-related
pathological features and clinical disease than for the
other two genotypes. The differences in PrPSc deposition
in the HuMM and HuMV lines suggest that the codon-
129 polymorphism in human beings is likely to affect
the distribution of PrPSc deposition in the brain.
Moreover, the similar numbers that scored positive for
PrP deposition in each of the MM and MV groups (11/15
and 11/13 respectively) suggest that the two genotypes
might be equally susceptible to vCJD, but with different
incubation periods. Titration experiments are needed to
fully compare the susceptibility of each line. The single
HuVV mouse positive for PrPSc shows that VV
individuals may be susceptible to vCJD with very long
incubation times, including a lengthy subclinical phase.
Transmission studies from all three genotype mice are
now underway to examine the infectious nature of the
disease and determine any alterations in the strain
characteristics on passage through human transgenic
mice. By contrast with published data suggesting that
VV individuals cannot propagate the vCJD biochemical
phenotype,15 the data presented here suggest that the
PrPSc type will remain a useful diagnostic feature of
secondary vCJD infection irrespective of codon-129
genotype, as has been observed for the two extant cases
of transfusion-associated vCJD infection. 5,27
Transmission of vCJD to the three lines of human
transgenic mice indicates that the human population
could be at significantly heightened risk of developing
disease after iatrogenic exposure to vCJD. Secondary
transmission of vCJD has partly removed the cattle-to-
human species barrier and has resulted in an agent that
can be transmitted from human to human with relative
efficiency. Transmission studies in cynomolgus macaques
provide further evidence for this agent adaptation as they
show reduction in incubation times after serial passage
of BSE.28 Our BSE inoculation at 10-1 dilution was
compared with vCJD inoculation at 10-2 because the latter
inoculum was found to be toxic to the mice at 10-1. Use of
a higher dose ofvCJD inoculum would have maintained
or increased the transmission efficiency of vCJD and
enhanced the current findings.
Our findings raise concerns relevant to the possibility
of secondary transmission of vCJD through blood
transfusion, fractionated blood products, or contaminated
surgical instruments. For this study mice were injected
intracerebrally, whereas the probable human exposure to
these agents is by peripheral routes (eg, oral or
intravenous), and thus human-to-human exposures
might be significantly less efficient. However, it is difficult
to know for sure what the practical implications might be
in human beings. Peripheral route challenge is in
progress; however, BSE transmission studies in primates
have shown the intravenous route to be as efficient as the
intracerebral route, with an extension of the incubation
time.28
Although all cases of vC]D up to now have been
observed in the MM genotype, this model of human-to-
human vCJD transmission suggests that other genotypes
are also susceptible. In our experimental setting, all
PRNP codon-129 genotypes are susceptible to vCJD
infection; however, progressive development of
pathological TSE features (vacuolation and PrP
deposition) is more rapid in the MM-genotype mice. An
explanation for this finding might be provided by in-vitro
conversion of recombinant human PrP by BSE and vCJD
agents, which has shown that PrP with methionine at
position 129 is more efficiently converted than PrP with
valine, and that conversion by vCJD is significantly more
efficient than by BSE.29 Long incubation periods during
which PrPSc is deposited predicts that, in human beings,
infection could be present in all genotypes for a significant
period before clinical onset. Incubation periods of more
than 30 years have been reported in the human TSE
disease kuru.30
The possibility that an MV or VV genotype could result
in a phenotype distinct from that recognised in vCJD
draws attention to the importance of systematic
assessment of the clinical, genetic, pathological, and
biochemical features of all human prion diseases. Our
findings indicate that for human-to-human vCJD infection
it should be assumed that all codon-129
genotype individuals (not just MM) can be infected, that
long incubation times can occur, and that a significant
level of subclinical disease might be present in the population.
Contributors
MTB, PH, and CP did immunocytochemical and western blot analysis;
JCM, NT, HNB, and LA produced the transgenic mouse lines; JWI
supplied vCJD case material and reviewed the neuropathology; VT did
the mouse inoculations; and MTB, PH, MWH, RGW, JWI, and JCM
prepared the manuscript.
Conflicts of interest
We have no conflicts of interest.
Acknowledgments
snip...
http://www.thelancet.com/journals/laneur/article/PIIS1474442206704136/abstract?isEOP=true
Too late to shut the gate on this killer
May 15, 2006
As "mad cow" disease spreads outward from Britain, a silent epidemic of carriers in humans has begun to emerge. Jennifer Cooke reports
http://www.smh.com.au/news/world/too-late-to-shut-the-gate-on-this-killer/2006/05/14/1147545210765.html
http://www.smh.com.au/news/world/too-late-to-shut-the-gate-on-this-killer/2006/05/14/1147545210765.html?page=2
http://www.smh.com.au/news/world/too-late-to-shut-the-gate-on-this-killer/2006/05/14/1147545210765.html?page=3
http://www.smh.com.au/news/world/too-late-to-shut-the-gate-on-this-killer/2006/05/14/1147545210765.html?page=4
TSS
Flounder just posted a worrisome article on CJD family site which I assume he'll post here shortly. I met with one of the activist family members of the vCJD families while in Scotland, who has become a friend of mine. They are quite concerned about secondary transmission of vCJD via iatrogenic means such as dentistry/surgery/tissue and blood donation now that it has been shown in the past months that MV and VV individuals are in fact susceptible but that their incubation periods are longer or that they remain asymptomatic throughout their lifespans. ....
HERE they are...............terry
SEAC
Position Statement
--------------------------------------------------------------------------------
Position statement vCJD and Endodontic dentistry
Issue
1. The Department of Health (DH) asked SEAC to advise on the findings and implications of a preliminary risk assessment of potential vCJD transmission via endodontic procedures (dental procedures involved in the maintenance of dental pulp and the treatment of the pulp cavity) 1. This is particularly pertinent because of the large number of endodontic procedures undertaken in the UK.
Background
2. There are no reported definite or suspected cases of vCJD transmission arising from dental procedures. However, prions are more resistant than other types of infectious agent to the conventional cleaning and sterilisation practices used to decontaminate dental instruments 2. Therefore, should dental instruments become contaminated from tissues in the oral cavity of infected individuals, there is a risk of transmission to subsequent patients.
3. A quantitative DH risk assessment 3, accepted by SEAC in 2003, considered two possible mechanisms for the transfer of vCJD infectivity via dental instruments: (i) accidental abrasion of the lingual tonsil, known to carry infectivity in vCJD cases; and (ii) contact with dental pulp that evidence from animal studies suggested may be infective. On the basis of the information available, the DH analysis suggested that the risk of transmission to individual patients via accidental abrasion of the lingual tonsil is very low. Furthermore, should dental pulp be infective, the risk of transmission via endodontic procedures, although higher, is also low. Although a very large number of dental procedures are conducted, the relative risk to public health from potential transmission via dental, compared with hospital, surgery was considered to be relatively low.
4. In 2006, SEAC considered a new preliminary risk assessment by DH of the risks of vCJD transmission via endodontic procedures, taking into account new information on decontamination of dental instruments, the potential infectivity of dental pulp, and the possible existence of subclinical vCJD carrier cases.
Endodontic instruments
5. Evidence suggests that the files and reamers used in endodontic procedures are reused and are difficult to reliably decontaminate 4. Appreciable quantities of residual material remain adherent to the surface after normal cleaning and sterilisation 5. Thus, there is potential for transfer of dental pulp between patients undergoing endodontic procedures.
vCJD infectivity in dental tissues
6. There are no data on vCJD infectivity in dental pulp. Although no abnormal prions were found in a study of dental tissues, including dental pulp, from vCJD cases 6, dental pulp includes blood and peripheral nerve tissue known to carry vCJD infectivity 7,8. In addition, appreciable infectivity has been found in the dental pulp of hamsters with hamster scrapie 9. Although it is possible that the peripheral nerve may only become infective close to, or after, the onset of clinical vCJD, inflammation may promote the propagation of prions 10. Thus, although the data are limited and indirect, it is reasonable to assume that the dental pulp of individuals subclinically-infected with vCJD may be infectious although the level of infectivity is unknown. Studies underway will provide direct data on the infectivity in dental tissues from vCJD cases.
Subclinical carrier state
7. A study of humanised mice showed that vCJD infections may not always progress to clinical disease within the normal lifespan of the animals 11. Another study suggested that prion infections in mice that remain at a subclinical level can be transmitted to other mice, resulting in clinical disease 12. Thus, there is evidence to suggest that individuals infected with the BSE / vCJD agent may remain in a subclinical infection carrier state instead of developing vCJD. A discrepancy between prevalence estimates based on a survey of abnormal prion protein in appendix and tonsil tissue and data on vCJD cases supports this hypothesis 13. As no diagnostic test exists to identify such individuals, they could over the course of their lives be potential sources of numerous secondary infections arising from invasive medical or dental procedures.
8. The prevalence of subclinical infection in the UK population is uncertain. A recent estimate suggests the number of subclinical carriers may be of the order of several thousand 14. SEAC has strongly recommended that further studies to ascertain better the prevalence of vCJD infection be urgently considered 15.
Transmission risks
9. The new DH analysis suggests that, on the basis that residual dental pulp on endodontic files and reamers is transferred relatively efficiently to patients on reuse, dental pulp is as infective as peripheral nerve tissue and a subclinical carrier population for vCJD exists, a self-sustaining vCJD epidemic arising from endodontic surgery is plausible. There are uncertainties about the efficiency of vCJD transmission via endodontic procedures, the vCJD infectivity of dental pulp and the existence of a subclinical infection carrier state. However, even if a self-sustaining epidemic were not possible, clusters of vCJD infections could arise from the use of instruments contaminated with the vCJD agent from endodontic procedures on infected patients. Interactions between this and other routes of secondary transmission, such as blood transfusion and hospital surgery, would make a self-sustaining epidemic more likely.
Potential risk reduction measures
10. Endodontic files and reamers have a limited lifespan, restricting the number of possible secondary transmissions. Improving the effectiveness of procedures used to decontaminate dental instruments would reduce the risk of transmission. Restricting endodontic files and reamers to single use would prevent potential secondary transmission via these instruments.
Conclusions
11. A preliminary risk assessment produced by DH suggests that vCJD transmission via endodontic dentistry may, under certain hypothetical but plausible scenarios, be sufficient to sustain a secondary vCJD epidemic. However, there are uncertainties around the data and assumptions underpinning the assessment. Research underway will address some of these uncertainties and allow the risk assessment to be refined. Once the research is complete and / or other data become available, the risks should be reassessed. A watching brief should be maintained.
12. It is unclear whether or not vCJD infectivity can be transmitted via endodontic files and reamers. However, given the plausibility of such a scenario and the large number of procedures undertaken annually, it would be prudent to consider restricting these instruments to single use as a precautionary measure. Since sufficiently rigorous decontamination of these instruments is difficult, single use of these instruments would eliminate this risk, should it exist.
SEAC
May 2006
--------------------------------------------------------------------------------
1. Department of Health. Dentistry and vCJD: the implications of a “carrier state” for a self-sustaining epidemic due to endodontic dentistry. A Preliminary Risk Assessment. Unpublished.
2. Smith et al. (2003) Prions and the oral cavity. J. Dent. Res. 82, 769-775.
3. Department of Health. (2003) Risk assessment for vCJD and dentistry.
4. Letters et al. (2005) A study of visual and blood contamination on reprocessed endodontic files from general dental practice. Br. Dent. J. 199, 522-525.
5. Smith et al. (2005) Residual protein levels on reprocessed dental instruments. J. Hosp. Infect. 61, 237-241.
6. Head et al. (2003) Investigation of PrPres in dental tissues in variant CJD. Br. Dent. J. 195, 339-343.
7. SEAC 91 minutes paragraph 9. www.seac.gov.uk/papers/papers.htm
8. Department of Health (2005) Assessing the risk of vCJD transmission via surgery: an interim view. Unpublished.
9. Ingrosso et al. (1999) Transmission of the 263K scrapie strain by the dental route. J. Gen. Virol. 80, 3043-3047.
10. Heikenwalder et al. (2005) Chronic lymphocytic inflammation specifies the organ tropism of prions. Science. 307, 1107-1110.
11. Bishop et al. (2006) Predicting susceptibility and incubation time of human-to-human transmission of vCJD. Lancet Neurology.
12. Hill et al. (2000) Species-barrier-independent prion replication in apparently resistant species. Proc. Natl. Acad. Sci. USA. 97, 10248-10253.
13. SEAC Epidemiology Subgroup (2005) Position statement on the vCJD epidemic. www.seac.gov.uk/statements/state260106subgroup.htm
14. Clarke & Ghani. (2005) Projections of future course of the primary vCJD epidemic in the UK: inclusion of subclinical infection and the possibility of wider genetic susceptibility. R. J. Soc. Interface.
15. SEAC (2005) SEAC response to the SEAC Epidemiology Subgroup statement on the vCJD epidemic. www.seac.gov.uk/statements/state260106.htm
Page updated: 8th May 2006
http://www.seac.gov.uk/statements/statement0506.htm
Subject: Predicting susceptibility and incubation time of human-to-human transmission of vCJD
Date: March 27, 2006 at 6:24 am PST
Lancet Neurology DOI:10.1016/S1474-4422(06)70413-6
Predicting susceptibility and incubation time of human-to-human transmission of vCJD
MT Bishop a, P Hart b, L Aitchison b, HN Baybutt b, C Plinston b, V Thomson b, NL Tuzi b, MW Head a, JW Ironside a, RG Will a and JC Manson b
Summary
Background
Identification of possible transmission of variant Creutzfeldt-Jakob disease (vCJD) via blood transfusion has caused concern over spread of the disease within the human population. We aimed to model iatrogenic spread to enable a comparison of transmission efficiencies of vCJD and bovine spongiform encephalopathy (BSE) and an assessment of the effect of the codon-129 polymorphism on human susceptibility.
Methods
Mice were produced to express human or bovine prion protein (PrP) by direct replacement of the mouse PrP gene. Since the human PrP gene has variation at codon 129, with MM, VV, and MV genotypes, three inbred lines with an identical genetic background were produced to express human PrP with the codon-129 MM, MV, and VV genotypes. Mice were inoculated with BSE or vCJD and assessed for clinical and pathological signs of disease.
Findings
BSE was transmitted to the bovine line but did not transmit to the human lines. By contrast, vCJD was transmitted to all three human lines with different pathological characteristics for each genotype and a gradation of transmission efficiency from MM to MV to VV.
Interpretation
Transmission of BSE to human beings is probably restricted by the presence of a significant species barrier. However, there seems to be a substantially reduced barrier for human-to-human transmission of vCJD. Moreover, all individuals, irrespective of codon-129 genotype, could be susceptible to secondary transmission of vCJD through routes such as blood transfusion. A lengthy preclinical disease is predicted by these models, which may represent a risk for further disease transmission and thus a significant public-health issue.
Affiliations
a. National CJD Surveillance Unit, Bryan Matthews Building, Western General Hospital, Edinburgh, UK
b. Institute for Animal Health, Neuropathogenesis Unit, King's Buildings, Edinburgh, UK
Correspondence to: Prof J C Manson, Institute for Animal Health, Neuropathogenesis Unit, Ogston Building, King's Buildings, West Mains Road, Edinburgh EH9 3JF, UK
SNIP...
further into this study;
SNIP...
Discussion
Although the cattle BSE epidemic in the UK has
amounted to more than 180 000 cases since the 1980s,
the extent of the human vCJD epidemic has so far
remained limited with the total number of cases
worldwide currently at 190. One explanation for this
apparent discrepancy is that there exists a significant
species barrier between cattle and human beings, which
limits the susceptibility of the human population to
BSE. The data shown here suggest that this could
indeed be the case since BSE was readily transmissible
to the bovine transgenic mice but not to the human
transgenic mice. However, once BSE has passed
through human beings in the form of vCJD, the
transmissibility of this TSE strain is altered for the
human population.
All the human transgenic lines inoculated with BSE
were negative for TSE transmission, which suggests that
either the human transgenic lines are relatively resistant
to transmission of BSE or the incubation time is longer
than the length of the experiment (approximately
700 days). BSE transmission previously observed by
others, in human transgenic lines overexpressing the
human prion protein, could be due to overexpression of
the PrP gene and may not therefore give a true reflection
of the species barrier between BSE and human
beings.15,25,26 This apparent resistance of human transgenic
mice to BSE could be explained by a large species barrier
and this in turn could explain the low number of vCJD
cases in the human population.
vCJD was transmitted to all three human lines with
different pathological characteristics for each genotype,
and a gradation of transmission efficiency from MM to
MV to VV. The greater transmission efficiency in HuMM
mice suggests that homozygosity for methionine at
codon 129 leads to earlier onset of TSE-related
pathological features and clinical disease than for the
other two genotypes. The differences in PrPSc deposition
in the HuMM and HuMV lines suggest that the codon-
129 polymorphism in human beings is likely to affect
the distribution of PrPSc deposition in the brain.
Moreover, the similar numbers that scored positive for
PrP deposition in each of the MM and MV groups (11/15
and 11/13 respectively) suggest that the two genotypes
might be equally susceptible to vCJD, but with different
incubation periods. Titration experiments are needed to
fully compare the susceptibility of each line. The single
HuVV mouse positive for PrPSc shows that VV
individuals may be susceptible to vCJD with very long
incubation times, including a lengthy subclinical phase.
Transmission studies from all three genotype mice are
now underway to examine the infectious nature of the
disease and determine any alterations in the strain
characteristics on passage through human transgenic
mice. By contrast with published data suggesting that
VV individuals cannot propagate the vCJD biochemical
phenotype,15 the data presented here suggest that the
PrPSc type will remain a useful diagnostic feature of
secondary vCJD infection irrespective of codon-129
genotype, as has been observed for the two extant cases
of transfusion-associated vCJD infection. 5,27
Transmission of vCJD to the three lines of human
transgenic mice indicates that the human population
could be at significantly heightened risk of developing
disease after iatrogenic exposure to vCJD. Secondary
transmission of vCJD has partly removed the cattle-to-
human species barrier and has resulted in an agent that
can be transmitted from human to human with relative
efficiency. Transmission studies in cynomolgus macaques
provide further evidence for this agent adaptation as they
show reduction in incubation times after serial passage
of BSE.28 Our BSE inoculation at 10-1 dilution was
compared with vCJD inoculation at 10-2 because the latter
inoculum was found to be toxic to the mice at 10-1. Use of
a higher dose ofvCJD inoculum would have maintained
or increased the transmission efficiency of vCJD and
enhanced the current findings.
Our findings raise concerns relevant to the possibility
of secondary transmission of vCJD through blood
transfusion, fractionated blood products, or contaminated
surgical instruments. For this study mice were injected
intracerebrally, whereas the probable human exposure to
these agents is by peripheral routes (eg, oral or
intravenous), and thus human-to-human exposures
might be significantly less efficient. However, it is difficult
to know for sure what the practical implications might be
in human beings. Peripheral route challenge is in
progress; however, BSE transmission studies in primates
have shown the intravenous route to be as efficient as the
intracerebral route, with an extension of the incubation
time.28
Although all cases of vC]D up to now have been
observed in the MM genotype, this model of human-to-
human vCJD transmission suggests that other genotypes
are also susceptible. In our experimental setting, all
PRNP codon-129 genotypes are susceptible to vCJD
infection; however, progressive development of
pathological TSE features (vacuolation and PrP
deposition) is more rapid in the MM-genotype mice. An
explanation for this finding might be provided by in-vitro
conversion of recombinant human PrP by BSE and vCJD
agents, which has shown that PrP with methionine at
position 129 is more efficiently converted than PrP with
valine, and that conversion by vCJD is significantly more
efficient than by BSE.29 Long incubation periods during
which PrPSc is deposited predicts that, in human beings,
infection could be present in all genotypes for a significant
period before clinical onset. Incubation periods of more
than 30 years have been reported in the human TSE
disease kuru.30
The possibility that an MV or VV genotype could result
in a phenotype distinct from that recognised in vCJD
draws attention to the importance of systematic
assessment of the clinical, genetic, pathological, and
biochemical features of all human prion diseases. Our
findings indicate that for human-to-human vCJD infection
it should be assumed that all codon-129
genotype individuals (not just MM) can be infected, that
long incubation times can occur, and that a significant
level of subclinical disease might be present in the population.
Contributors
MTB, PH, and CP did immunocytochemical and western blot analysis;
JCM, NT, HNB, and LA produced the transgenic mouse lines; JWI
supplied vCJD case material and reviewed the neuropathology; VT did
the mouse inoculations; and MTB, PH, MWH, RGW, JWI, and JCM
prepared the manuscript.
Conflicts of interest
We have no conflicts of interest.
Acknowledgments
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http://www.thelancet.com/journals/laneur/article/PIIS1474442206704136/abstract?isEOP=true
Too late to shut the gate on this killer
May 15, 2006
As "mad cow" disease spreads outward from Britain, a silent epidemic of carriers in humans has begun to emerge. Jennifer Cooke reports
http://www.smh.com.au/news/world/too-late-to-shut-the-gate-on-this-killer/2006/05/14/1147545210765.html
http://www.smh.com.au/news/world/too-late-to-shut-the-gate-on-this-killer/2006/05/14/1147545210765.html?page=2
http://www.smh.com.au/news/world/too-late-to-shut-the-gate-on-this-killer/2006/05/14/1147545210765.html?page=3
http://www.smh.com.au/news/world/too-late-to-shut-the-gate-on-this-killer/2006/05/14/1147545210765.html?page=4
TSS