Wednesday, August 24, 2011

There Is No Safe Dose of Prions

There Is No Safe Dose of Prions

Helen R. Fryer, Angela R. McLean*

The Institute for Emerging Infections, Oxford Martin School, Department of Zoology, Oxford University, Oxford, United Kingdom


Understanding the circumstances under which exposure to transmissible spongiform encephalopathies (TSEs) leads to infection is important for managing risks to public health. Based upon ideas in toxicology and radiology, it is plausible that exposure to harmful agents, including TSEs, is completely safe if the dose is low enough. However, the existence of a threshold, below which infection probability is zero has never been demonstrated experimentally. Here we explore this question by combining data and mathematical models that describe scrapie infections in mice following experimental challenge over a broad range of doses. We analyse data from 4338 mice inoculated at doses ranging over ten orders of magnitude. These data are compared to results from a within-host model in which prions accumulate according to a stochastic birth-death process. Crucially, this model assumes no threshold on the dose required for infection. Our data reveal that infection is possible at the very low dose of a 1000 fold dilution of the dose that infects half the challenged animals (ID50). Furthermore, the dose response curve closely matches that predicted by the model. These findings imply that there is no safe dose of prions and that assessments of the risk from low dose exposure are right to assume a linear relationship between dose and probability of infection. We also refine two common perceptions about TSE incubation periods: that their mean values decrease linearly with logarithmic decreases in dose and that they are highly reproducible between hosts. The model and data both show that the linear decrease in incubation period holds only for doses above the ID50. Furthermore, variability in incubation periods is greater than predicted by the model, not smaller. This result poses new questions about the sources of variability in prion incubation periods. It also provides insight into the limitations of the incubation period assay.

Citation: Fryer HR, McLean AR (2011) There Is No Safe Dose of Prions. PLoS ONE 6(8): e23664. doi:10.1371/journal.pone.0023664

Editor: Noriyuki Nishida, Nagasaki University Graduate School of Biomedical Sciences, Japan

Received May 6, 2011; Accepted July 21, 2011; Published August 15, 2011

Copyright: 2011 Fryer, McLean. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This research was supported by the Oxford Martin School ( and the Foods Standards Agency (, grant number M03027. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.

* E-mail:


Discussion In this study we first asked whether there exists a threshold dose of prions below which the probability of infection is zero. By comparing the scrapie dose-response curve observed in mice to model predictions we found no evidence that such a threshold exists. As the stochastic-birth death model predicts, the probability of infection simply becomes smaller as the dose decreases. Furthermore, we find evidence to support the assumption of a linear relationship between dose and probability of infection in assessing the risk from low dose exposure. Use of a linear relation for doses above the ID50 will lead to overestimation of the risk. Although we find no evidence of a threshold, it must be emphasized that we cannot rule out this possibility. Though it was to be expected because of the limited sample size, no mice were infected at the lowest relative dose tested (–4), therefore a threshold may exist at this dose or lower. However, acquiring data to investigate this question further would require an unfeasibly large number of test animals. Furthermore, the observation of infection at a dose 1000 times more dilute than the ID50 shows that infection is still possible at very low doses. In practical terms this is low enough to regard there to be no safe dose.

Previous modelling work has focussed on understanding the molecular form of a prion and its mechanism of replication. We are not proposing a specific form or replication mechanism, rather we ask whether data on infection probabilities are consistent with the simplest model of replication that assumes no threshold dose. If evidence did emerge of a threshold dose, one could tie it to the hypothesis that the smallest infectious agent involved in TSEs (a prion) is a polymer consisting of multiple PrPSc monomers above a critical polymer length [4-6] . It is thought that such a polymer replicates when it breaks into two or more polymers, each larger than the critical length, before undergoing rapid monomer addition. This is currently the most widely accepted mechanism of prion replication. However, it is noteworthy that a threshold polymer length is not the same as a threshold dose of prions, since a prion is normally regarded as one infectious agent (i.e. a polymer), not one PrPSc monomer. Dilution of infectious substrate would not necessarily split up prion polymers into units lower than the critical polymer length, thus, the polymer breakage-addition model of prion replication is consistent with our finding of no threshold for infectiousness.

In this study we also asked how TSE incubation periods change according to the inoculating dose. First, we asked whether it is true that mean incubation period decreases linearly with logarithmic decrease in dose. The model predicts that this relationship holds only at doses higher than the ID50. For doses below the ID50, mean incubation period is predicted to be invariant to dose. The murine data are consistent with these predictions. This finding delineates the situation in which there is a linear relationship between dose and provides insight into the limitations of the incubation period assay. Specifically, it suggests that the incubation period assay should be unable to distinguish between different doses below the ID50.

Second, we investigated variability in incubation periods and asked whether TSE incubation periods are highly reproducible. We revealed that they are markedly more variable than predicted by the model. Our findings lead us to question why this is so, especially given that the dose response curve and the mean incubation periods are in close agreement with the model. Could the mechanism of prion growth be incorrect or does the inconsistency lie with data collection or the relationship between prion numbers and clinical symptoms? The most popular model for prion growth, based upon polymer breakage and expansion, is underpinned by exponential growth dynamics and would predict the same variability as seen here. That model therefore also cannot explain the effect that we see. In regards to data collection, some variability is likely to arise from the difficulty in spotting symptoms and from small experimental variability in the dose of the inoculum. It must also be noted that some inaccuracies in the data could arise because the duration of the experiments was finite. At the end of each experiment all surviving mice were culled and examined for the presence of pathological lesion without symptoms might have progressed to disease if the experiments had run for longer. However this does not explain the high variability in incubations periods as if the data were not censored in this way, such unusually long incubation periods would only increase variability, not reduce it.

It is not difficult to propose ways in which prion infection is more complex than a stochastic birth-death process in a homogeneous environment. All the heterogeneities of the in vivo situation: spatial, tissue, temporal and genetic [35] could add to the variability in incubation periods. It is indeed remarkable that such a simple model so clearly reproduces the infection probability and mean incubation period across thousands of mice challenged under such a range of experimental conditions. The intellectual challenge posed by this analysis is to understand what processes are driving the observed variability in incubation periods whilst conserving the infection probabilities and mean incubation periods that are so well explained by the simple model presented here.

Wednesday, August 24, 2011

All Clinically-Relevant Blood Components Transmit Prion Disease following a Single Blood Transfusion: A Sheep Model of vCJD

Wednesday, June 29, 2011

TSEAC Meeting August 1, 2011 donor deferral Saudi Arabia vCJD risk blood and blood products


Transmission Results in Squirrel Monkeys Inoculated with Human sCJD, vCJD, and GSS Blood Specimens: the Baxter Study

Brown, P1; Gibson, S2; Williams, L3; Ironside, J4; Will, R4; Kreil, T5; Abee, C3 1Fondation Alliance BioSecure, France; 2University of South Alabama, USA; 3University of Texas MD Anderson Cancer Center, USA; 4Western General Hospital, UK; 5Baxter BioSience, Austria

Background: Rodent and sheep models of Transmissible Spongiform Encephalopathy (TSE) have documented blood infectivity in both the pre-clinical and clinical phases of disease. Results in a (presumably more appropriate) non-human primate model have not been reported.

Objective: To determine if blood components (red cells, white cells, platelets, and plasma) from various forms of human TSE are infectious.

Methods: Blood components were inoculated intra-cerebrally (0.1 ml) and intravenously (0.5 ml) into squirrel monkeys from 2 patients with sporadic Creutzfeldt- Jakob disease (sCJD) and 3 patients with variant Creutzfeldt-Jakob disease (vCJD). Additional monkeys were inoculated with buffy coat or plasma samples from chimpanzees infected with either sCJD or Gerstmann-Sträussler-Scheinker disease (GSS). Animals were monitored for a period of 5 years, and all dying or sacrificed animals had post-mortem neuropathological examinations and Western blots to determine the presence or absence of the misfolded prion protein (PrPTSE).

Results: No transmissions occurred in any of the animals inoculated with blood components from patients with sporadic or variant CJD. All donor chimpanzees (sCJD and GSS) became symptomatic within 6 weeks of their pre-clinical phase plasmapheresis, several months earlier than the expected onset of illness. One monkey inoculated with purified leukocytes from a pre-clinical GSS chimpanzee developed disease after 36 months.

Conclusion: No infectivity was found in small volumes of blood components from 4 patients with sporadic CJD and 3 patients with variant CJD. ***However, a single transmission from a chimpanzee-passaged strain of GSS shows that infectivity may be present in leukocytes, and the shock of general anaesthesia and plasmspheresis appears to have triggered the onset of illness in pre-clinical donor chimpanzees.

Saturday, September 5, 2009



Friday, August 12, 2011

Creutzfeldt-Jakob disease (CJD) biannual update (2011/2), Incidents Panel, National Anonymous Tonsil Archive

Sunday, August 21, 2011

The British disease, or a disease gone global, The TSE Prion Disease

Thursday, August 4, 2011

Terry Singeltary Sr. on the Creutzfeldt-Jakob Disease Public Health Crisis, Date aired: 27 Jun 2011

see video here ;

Saturday, June 25, 2011

Transmissibility of BSE-L and Cattle-Adapted TME Prion Strain to Cynomolgus Macaque

"BSE-L in North America may have existed for decades"

Over the next 8-10 weeks, approximately 40% of all the adult mink on the farm died from TME.


The rancher was a ''dead stock'' feeder using mostly (>95%) downer or dead dairy cattle...

Sunday, June 26, 2011

Risk Analysis of Low-Dose Prion Exposures in Cynomolgus Macaque

Thursday, June 23, 2011

Experimental H-type bovine spongiform encephalopathy characterized by plaques and glial- and stellate-type prion protein deposits

Thursday, July 21, 2011

A Second Case of Gerstmann-Sträussler-Scheinker Disease Linked to the G131V Mutation in the Prion Protein Gene in a Dutch Patient Journal of Neuropathology & Experimental Neurology:

August 2011 - Volume 70 - Issue 8 - pp 698-702

Saturday, August 14, 2010

BSE Case Associated with Prion Protein Gene Mutation (g-h-BSEalabama) and VPSPr PRIONPATHY

(see mad cow feed in COMMERCE IN ALABAMA...TSS)

Wednesday, June 15, 2011

Galveston, Texas - Isle port moves through thousands of heifers headed to Russia, none from Texas, Alabama, or Washington, due to BSE risk factor

Saturday, July 23, 2011


Saturday, November 6, 2010

TAFS1 Position Paper on Position Paper on Relaxation of the Feed Ban in the EU Berne, 2010 TAFS


Archive Number 20101206.4364 Published Date 06-DEC-2010 Subject PRO/AH/EDR> Prion disease update 2010 (11)

PRION DISEASE UPDATE 2010 (11),F2400_P1001_PUB_MAIL_ID:1000,86129


Molecular characterization of BSE in Canada

Jianmin Yang1, Sandor Dudas2, Catherine Graham2, Markus Czub3, Tim McAllister1, Stefanie Czub1 1Agriculture and Agri-Food Canada Research Centre, Canada; 2National and OIE BSE Reference Laboratory, Canada; 3University of Calgary, Canada

Background: Three BSE types (classical and two atypical) have been identified on the basis of molecular characteristics of the misfolded protein associated with the disease. To date, each of these three types have been detected in Canadian cattle.

Objectives: This study was conducted to further characterize the 16 Canadian BSE cases based on the biochemical properties of there associated PrPres. Methods: Immuno-reactivity, molecular weight, glycoform profiles and relative proteinase K sensitivity of the PrPres from each of the 16 confirmed Canadian BSE cases was determined using modified Western blot analysis.

Results: Fourteen of the 16 Canadian BSE cases were C type, 1 was H type and 1 was L type. The Canadian H and L-type BSE cases exhibited size shifts and changes in glycosylation similar to other atypical BSE cases. PK digestion under mild and stringent conditions revealed a reduced protease resistance of the atypical cases compared to the C-type cases. N terminal- specific antibodies bound to PrPres from H type but not from C or L type. The C-terminal-specific antibodies resulted in a shift in the glycoform profile and detected a fourth band in the Canadian H-type BSE.

Discussion: The C, L and H type BSE cases in Canada exhibit molecular characteristics similar to those described for classical and atypical BSE cases from Europe and Japan. This supports the theory that the importation of BSE contaminated feedstuff is the source of C-type BSE in Canada.

*** It also suggests a similar cause or source for atypical BSE in these countries.



To minimise the risk of farmers' claims for compensation from feed compounders.

To minimise the potential damage to compound feed markets through adverse publicity.

To maximise freedom of action for feed compounders, notably by maintaining the availability of meat and bone meal as a raw material in animal feeds, and ensuring time is available to make any changes which may be required.




MAFF remains under pressure in Brussels and is not skilled at handling potentially explosive issues.

5. Tests _may_ show that ruminant feeds have been sold which contain illegal traces of ruminant protein. More likely, a few positive test results will turn up but proof that a particular feed mill knowingly supplied it to a particular farm will be difficult if not impossible.

6. The threat remains real and it will be some years before feed compounders are free of it. The longer we can avoid any direct linkage between feed milling _practices_ and actual BSE cases, the more likely it is that serious damage can be avoided. ...

SEE full text ;

Saturday, August 14, 2010

BSE Case Associated with Prion Protein Gene Mutation (g-h-BSEalabama) and VPSPr PRIONPATHY

(see mad cow feed in COMMERCE IN ALABAMA...TSS)

Sunday, August 21, 2011

Classical Bovine Spongiform Encephalopathy by Transmission of H-Type Prion in Homologous Prion Protein Context

Volume 17, Number 9-September 2011


Thursday, July 28, 2011

An Update on the Animal Disease Traceability Framework July 27, 2011

Thursday, July 14, 2011

Histopathological Studies of "CH1641-Like" Scrapie Sources Versus Classical Scrapie and BSE Transmitted to Ovine Transgenic Mice (TgOvPrP4)

Monday, June 20, 2011 2011

Annual Conference of the National Institute for Animal Agriculture ATYPICAL NOR-98 LIKE SCRAPIE UPDATE USA

Thursday, June 2, 2011

USDA scrapie report for April 2011 NEW ATYPICAL NOR-98 SCRAPIE CASES Pennsylvania AND California

Monday, June 27, 2011

Comparison of Sheep Nor98 with Human Variably Protease-Sensitive Prionopathy and Gerstmann-Sträussler-Scheinker Disease

Monday, June 27, 2011

Zoonotic Potential of CWD: Experimental Transmissions to Non-Human Primates


Wednesday, September 08, 2010


Wednesday, January 5, 2011



David W. Colby1,* and Stanley B. Prusiner1,2

Wednesday, July 06, 2011

Swine Are Susceptible to Chronic Wasting Disease by Intracerebral Inoculation

Thursday, April 03, 2008

A prion disease of cervids: Chronic wasting disease

2008 1: Vet Res. 2008 Apr 3;39(4):41

A prion disease of cervids: Chronic wasting disease

Sigurdson CJ.


*** twenty-seven CJD patients who regularly consumed venison were reported to the Surveillance Center***,


full text ;



Perceptions of unconventional slow virus diseases of animals in the USA

Spraker suggested an interesting explanation for the occurrence of CWD. The deer pens at the Foot Hills Campus were built some 30-40 years ago by a Dr. Bob Davis. At or about that time, allegedly, some scrapie work was conducted at this site. When deer were introduced to the pens they occupied ground that had previously been occupied by sheep. Whether they were scrapie infected sheep or not is unclear.


October 1994

Mr R.N. Elmhirst Chairman British Deer Farmers Association Holly Lodge Spencers Lane BerksWell Coventry CV7 7BZ

Dear Mr Elmhirst,


Thank you for your recent letter concerning the publication of the third annual report from the CJD Surveillance Unit. I am sorry that you are dissatisfied with the way in which this report was published.

The Surveillance Unit is a completely independant outside body and the Department of Health is committed to publishing their reports as soon as they become available. In the circumstances it is not the practice to circulate the report for comment since the findings of the report would not be amended. In future we can ensure that the British Deer Farmers Association receives a copy of the report in advance of publication.

The Chief Medical Officer has undertaken to keep the public fully informed of the results of any research in respect of CJD. This report was entirely the work of the unit and was produced completely independantly of the the Department.

The statistical results reqarding the consumption of venison was put into perspective in the body of the report and was not mentioned at all in the press release. Media attention regarding this report was low key but gave a realistic presentation of the statistical findings of the Unit. This approach to publication was successful in that consumption of venison was highlighted only once by the media ie. in the News at one television proqramme.

I believe that a further statement about the report, or indeed statistical links between CJD and consumption of venison, would increase, and quite possibly give damaging credence, to the whole issue. From the low key media reports of which I am aware it seems unlikely that venison consumption will suffer adversely, if at all.

Sunday, July 03, 2011

Prion Disease Detection, PMCA Kinetics, and IgG in Urine from Naturally/Experimentally Infected Scrapie Sheep and Preclinical/Clinical CWD Deer


Thursday, May 26, 2011

Travel History, Hunting, and Venison Consumption Related to Prion Disease Exposure, 2006-2007 FoodNet Population Survey

Journal of the American Dietetic Association Volume 111, Issue 6 , Pages 858-863, June 2011.

NOR IS THE FDA recalling this CWD positive elk meat for the well being of the dead elk ;

Wednesday, March 18, 2009

Noah's Ark Holding, LLC, Dawson, MN RECALL Elk products contain meat derived from an elk confirmed to have CWD NV, CA, TX, CO, NY, UT, FL, OK RECALLS AND FIELD CORRECTIONS: FOODS CLASS II

Monday, August 8, 2011

Susceptibility of Domestic Cats to CWD Infection


No comments:

Post a Comment