Subject: SEAC Position statement on TSE infectivity in blood
Date: August 2, 2006 at 6:37 am PST
SEAC
Position Statement
--------------------------------------------------------------------------------
Position statement on TSE infectivity in blood
Issue
1. 1. The UK blood services and Department of Health (DH) asked SEAC to consider data on the nature of transmissible spongiform encephalopathy (TSE) infectivity in blood and the implications for transmission of variant Creutzfeldt-Jakob disease (vCJD) via transfusion of blood products. The committee considered four specific issues:
(i) the level of TSE infectivity in whole blood and the distribution of infectivity amongst individual components of blood,
(ii) the change in the level of TSE infectivity in blood over the course of the incubation period of disease,
(iii) the relative efficiencies of the intracranial (ic) and intravenous (iv) routes of inoculation,
(iv) the dose-response relationship for TSE infection.
Background
2. Blood has been shown to carry TSE infectivity in a number of different animal models 1-4. Three cases of probable vCJD transmission via transfusion of non-leucodepleted red blood cells provide strong evidence that blood from humans infected with vCJD can carry the infectious agent during the pre-clinical stage of the disease 5-7.
3. Precautionary measures, including leucodepletion and importation of fresh frozen plasma for children, have been implemented by the UK blood services to reduce the risk of vCJD transmission via blood transfusion. Additional blood processing technologies that may further reduce transmission risks are under consideration. Assessment of the potential effectiveness of new technologies relies on assumptions about the nature of the infectivity in blood, particularly the level and distribution of vCJD infectivity in blood components. However, there is much uncertainty about the nature, level and distribution of infectivity in blood. An assessment produced by Det Norske Veritas Consulting (DNV) and reviewed by SEAC provides a working model for the level of vCJD infectivity in blood and the distribution of infectivity between blood components 8.
4. At SEAC 92, SEAC reassessed some of the assumptions in the DNV risk assessment by consideration of recent published literature and unpublished data presented by a number of researchers 9-13. Many of the available data were derived from animal studies that have used prion strains, inocula and routes of administration that may not be directly applicable to the human blood transfusion situation. Most of the data are from studies of infectivity in hamster blood infected with hamster scrapie 3,10,13,14 and mice infected with mouse adapted vCJD 2. Many of the hamster studies have not yet been published and therefore, have not been subject to the usual peer review process. Extrapolation of data from studies of hamster scrapie to vCJD is complicated by differences in the pathogenesis of these diseases, particularly the low level of lymphoreticular system (LRS) involvement in the pathogenesis of hamster scrapie in contrast to vCJD. Limited data, that may be more relevant, are available from ongoing studies of the infectivity in the blood of sheep experimentally infected with BSE or scrapie as the pathogenesis of these diseases involve the LRS in this model 11.
Level of TSE infectivity in whole blood
5. The levels of infectivity reported in rodent studies to examine the infectivity in blood of animals with TSEs vary widely, ranging from about one to 300 infectious doses*(ID)/mL of blood. One large unpublished study 10 involving a series of experiments to measure the infectivity in samples of pooled blood from large groups of hamsters with hamster scrapie suggests a mean level of infectivity of around 10 ID/mL of blood (range of two to 24 ID/mL of blood). In a published study, levels of mouse adapted vCJD infectivity within this lower range were found in blood components from mice at late pre-clinical or clinical stages of infection 2. There are no data on the infectivity in the blood of humans with vCJD to assess the relevance of these data to humans.
Origin of blood infectivity
6. The source of infectivity in blood is not understood. Unpublished comparisons of the infectivity in blood from intact and splenectomised hamsters suggest that the spleen is not the source of infectivity in blood 10. Unpublished comparisons 10 of the rate of increase of infectivity in pooled blood and brain from infected hamsters during the incubation period of hamster scrapie suggest that it is not the result of leakage from the central nervous system (CNS) into the blood supply 10. However, a single published study that measured abnormal prion protein (PrPSc) in the buffy coat (white blood cells and platelets) from single hamsters infected with hamster scrapie suggests that PrPSc concentrations in blood are bimodal with a peak in the pre-clinical phase from peripheral replication in the spleen and other lymphoid tissues, followed by a larger rise in PrPSc concentrations leading into the clinical stage of the disease from leakage from the CNS 13.
Distribution of infectivity in blood components
7. Published and unpublished data from studies of the infectivity in components of blood from hamsters with hamster scrapie show that around one half of the infectivity in blood can be removed by depleting blood of white blood cells 3, and that the infectivity associated with the white blood cells can be substantially depleted by extensive washing 10. In addition, infectivity is not, or is minimally, associated with platelets 14 or red blood cells 10. These data suggest, at least in this model of TSE infection, that infectivity may be distributed equally between plasma and white blood cells but is weakly bound to white blood cells. Data from published experiments to measure mouse adapted vCJD infectivity in components of blood taken at the late pre-clinical or clinical stages of disease also suggest that infectivity is principally associated with plasma and white blood cells, minimally associated with red blood cells but that there may be some association with platelets 2. The buffy coat from sheep with scrapie or BSE has also been shown to transmit infection by transfusion to healthy recipient sheep 1,11. It is possible that there are inter-species and inter-strain differences in the distribution of TSE infectivity in blood components. Therefore, additional research to examine the infectivity in blood components, particularly from models using TSE strains closely related to vCJD, will allow assessment of the relevance of these data to humans infected with vCJD.
Change in infectivity during the incubation period
8. A number of studies in animals have examined the level of infectivity in blood during both the pre-clinical and clinical stages of TSE infection 2,9,11. An unpublished study 10 examined the infectivity in the blood of hamsters after ic inoculation with hamster scrapie at a number of time points during the preclinical stage of infection. Infectivity was first detected at the mid-point of the incubation period with the level of infectivity increasing linearly towards the clinical stage of infection. Extrapolation of these data suggests infectivity may first appear in blood at around a third of the way into the incubation period. Similar findings were obtained when the experiment was repeated using oral inoculation. Although the relationship between PrPSc and infectivity is unclear, PrPSc concentrations in the blood of hamsters infected with hamster scrapie show a bimodal profile (as described in paragraph 7) 13. Studies of mouse adapted vCJD 2,9 and sheep infected with scrapie or BSE 1,11 only examined the level of infectivity at one point during the preclinical stage of infection but show that blood is infectious during the second half of the incubation period. Two cases of probable blood transfusion associated transmission of vCJD from blood donors 20 months 5 and 3.5 years 7 prior to the onset of disease have been identified, indicating that human blood can be infectious in the preclinical phase. More extensive data, particularly from models using TSE strains closely related to vCJD, will inform on the relevance of the findings in the hamster scrapie model to changes in infectivity in the incubation period of vCJD in humans.
Relative efficiency of the ic and iv routes of transmission
9. The efficiency of transmission varies depending on the route of administration, host, TSE strain, source of inoculum and how it is prepared. Most measurements of TSE infectivity are derived from bioassays using the ic route of administration. Since the efficiencies of the ic and iv routes of transmission may not be equivalent, the infectivity of an inoculum measured by the ic route may not reflect the infectivity of the same inoculum administered by the iv route, this latter route being the most relevant to blood transfusion. A small number of studies using different animal models have compared the infectivity of brain homogenate or purified blood components administered by the ic or iv routes. These studies suggest that the efficiency of transmission by the iv route is between 10% and 100% of the efficiency of ic route 2,4,9,14,15. One unpublished study 10, comparing the efficiency of iv transfusion of intact whole blood and ic inoculation of sonicated whole blood from hamsters with hamster scrapie showed the iv route to be considerably less efficient than suggested by this range. In addition, a published study 2 showed that mouse adapted vCJD could be transmitted equally efficiently from inoculation of buffy coat from infected animals by the ic or iv routes but the transmission efficiency from inoculation of plasma was lower by the iv compared with the ic route. These studies suggest that the form of inoculum may strongly influence the relative transmission efficiencies of inoculation by different routes.
Dose-response relationship
10. Evidence from animal studies 10 suggests that TSE infectivity is quantal in nature. An infectious dose diluted by distribution to a number of individuals reduces the risk of transmission to an individual. However, at the population level, one of the exposed individuals would be still be expected to become infected. Higher doses split between individuals would lead to more than one infection. The implication of this relationship between dose and probability of infection for strategies to reduce the public health risks in relation to blood transfusion is that pooling blood to dilute infectivity does not decrease the risks to public health. Indeed, depending on the dose, pooling is likely to increase the risks to public health. Strategies to remove or inactivate infectivity in blood would reduce the risks of transmission to both the individual and at the population level.
Conclusions
11. The available data show that blood is infectious during the preclinical stage of vCJD. Although the precise time in the incubation period of vCJD at which blood becomes infectious is unclear, data from animal models suggests it may be infectious from at least, if not before, the middle of the incubation period. The source of infectivity in blood is not understood. Data from rodent studies suggests that infectivity in whole blood is around 10 ID/mL and that it mostly resides in the plasma and white blood cell components with infectivity associated with white blood cells substantially depleted by extensive washing. However, additional information from other animal models is required to assess whether these findings may be closely representative of vCJD infectivity in human blood. It is clear that an infectious dose in blood can be disseminated but not diluted by distribution to a large number of recipients. Consequently, pooling of potentially infectious material, or in other ways disseminating infectious material between a number of recipients, will not reduce the number of people infected, and is likely to increase the number of people infected.
SEAC
July 2006
--------------------------------------------------------------------------------
References
1. Hunter et al. (2002) Transmission of prion diseases by blood transfusion. J. Gen. Virol. 83, 2897-2905.
2. Cervenakova et al. (2003) Similar levels of infectivity in the blood of mice infected with human-derived vCJD and GSS strains of transmissible spongiform encephalopathy. Transfusion. 43, 1687-1694.
3. Gregori et al. (2004) Effectiveness of leucodepletion for removal of infectivity of transmissible spongiform encephalopathies from blood. Lancet. 364, 529-531.
4. Hertzog et al. (2004) Tissue distribution of bovine spongiform encephalopathy agent in primates after intravenous or oral infection. Lancet. 363, 422-428.
5. Health protection Agency (2006): New case of variant CJD associated with blood transfusion.
6. Peden et al. (2004) Preclinical vCJD after blood transfusion in a PRNP codon 129 heterozygous patient. Lancet. 364, 527-529
7. Llewelyn et al. (2004) Possible transmission of variant Creutzfeldt-Jakob disease by blood transfusion. Lancet. 363, 411-412.
8. DNV Consulting (2003) Risk assessment of exposure to vCJD infectivity in blood and blood products.
9. Unpublished data from the Jerome H. Holland Laboratory for Biomedical Sciences, American Red Cross, Maryland, USA presented by Dr L Cervenakova.
10. Unpublished data from the VA Medical Center, University of Maryland, Baltimore, USA presented by Dr R Rohwer.
11. Unpublished data from the Institute for Animal Health, Compton, Berkshire, UK presented by Professor J. Manson.
12. Unpublished data from the Laboratory for Prion Pathogenesis, Atomic Energy Commission, Service de Neurovirologie, Cedex, France presented by Professor C Lasmézas.
13. Saá et al. (2006) Presymptomatic detection of prions in blood. Science. 313, 92-94.
14. Holada et al. (2002) Scrapie infectivity in hamster blood is not associated with platelets. J. Virol. 76, 4649-4650.
15. Kimberlin (1991) An overview of bovine spongiform encephalopathy. Dev. Biol. Stand. 75, 75-82.
16. Brown et al. (1998) The distribution of infectivity in blood components and plasma derivatives in experimental models of transmissible spongiform encephalopathy. Transfusion 38, 810-816.
Page updated: 1st August 2006
http://www.seac.gov.uk/statements/statement0806.htm
P.S. hey there timh, like muscle, they once thought too that blood would not transmit, until it did. amazing what you can find out when you pull your head out of your .......................out of the sand..........TSS
Date: August 2, 2006 at 6:37 am PST
SEAC
Position Statement
--------------------------------------------------------------------------------
Position statement on TSE infectivity in blood
Issue
1. 1. The UK blood services and Department of Health (DH) asked SEAC to consider data on the nature of transmissible spongiform encephalopathy (TSE) infectivity in blood and the implications for transmission of variant Creutzfeldt-Jakob disease (vCJD) via transfusion of blood products. The committee considered four specific issues:
(i) the level of TSE infectivity in whole blood and the distribution of infectivity amongst individual components of blood,
(ii) the change in the level of TSE infectivity in blood over the course of the incubation period of disease,
(iii) the relative efficiencies of the intracranial (ic) and intravenous (iv) routes of inoculation,
(iv) the dose-response relationship for TSE infection.
Background
2. Blood has been shown to carry TSE infectivity in a number of different animal models 1-4. Three cases of probable vCJD transmission via transfusion of non-leucodepleted red blood cells provide strong evidence that blood from humans infected with vCJD can carry the infectious agent during the pre-clinical stage of the disease 5-7.
3. Precautionary measures, including leucodepletion and importation of fresh frozen plasma for children, have been implemented by the UK blood services to reduce the risk of vCJD transmission via blood transfusion. Additional blood processing technologies that may further reduce transmission risks are under consideration. Assessment of the potential effectiveness of new technologies relies on assumptions about the nature of the infectivity in blood, particularly the level and distribution of vCJD infectivity in blood components. However, there is much uncertainty about the nature, level and distribution of infectivity in blood. An assessment produced by Det Norske Veritas Consulting (DNV) and reviewed by SEAC provides a working model for the level of vCJD infectivity in blood and the distribution of infectivity between blood components 8.
4. At SEAC 92, SEAC reassessed some of the assumptions in the DNV risk assessment by consideration of recent published literature and unpublished data presented by a number of researchers 9-13. Many of the available data were derived from animal studies that have used prion strains, inocula and routes of administration that may not be directly applicable to the human blood transfusion situation. Most of the data are from studies of infectivity in hamster blood infected with hamster scrapie 3,10,13,14 and mice infected with mouse adapted vCJD 2. Many of the hamster studies have not yet been published and therefore, have not been subject to the usual peer review process. Extrapolation of data from studies of hamster scrapie to vCJD is complicated by differences in the pathogenesis of these diseases, particularly the low level of lymphoreticular system (LRS) involvement in the pathogenesis of hamster scrapie in contrast to vCJD. Limited data, that may be more relevant, are available from ongoing studies of the infectivity in the blood of sheep experimentally infected with BSE or scrapie as the pathogenesis of these diseases involve the LRS in this model 11.
Level of TSE infectivity in whole blood
5. The levels of infectivity reported in rodent studies to examine the infectivity in blood of animals with TSEs vary widely, ranging from about one to 300 infectious doses*(ID)/mL of blood. One large unpublished study 10 involving a series of experiments to measure the infectivity in samples of pooled blood from large groups of hamsters with hamster scrapie suggests a mean level of infectivity of around 10 ID/mL of blood (range of two to 24 ID/mL of blood). In a published study, levels of mouse adapted vCJD infectivity within this lower range were found in blood components from mice at late pre-clinical or clinical stages of infection 2. There are no data on the infectivity in the blood of humans with vCJD to assess the relevance of these data to humans.
Origin of blood infectivity
6. The source of infectivity in blood is not understood. Unpublished comparisons of the infectivity in blood from intact and splenectomised hamsters suggest that the spleen is not the source of infectivity in blood 10. Unpublished comparisons 10 of the rate of increase of infectivity in pooled blood and brain from infected hamsters during the incubation period of hamster scrapie suggest that it is not the result of leakage from the central nervous system (CNS) into the blood supply 10. However, a single published study that measured abnormal prion protein (PrPSc) in the buffy coat (white blood cells and platelets) from single hamsters infected with hamster scrapie suggests that PrPSc concentrations in blood are bimodal with a peak in the pre-clinical phase from peripheral replication in the spleen and other lymphoid tissues, followed by a larger rise in PrPSc concentrations leading into the clinical stage of the disease from leakage from the CNS 13.
Distribution of infectivity in blood components
7. Published and unpublished data from studies of the infectivity in components of blood from hamsters with hamster scrapie show that around one half of the infectivity in blood can be removed by depleting blood of white blood cells 3, and that the infectivity associated with the white blood cells can be substantially depleted by extensive washing 10. In addition, infectivity is not, or is minimally, associated with platelets 14 or red blood cells 10. These data suggest, at least in this model of TSE infection, that infectivity may be distributed equally between plasma and white blood cells but is weakly bound to white blood cells. Data from published experiments to measure mouse adapted vCJD infectivity in components of blood taken at the late pre-clinical or clinical stages of disease also suggest that infectivity is principally associated with plasma and white blood cells, minimally associated with red blood cells but that there may be some association with platelets 2. The buffy coat from sheep with scrapie or BSE has also been shown to transmit infection by transfusion to healthy recipient sheep 1,11. It is possible that there are inter-species and inter-strain differences in the distribution of TSE infectivity in blood components. Therefore, additional research to examine the infectivity in blood components, particularly from models using TSE strains closely related to vCJD, will allow assessment of the relevance of these data to humans infected with vCJD.
Change in infectivity during the incubation period
8. A number of studies in animals have examined the level of infectivity in blood during both the pre-clinical and clinical stages of TSE infection 2,9,11. An unpublished study 10 examined the infectivity in the blood of hamsters after ic inoculation with hamster scrapie at a number of time points during the preclinical stage of infection. Infectivity was first detected at the mid-point of the incubation period with the level of infectivity increasing linearly towards the clinical stage of infection. Extrapolation of these data suggests infectivity may first appear in blood at around a third of the way into the incubation period. Similar findings were obtained when the experiment was repeated using oral inoculation. Although the relationship between PrPSc and infectivity is unclear, PrPSc concentrations in the blood of hamsters infected with hamster scrapie show a bimodal profile (as described in paragraph 7) 13. Studies of mouse adapted vCJD 2,9 and sheep infected with scrapie or BSE 1,11 only examined the level of infectivity at one point during the preclinical stage of infection but show that blood is infectious during the second half of the incubation period. Two cases of probable blood transfusion associated transmission of vCJD from blood donors 20 months 5 and 3.5 years 7 prior to the onset of disease have been identified, indicating that human blood can be infectious in the preclinical phase. More extensive data, particularly from models using TSE strains closely related to vCJD, will inform on the relevance of the findings in the hamster scrapie model to changes in infectivity in the incubation period of vCJD in humans.
Relative efficiency of the ic and iv routes of transmission
9. The efficiency of transmission varies depending on the route of administration, host, TSE strain, source of inoculum and how it is prepared. Most measurements of TSE infectivity are derived from bioassays using the ic route of administration. Since the efficiencies of the ic and iv routes of transmission may not be equivalent, the infectivity of an inoculum measured by the ic route may not reflect the infectivity of the same inoculum administered by the iv route, this latter route being the most relevant to blood transfusion. A small number of studies using different animal models have compared the infectivity of brain homogenate or purified blood components administered by the ic or iv routes. These studies suggest that the efficiency of transmission by the iv route is between 10% and 100% of the efficiency of ic route 2,4,9,14,15. One unpublished study 10, comparing the efficiency of iv transfusion of intact whole blood and ic inoculation of sonicated whole blood from hamsters with hamster scrapie showed the iv route to be considerably less efficient than suggested by this range. In addition, a published study 2 showed that mouse adapted vCJD could be transmitted equally efficiently from inoculation of buffy coat from infected animals by the ic or iv routes but the transmission efficiency from inoculation of plasma was lower by the iv compared with the ic route. These studies suggest that the form of inoculum may strongly influence the relative transmission efficiencies of inoculation by different routes.
Dose-response relationship
10. Evidence from animal studies 10 suggests that TSE infectivity is quantal in nature. An infectious dose diluted by distribution to a number of individuals reduces the risk of transmission to an individual. However, at the population level, one of the exposed individuals would be still be expected to become infected. Higher doses split between individuals would lead to more than one infection. The implication of this relationship between dose and probability of infection for strategies to reduce the public health risks in relation to blood transfusion is that pooling blood to dilute infectivity does not decrease the risks to public health. Indeed, depending on the dose, pooling is likely to increase the risks to public health. Strategies to remove or inactivate infectivity in blood would reduce the risks of transmission to both the individual and at the population level.
Conclusions
11. The available data show that blood is infectious during the preclinical stage of vCJD. Although the precise time in the incubation period of vCJD at which blood becomes infectious is unclear, data from animal models suggests it may be infectious from at least, if not before, the middle of the incubation period. The source of infectivity in blood is not understood. Data from rodent studies suggests that infectivity in whole blood is around 10 ID/mL and that it mostly resides in the plasma and white blood cell components with infectivity associated with white blood cells substantially depleted by extensive washing. However, additional information from other animal models is required to assess whether these findings may be closely representative of vCJD infectivity in human blood. It is clear that an infectious dose in blood can be disseminated but not diluted by distribution to a large number of recipients. Consequently, pooling of potentially infectious material, or in other ways disseminating infectious material between a number of recipients, will not reduce the number of people infected, and is likely to increase the number of people infected.
SEAC
July 2006
--------------------------------------------------------------------------------
References
1. Hunter et al. (2002) Transmission of prion diseases by blood transfusion. J. Gen. Virol. 83, 2897-2905.
2. Cervenakova et al. (2003) Similar levels of infectivity in the blood of mice infected with human-derived vCJD and GSS strains of transmissible spongiform encephalopathy. Transfusion. 43, 1687-1694.
3. Gregori et al. (2004) Effectiveness of leucodepletion for removal of infectivity of transmissible spongiform encephalopathies from blood. Lancet. 364, 529-531.
4. Hertzog et al. (2004) Tissue distribution of bovine spongiform encephalopathy agent in primates after intravenous or oral infection. Lancet. 363, 422-428.
5. Health protection Agency (2006): New case of variant CJD associated with blood transfusion.
6. Peden et al. (2004) Preclinical vCJD after blood transfusion in a PRNP codon 129 heterozygous patient. Lancet. 364, 527-529
7. Llewelyn et al. (2004) Possible transmission of variant Creutzfeldt-Jakob disease by blood transfusion. Lancet. 363, 411-412.
8. DNV Consulting (2003) Risk assessment of exposure to vCJD infectivity in blood and blood products.
9. Unpublished data from the Jerome H. Holland Laboratory for Biomedical Sciences, American Red Cross, Maryland, USA presented by Dr L Cervenakova.
10. Unpublished data from the VA Medical Center, University of Maryland, Baltimore, USA presented by Dr R Rohwer.
11. Unpublished data from the Institute for Animal Health, Compton, Berkshire, UK presented by Professor J. Manson.
12. Unpublished data from the Laboratory for Prion Pathogenesis, Atomic Energy Commission, Service de Neurovirologie, Cedex, France presented by Professor C Lasmézas.
13. Saá et al. (2006) Presymptomatic detection of prions in blood. Science. 313, 92-94.
14. Holada et al. (2002) Scrapie infectivity in hamster blood is not associated with platelets. J. Virol. 76, 4649-4650.
15. Kimberlin (1991) An overview of bovine spongiform encephalopathy. Dev. Biol. Stand. 75, 75-82.
16. Brown et al. (1998) The distribution of infectivity in blood components and plasma derivatives in experimental models of transmissible spongiform encephalopathy. Transfusion 38, 810-816.
Page updated: 1st August 2006
http://www.seac.gov.uk/statements/statement0806.htm
P.S. hey there timh, like muscle, they once thought too that blood would not transmit, until it did. amazing what you can find out when you pull your head out of your .......................out of the sand..........TSS