High incidence of scrapie induced by repeated injections of subinfectious prion doses

Chronic Wasting Disease: State of the Science

Chronic wasting disease (CWD) is a prion disease affecting cervid species, both free-ranging and captive populations. As the geographic range continues to expand and disease prevalence continues to increase, CWD will have an impact on cervid populations, local economies, and ecosystem health. Mitigation of this "wicked" disease will require input from many different stakeholders including hunters, landowners, research biologists, wildlife managers, and others, working together. The NC1209 (North American interdisciplinary chronic wasting disease research consortium) is composed of scientists from different disciplines involved with investigating and managing CWD. Leveraging this broad breadth of expertise, the Consortium has created a state-of-the-science review of five key aspects of CWD, including current diagnostic capabilities for detecting prions, requirements for validating these diagnostics, the role of environmental transmission in CWD dynamics, and potential zoonotic risks associated with CWD. The goal of this review is to increase stakeholders', managers', and decision-makers' understanding of this disease informed by current scientific knowledge.

Failure To Detect Prion Infectivity in Ticks following Prion-Infected Blood Meal

Chronic wasting disease (CWD) is an emerging and fatal contagious prion disease that affects cervids, including mule deer, white-tailed deer, black-tailed deer, red deer reindeer, elk, and moose. CWD prions are widely distributed throughout the bodies of CWD-infected animals and are found in the nervous system, lymphoid tissues, muscle, blood, urine, feces, and antler velvet. The mechanism of CWD transmission in natural settings is unknown. Potential mechanisms of transmission include horizontal, maternal, or environmental routes. Due to the presence of prions in the blood of CWD-infected animals, the potential exists for invertebrates that feed on mammalian blood to contribute to the transmission of CWD. The geographic range of the Rocky Mountain Wood tick, Dermancentor andersoni, overlaps with CWD throughout the northwest United States and southwest Canada, raising the possibility that D. andersoni parasitization of cervids may be involved in CWD transmission. We investigated this possibility by examining the blood meal of D. andersoni that fed upon prion-infected hamsters for the presence of prion infectivity by animal bioassay. None of the hamsters inoculated with a D. andersoni blood meal that had been ingested from prion-infected hamsters developed clinical signs of prion disease or had evidence for a subclinical prion infection. Overall, the data do not demonstrate a role for D. andersoni in the transmission of prion disease.IMPORTANCE Chronic wasting disease (CWD) is an emerging prion disease that affects cervids, including mule deer, white-tailed deer, black-tailed deer, red deer reindeer, elk, and moose. The mechanism of CWD transmission in unknown. Due to the presence of prions in the blood of CWD-infected animals, it is possible for invertebrates that feed on cervid blood to contribute to the transmission of CWD. We examined the blood meal of D. andersoni, a tick with a similar geographic range as cervids, that fed upon prion-infected hamsters for the presence of prion infectivity by animal bioassay. None of the D. andersoni blood meals that had been ingested from prion-infected hamsters yielded evidence of prion infection. Overall, the data do not support a role of D. andersoni in the transmission of prion disease.

Very low oral exposure to prions of brain or saliva origin can transmit chronic wasting disease

The minimum infectious dose required to induce CWD infection in cervids remains unknown, as does whether peripherally shed prions and/or multiple low dose exposures are important factors in CWD transmission. With the goal of better understand CWD infection in nature, we studied oral exposures of deer to very low doses of CWD prions and also examined whether the frequency of exposure or prion source may influence infection and pathogenesis. We orally inoculated white-tailed deer with either single or multiple divided doses of prions of brain or saliva origin and monitored infection by serial longitudinal tissue biopsies spanning over two years. We report that oral exposure to as little as 300 nanograms (ng) of CWD-positive brain or to saliva containing seeding activity equivalent to 300 ng of CWD-positive brain, were sufficient to transmit CWD disease. This was true whether the inoculum was administered as a single bolus or divided as three weekly 100 ng exposures. However, when the 300 ng total dose was apportioned as 10, 30 ng doses delivered over 12 weeks, no infection occurred. While low-dose exposures to prions of brain or saliva origin prolonged the time from inoculation to first detection of infection, once infection was established, we observed no differences in disease pathogenesis. These studies suggest that the CWD minimum infectious dose approximates 100 to 300 ng CWD-positive brain (or saliva equivalent), and that CWD infection appears to conform more with a threshold than a cumulative dose dynamic.

Susceptibility to Creutzfeldt-Jakob disease after human growth hormone treatment in France

Objective

To identify, among the available data concerning host characteristics and exposure, risk factors influencing the susceptibility for developing iatrogenic Creutzfeldt-Jakob disease (iCJD) in a cohort of patients treated with human cadaver-sourced growth hormone (hGH) in France.

Methods

This study included all 1,443 individuals treated in France with hGH from January 1982 to December 1985, out of which 119 cases of hGH-iCJD have been identified so far. We applied a 3 sequential step Cox analysis involving univariable, stepwise, and nonstepwise multivariable procedures. The variables studied were sex, age at hGH treatment initiation, and treatment characteristics (batches and doses).

Results

While no effect of age at treatment initiation was observed, a significant effect of sex on disease susceptibility was unexpectedly evidenced with a 2-fold increase of disease occurrence in male patients. This effect did not depend on differences of exposure between male and female patients. We identified 4 categories of hormone batches from high risk to no association with susceptibility. A relationship between the dose received from at-risk hormone batches and the attack rate (number of patients developing the disease among exposed individuals) was demonstrated.

Conclusions

Studying the hGH-exposed patients in France provides epidemiologic evidence of a relationship between dose of inoculum and disease occurrence in humans and suggests an unexpected effect of sex on individual susceptibility.

Iatrogenic Creutzfeldt-Jakob disease with Amyloid-β pathology: an international study

The presence of pathology related to the deposition of amyloid-β (Aβ) has been recently reported in iatrogenic Creutzfeldt-Jakob disease (iCJD) acquired from inoculation of growth hormone (GH) extracted from human cadaveric pituitary gland or use of cadaveric dura mater (DM) grafts.To investigate this phenomenon further, a cohort of 27 iCJD cases - 21 with adequate number of histopathological sections - originating from Australia, France, Italy, and the Unites States, were examined by immunohistochemistry, amyloid staining, and Western blot analysis of the scrapie prion protein (PrPSc), and compared with age-group matched cases of sporadic CJD (sCJD), Alzheimer disease (AD) or free of neurodegenerative diseases (non-ND).Cases of iCJD and sCJD shared similar profiles of proteinase K-resistant PrPSc with the exception of iCJD harboring the "MMi" phenotype. Cerebral amyloid angiopathy (CAA), either associated with, or free of, Thioflavin S-positive amyloid core plaques (CP), was observed in 52% of 21 cases of iCJD, which comprised 37.5% and 61.5% of the cases of GH- and DM-iCJD, respectively. If only cases younger than 54 years were considered, Aβ pathology affected 41%, 2% and 0% of iCJD, sCJD and non-ND, respectively. Despite the patients' younger age CAA was more severe in iCJD than sCJD, while Aβ diffuse plaques, in absence of Aβ CP, populated one third of sCJD. Aβ pathology was by far most severe in AD. Tau pathology was scanty in iCJD and sCJD.In conclusion, (i) despite the divergences in the use of cadaveric GH and DM products, our cases combined with previous studies showed remarkably similar iCJD and Aβ phenotypes indicating that the occurrence of Aβ pathology in iCJD is a widespread phenomenon, (ii) CAA emerges as the hallmark of the Aβ phenotype in iCJD since it is observed in nearly 90% of all iCJD with Aβ pathology reported to date including ours, and it is shared by GH- and DM-iCJD, (iii) although the contributions to Aβ pathology of other factors, including GH deficiency, cannot be discounted, our findings increase the mounting evidence that this pathology is acquired by a mechanism resembling that of prion diseases.

PrPSc formation and clearance as determinants of prion tropism

Prion strains are characterized by strain-specific differences in neuropathology but can also differ in incubation period, clinical disease, host-range and tissue tropism. The hyper (HY) and drowsy (DY) strains of hamster-adapted transmissible mink encephalopathy (TME) differ in tissue tropism and susceptibility to infection by extraneural routes of infection. Notably, DY TME is not detected in the secondary lymphoreticular system (LRS) tissues of infected hosts regardless of the route of inoculation. We found that similar to the lymphotropic strain HY TME, DY TME crosses mucosal epithelia, enters draining lymphatic vessels in underlying laminae propriae, and is transported to LRS tissues. Since DY TME causes disease once it enters the peripheral nervous system, the restriction in DY TME pathogenesis is due to its inability to establish infection in LRS tissues, not a failure of transport. To determine if LRS tissues can support DY TME formation, we performed protein misfolding cyclic amplification using DY PrPSc as the seed and spleen homogenate as the source of PrPC. We found that the spleen environment can support DY PrPSc formation, although at lower rates compared to lymphotropic strains, suggesting that the failure of DY TME to establish infection in the spleen is not due to the absence of a strain-specific conversion cofactor. Finally, we provide evidence that DY PrPSc is more susceptible to degradation when compared to PrPSc from other lymphotrophic strains. We hypothesize that the relative rates of PrPSc formation and clearance can influence prion tropism.

Longitudinal Detection of Prion Shedding in Saliva and Urine by Chronic Wasting Disease-Infected Deer by Real-Time Quaking-Induced Conversion

Chronic wasting disease (CWD) is an emergent, rapidly spreading prion disease of cervids. Shedding of infectious prions in saliva and urine is thought to be an important factor in CWD transmission. To help to elucidate this issue, we applied an in vitro amplification assay to determine the onset, duration, and magnitude of prion shedding in longitudinally collected saliva and urine samples from CWD-exposed white-tailed deer. We detected prion shedding as early as 3 months after CWD exposure and sustained shedding throughout the disease course. We estimated that the 50% lethal dose (LD50) for cervidized transgenic mice would be contained in 1 ml of infected deer saliva or 10 ml of urine. Given the average course of infection and daily production of these body fluids, an infected deer would shed thousands of prion infectious doses over the course of CWD infection. The direct and indirect environmental impacts of this magnitude of prion shedding on cervid and noncervid species are surely significant.

IMPORTANCE

Chronic wasting disease (CWD) is an emerging and uniformly fatal prion disease affecting free-ranging deer and elk and is now recognized in 22 U.S. states and 2 Canadian provinces. It is unique among prion diseases in that it is transmitted naturally through wild populations. A major hypothesis to explain CWD's florid spread is that prions are shed in excreta and transmitted via direct or indirect environmental contact. Here we use a rapid in vitro assay to show that infectious doses of CWD prions are in fact shed throughout the multiyear disease course in deer. This finding is an important advance in assessing the risks posed by shed CWD prions to animals as well as humans.

Mitigation of prion infectivity and conversion capacity by a simulated natural process--repeated cycles of drying and wetting

Prions enter the environment from infected hosts, bind to a wide range of soil and soil minerals, and remain highly infectious. Environmental sources of prions almost certainly contribute to the transmission of chronic wasting disease in cervids and scrapie in sheep and goats. While much is known about the introduction of prions into the environment and their interaction with soil, relatively little is known about prion degradation and inactivation by natural environmental processes. In this study, we examined the effect of repeated cycles of drying and wetting on prion fitness and determined that 10 cycles of repeated drying and wetting could reduce PrP^Sc abundance, PMCA amplification efficiency and extend the incubation period of disease. Importantly, prions bound to soil were more susceptible to inactivation by repeated cycles of drying and wetting compared to unbound prions, a result which may be due to conformational changes in soil-bound PrP^Sc or consolidation of the bonding between PrP^Sc and soil. This novel finding demonstrates that naturally-occurring environmental process can degrade prions.

Prion diseases such as chronic wasting disease and scrapie are emerging in North America at an increasing rate. Infectious prions are introduced into the environment from both living and dead animals where they can bind to soil. Little information is available on the effect of prion inactivation under conditions that would be found in the natural environment. In this study, we exposed both unbound and soil-bound prions to repeated cycles of drying and wetting to simulate ambient environmental conditions. We found evidence of prion inactivation in both unbound and soil bound prions. The influence of repeated cycles of drying and wetting are dependent on the prion strain and soil type used and, interestingly, prions bound to soil were more susceptible to inactivation. This is the first report of natural environmental processes mitigating prion infectivity. This data suggests that the total environmental prion load is a balance between input and natural clearance.

Laboratory activities involving transmissible spongiform encephalopathy causing agents: risk assessment and biosafety recommendations in Belgium

Since the appearance in 1986 of epidemic of bovine spongiform encephalopathy (BSE), a new form of neurological disease in cattle which also affected human beings, many diagnostic and research activities have been performed to develop detection and therapeutic tools. A lot of progress was made in better identifying, understanding and controlling the spread of the disease by appropriate monitoring and control programs in European countries. This paper reviews the recent knowledge on pathogenesis, transmission and persistence outside the host of prion, the causative agent of transmissible spongiform encephalopathies (TSE) in mammals with a particular focus on risk (re)assessment and management of biosafety measures to be implemented in diagnostic and research laboratories in Belgium. Also, in response to the need of an increasing number of European diagnostic laboratories stopping TSE diagnosis due to a decreasing number of TSE cases reported in the last years, decontamination procedures and a protocol for decommissioning TSE diagnostic laboratories is proposed.

A quantitative assessment of the prion risk associated with wastewater from carcass-handling facilities

Wastewater from facilities processing livestock that may harbor transmissible spongiform encephalopathies (TSEs) infectivity is permitted under license for application to land where susceptible livestock may have access. Several previous risk assessments have investigated the risk of bovine spongiform encephalopathy (BSE) associated with wastewater effluents; however, the risk of exposure to classical scrapie and atypical scrapie has not been assessed. With the prevalence of certain TSEs (BSE in cattle and classical scrapie in sheep) steadily in decline, and with considerable changes in the structure of carcass-processing industries in Great Britain, a reappraisal of the TSE risk posed by wastewater is required. Our results indicate that the predicted number of new TSE infections arising from the spreading of wastewater on pasture over one year would be low, with a mean of one infection every 1,000 years for BSE in cattle (769, 555,556), and one infection every 30 years (16, 2,500), and 33 years (16, 3,333) for classical and atypical scrapie, respectively. It is assumed that the values and assumptions used in this risk assessment remain constant. For BSE in cattle the main contributors are abattoir and rendering effluent, contributing 35% and 22% of the total number of new BSE infections. For TSEs in sheep, effluent from small incinerators and rendering plants are the major contributors (on average 32% and 31% of the total number of new classical scrapie and atypical scrapie infections). This is a reflection of the volume of carcass material and Category 1 material flow through such facilities.

Detection of disease-associated prion protein in the posterior portion of the small intestine involving the continuous Peyer's patch in cattle orally infected with bovine spongiform encephalopathy agent

Twenty-eight calves were exposed to 5 g of homogenized brainstems confirmed as bovine spongiform encephalopathy (BSE) agents. Two to five animals were sequentially killed for post-mortem analyses 20 months post-inoculation (MPI) at intervals of 6 or 12 months. Samples from animals challenged orally with BSE agents were examined by Western blot and immunohistochemical analyses. Immunolabelled, disease-associated prion protein (PrPsc) was detected in a small portion of follicles in the continuous Peyer's patch from the posterior portion of the small intestine involving the entire ileum and the posterior jejunum but not in the discrete Peyer's patches in the remaining jejunum in preclinical animals at 20, 36, and 48 MPI. The PrPsc-positive cells corresponded to tingible body macrophages on double immunofluorescence labelling. In addition, PrPsc accumulated in 7 of 14 animals in the central nervous system (CNS) after 34 MPI, and five of them developed clinical signs and were killed at 34, 46, 58, and 66 MPI. Two preclinical animals killed at 36 and 48 MPI presented the earliest detectable and smallest deposition of immunolabelled PrPsc in the dorsal motor nucleus of the vagus nerve, the spinal trigeminal nucleus of the medulla oblongata at the obex region, and/or the intermediolateral nucleus of the 13th thoracic segment of the spinal cord. Based on serial killing, no PrPsc was detectable in the CNS, including the medulla oblongata at the obex level, before 30 MPI, by Western blot and immunohistochemical analyses. These results are important for understanding the pathogenesis of BSE.

Prion transmission: prion excretion and occurrence in the environment

Prion diseases range from being highly infectious, for example scrapie and CWD, which show facile transmission between susceptible individuals, to showing negligible horizontal transmission, such as BSE and CJD, which are spread via food or iatrogenically, respectively. Scrapie and CWD display considerable in vivo dissemination, with PrP(Sc) and infectivity being found in a range of peripheral tissues. This in vivo dissemination appears to facilitate the recently reported excretion of prion through multiple routes such as from skin, feces, urine, milk, nasal secretions, saliva and placenta. Furthermore, excreted scrapie and CWD agent is detected within environmental samples such as water and on the surfaces of inanimate objects. The cycle of "uptake of prion from the environment--widespread in vivo prion dissemination--prion excretion--prion persistence in the environment" is likely to explain the facile transmission and maintenance of these diseases within wild and farmed populations over many years.

In vitro amplification of prions from milk in the detection of subclinical infections

Prions can be amplified by serial protein misfolding cyclic amplification (sPMCA) from the milk of a high proportion of apparently healthy, scrapie exposed sheep with PRNP genotypes not previously associated with high disease penetrance. These data strongly suggest the widespread presence of subclinical scrapie infections within scrapie-exposed flocks containing sheep with a range of susceptible PRNP genotypes. These data also lead to the hypothesis that similar subclinical disease states may be common for other animal and human prion diseases. Furthermore, the application of sPMCA to milk provides a method to detect such subclinical disease. Here, we describe the high level amplification of bovine spongiform encephalopathy (BSE) prions from both ovine and bovine origin, a methodology that will facilitate the detection of any prions secreted within bovine and ovine milk during subclinical and clinical BSE disease.

Chronic wasting disease prions in elk antler velvet

Residue 226 of cervid prion proteins may be a determinant of CWD pathogenesis.

Chronic wasting disease (CWD) is a contagious, fatal prion disease of deer and elk that continues to emerge in new locations. To explore the means by which prions are transmitted with high efficiency among cervids, we examined prion infectivity in the apical skin layer covering the growing antler (antler velvet) by using CWD-susceptible transgenic mice and protein misfolding cyclic amplification. Our finding of prions in antler velvet of CWD-affected elk suggests that this tissue may play a role in disease transmission among cervids. Humans who consume antler velvet as a nutritional supplement are at risk for exposure to prions. The fact that CWD prion incubation times in transgenic mice expressing elk prion protein are consistently more rapid raises the possibility that residue 226, the sole primary structural difference between deer and elk prion protein, may be a major determinant of CWD pathogenesis.

Potential of active and passive immunizations for the prevention and therapy of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies are fatal neurodegenerative disorders that affect humans and certain animals and are caused by prions. In most cases, infection occurs by ingestion of prions. Their long-time persistence in the environment creates a reservoir of potentially infectious matter that renders the eradication of the disease problematic. Unfortunately, no cure is available to date. Yet, for both the treatment of infected and the protection of uninfected individuals, active and passive immunizations have been shown to have a beneficial effect on the course of the disease. The current review provides an overview of such antibody-based approaches and assesses their feasibility and potential in prophylaxis and therapy of transmissible spongiform encephalopathies.

The infectivity of transmissible spongiform encephalopathy agent at low doses: the importance of phospholipid

The issue of whether the mechanism of infection is independent or co-operative for low doses of transmissible spongiform encephalopathy (TSE) agent is critical for risk assessment. The susceptibility (and hence ID(50)) of individuals with the same prion protein (PrP) genotype may vary considerably with a small proportion being very susceptible. Assuming independent action, the incubation period (IP) would continue to increase until the dose is below the ID(50) of the most susceptible individuals in the experiment, at which point it would become constant. This may explain the observed increase in IP with decreasing dose below the apparent ID(50) in experiments with untreated TSE agent. In contrast, IPs for autoclaved or NaOH-treated TSE agent increase greatly at doses <ID(50) suggesting strong co-operative action, or even a threshold. It is proposed here that the unit of infectivity for prion disease is a nucleation seed comprised of PrP and host phospholipid (PL). PL would play a structural role through mediating protein/lipid interactions with PrP. Heating or alkali treatment destroys the PL breaking up the nucleation seeds, which require long IPs to reform at low doses. Replenishing those inactivated PLs with host PL would explain how the phenotypic effect of long IP at low dose is lost on subpassage. It is proposed here that strain thermostability is controlled by the nature and strength of the PrP/PL interactions, which are independent of the host PrP genotype. Although repeated oral exposure to low doses of scrapie is less harmful than a single large exposure, this effect may reflect interference by competition rather than diminished risks due to a co-operative effect, and is of little importance for 'one-off' low-dose environmental exposures.

Advanced survival models for risk-factor analysis in scrapie

Because of the confounding effects of long incubation duration and flock management, accurate epidemiological studies of scrapie outbreaks are difficult to carry out. In this study, 641 Manech red-faced sheep from six scrapie-affected field flocks in Pyrénées Atlantiques, France, were monitored for clinical scrapie over a 6–9 year period. Over this period, 170 scrapie clinical cases were recorded and half of the culled animals were submitted for post-mortem transmissible spongiform encephalopathy diagnosis to assess their infectious status. Collected data were analysed using a ‘mixture cure model’ approach, which allowed for the discriminating effect of PrP genotype and flock origin on incidence and incubation period. Simulations were performed to evaluate the applicability of such a statistical model to the collected data. As expected, ARR heterozygote sheep were less at risk of becoming infected than ARQ/ARQ individuals and had a greater age at clinical onset. Conversely, when compared with ARQ/ARQ, the VRQ haplotype was associated with an increased infection risk, but not a shorter incubation period. Considering the flock effect, we observed that a high incidence rate was not associated with shorter incubation periods and that the incubation period could be significantly different in flocks harbouring similar infection risks. These results strongly support the conclusion that other parameters, such as the nature of the agent or flock management, could interfere with epidemiological dynamics of the infection in scrapie-affected flocks.

Quantitative evaluation of prion inactivation comparing steam sterilization and chemical sterilants: proposed method for test standardization

Prions are notoriously resistant to inactivation. To prevent accidental transmission of variant Creutzfeldt-Jakob disease (vCJD), various decontamination procedures have been adopted for re-usable medical devices by the authorities of countries at risk. As the vCJD agent in humans has a wide tissue distribution, practical methods of prion decontamination urgently need to be standardized, as do other sterilization and disinfection procedures (European Committee for Standardization). This article proposes a method using a quantitative murine model, combining observations of the decrease in the infection rate, the increase in the incubation period and a simultaneously performed chemical protein fixation control. In terms of practical application, autoclaving at 134 degrees C for 18 min or 121 degrees C for 30 min, and 1N sodium hydroxide for 15 min reduced the transmission of infectivity by a factor of at least 10(6). Partial efficacy can also be identified by the methodology, particularly for liquid cold sterilants such as glutaraldehyde and peracetic acid solutions.