Experimental transmission of atypical scrapie to sheep
Marion M Simmons1 , Timm Konold1 , Hugh A Simmons3 , Yvonne I Spencer1 , Richard Lockey1 , John Spiropoulos1 , Sharon Everitt2 and Derek Clifford3
1Department of Pathology, Veterinary Laboratories Agency – Weybridge, Woodham Lane, New Haw, Addlestone, Surrey KT15 3NB, UK2Department of Molecular Pathogenesis and Genetics, Veterinary Laboratories Agency – Weybridge, Woodham Lane, New Haw, Addlestone, Surrey KT15 3NB, UK3Animal Services Unit, Veterinary Laboratories Agency – Weybridge, Woodham Lane, New Haw, Addlestone, Surrey KT15 3NB, UK
author email corresponding author email
BMC Veterinary Research 2007, 3:20doi:10.1186/1746-6148-3-20
The electronic version of this article is the complete one and can be found online at: http://www.biomedcentral.com/1746-6148/3/20
Received: 26 March 2007 Accepted: 28 August 2007 Published: 28 August 2007
© 2007 Crown copyright; licensee BioMed Central Ltd.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
AbstractBackgroundActive surveillance for transmissible spongiform encephalopathies in small ruminants has been an EU regulatory requirement since 2002. A number of European countries have subsequently reported cases of atypical scrapie, similar to previously published cases from Norway, which have pathological and molecular features distinct from classical scrapie. Most cases have occurred singly in flocks, associated with genotypes considered to be more resistant to classical disease. Experimental transmissibility of such isolates has been reported in certain ovinised transgenic mice, but has not previously been reported in the natural host. Information on the transmissibility of this agent is vital to ensuring that disease control measures are effective and proportionate.
ResultsThis report presents the successful experimental transmission, in 378 days, of atypical scrapie to a recipient sheep of homologous genotype with preservation of the pathological and molecular characteristics of the donor. This isolate also transmitted to ovinised transgenic mice (Tg338) with a murine phenotype indistinguishable from that of Nor 98.
ConclusionThis result strengthens the opinion that these cases result from a distinct strain of scrapie agent, which is potentially transmissible in the natural host under field conditions.
DiscussionTo our knowledge this is the first report of the successful experimental transmission of atypical scrapie within the natural host. The retention of the molecular and pathological characteristics of the scrapie agent following this experimental transmission supports the hypothesis that disease in the donor animal was caused by a stable strain of the TSE agent which has phenotypic characteristics distinct from those associated with classical scrapie. Although clinical signs were not recorded from the donor animal because it presented dead (fallen stock), the signs seen in the challenged animal were consistent with those reported for the small number of passive surveillance cases so far identified in the UK [14,15] and elsewhere [6,16], which were characterised by gait abnormalities, abnormal behaviour and a relative lack of pruritic signs.
The transmission characteristics of atypical scrapie from different countries in Tg338 mice  and emerging data from similar transmissions of the isolate in this report indicate that at least one strain of atypical scrapie compatible with/indistinguishable from Nor98 is widespread across Europe, despite its relatively recent identification.
Although it has now been demonstrated to be experimentally transmissible within and between species via the intracerebral route, there is little epidemiological evidence for ease of transmission of this strain under field conditions, where scrapie is most likely transmitted via the oral route. This raises the question of how it came to be so geographically widespread. One hypothesis is that this represents a spontaneous genetic disease in sheep similar to the familial forms of TSE in man (e.g. GSS, familial CJD and FFI etc.), in which the resultant disease can subsequently be transmitted experimentally [17,18].
Horizontal transmission of classical scrapie has been shown to occur in adult sheep exposed to an affected flock  and even in animals which have had contact only with a contaminated environment (GI Dexter and SJ Bellworthy, personal communication). The environmental persistence of the classical scrapie agent is also supported by epidemiological evidence from Icelandic repopulation studies [20,21]. However, the low molecular mass (<14 kD) band of PrPSc seen in Western blots of atypical isolates is associated with a reduction in PK resistance of this disease-associated protein. It is possible that this relative protease susceptibility may make the agent less robust in field conditions and thereby reduce the extent to which it is transmissible by natural routes, and so limit the number of cases. It could be argued that this in turn supports the hypothesis that the natural disease may arise spontaneously. However, although genetic studies have indicated a PrP genotype host range for atypical scrapie which, for the majority of cases, is almost fully distinct from that of the classical forms of disease, they have not identified a single genetic feature (such as the point mutations in some human forms ) which might account for this.
This relative PrPSc fragility  compared to the PrPSc in classical scrapie, may also account for the total absence of intracellular PrPSc immunolabelling. The different PK cleavage sites in PrP associated with different strains of TSE infecting small ruminants can be used to differentiate them, both in blots and by immunohistochemistry. Intracellular immunostaining is significantly reduced in sheep infected with BSE compared to classical scrapie, when the appropriate monoclonal antibodies are applied, and it has been speculated that this demonstrates a partial digestion of the PrPSc molecule by the endogenous proteases within the cells . If the PrPSc is more protease sensitive, then this loss of intracellular immunolabelling even with an antibody to the core of the protein (such as MAb 2G11) might indicate that cells can more effectively catabolise this abnormal form of the protein than more 'classical' forms, and therefore succumb to clinical disease much later, if at all. This may also account for the apparent absence of disease-related PrPSc in lymphoid tissues. It is interesting to note that in familial CJD (E200K mutation) also, PrP labelling was seen as a uniformly fine deposit throughout all cortical layers .
This study confirms that 'atypical scrapie' is transmissible within the natural host species, via the intracerebral route, without alteration of the pathological and molecular characteristics. This is the first report from a larger study which will explore more widely the effect of the PrP gene on the experimental susceptibility and resultant phenotype following intracerebral or oral challenge with this type of scrapie isolate.
ConclusionAt present the significance of this result, in terms of the transmissibility or pathogenicity under 'field conditions' of this agent strain in any species remains speculative, but it supports the need for appropriate control measures protecting both the animal and the human food chain to encompass atypical scrapie cases specifically.
Tissue distribution. For atypical scrapie, what is PrPres andinfectivity distribution within sheep of different genotypes,particularly with respect to SRM removal? For classicalscrapie and experimental BSE in sheep, tissue distribution ofinfectivity is widespread. Thus, even with SRM controls inplace, an infected sheep poses around 1000 times the risk tohuman health than does an infected cow22. Does thedistribution depend on whether infection is by the oral or21 Gubbins S. Prevalence of BSE in sheep: interpreting the results ofretrospective andprospective testing of sheep TSE cases. SEAC 84 open meeting22 paper presented to Food Standards Agency board on 9 December 2004.http://www.foodstandards.gov.uk/multimedia/pdfs/fsa041204.pdfAlso see paper SEAC/84/2 Annex 2: McLean, A.Page 13© SEAC 27 February 2006intracerebral route? Are some VRQ sheep carriers with noneurological symptoms?
SEAC SHEEP SUBGROUPPOSITION STATEMENT
Published online before print October 20, 2005Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0502296102Medical Sciences
A newly identified type of scrapie agent can naturally infect sheep with resistant PrP genotypes
( sheep prion transgenic mice )Annick Le Dur *, Vincent Béringue *, Olivier Andréoletti , Fabienne Reine *,Thanh Lan Laï *, Thierry Baron , Bjørn Bratberg ¶, Jean-Luc Vilotte ,Pierre Sarradin **, Sylvie L. Benestad ¶, and Hubert Laude **Virologie Immunologie Moléculaires and Génétique Biochimique etCytogénétique, Institut National de la Recherche Agronomique, 78350Jouy-en-Josas, France; Unité Mixte de Recherche, Institut National de laRecherche Agronomique-Ecole Nationale Vétérinaire de Toulouse, InteractionsHôte Agent Pathogène, 31066 Toulouse, France; Agence Française de SécuritéSanitaire des Aliments, Unité Agents Transmissibles Non Conventionnels,69364 Lyon, France; **Pathologie Infectieuse et Immunologie, InstitutNational de la Recherche Agronomique, 37380 Nouzilly, France; and¶Department of Pathology, National Veterinary Institute, 0033 Oslo, Norway
Edited by Stanley B. Prusiner, University of California, San Francisco, CA,and approved September 12, 2005 (received for review March 21, 2005)
Scrapie in small ruminants belongs to transmissible spongiform encephalopathies (TSEs), or prion diseases, a family of fatal neurodegenerative disorders that affect humans and animals and can transmit within and between species by ingestion or inoculation. Conversion of the host-encoded prion protein (PrP), normal cellular PrP (PrPc), into amisfolded form, abnormal PrP (PrPSc), plays a key role in TSE transmission and pathogenesis. The intensified surveillance of scrapie in the European Union, together with the improvement of PrPSc detection techniques, has led to the discovery of a growing number of so-called atypical scrapie cases. These include clinical Nor98 cases first identified in Norwegian sheep on the basis of unusual pathological and PrPSc molecular features and "cases" that produced discordant responses in the rapid tests currently applied tothe large-scale random screening of slaughtered or fallen animals. Worryingly, a substantial proportion of such cases involved sheep with PrP genotypes known until now to confer natural resistance to conventional scrapie. Here we report that both Nor98 and discordant cases, including three sheep homozygous for the resistant PrPARR allele (A136R154R171), efficiently transmitted the disease to transgenic mice expressing ovine PrP, and that they shared unique biological and biochemical features upon propagation in mice. These observations support the view that a truly infectious TSE agent, unrecognized until recently, infects sheep and goatflocks andmay have important implications in terms of scrapie control andpublic health.
Author contributions: H.L. designed research; A.L.D., V.B., O.A., F.R.,T.L.L., J.-L.V., and H.L. performed research; T.B., B.B., P.S., and S.L.B.contributed new reagents/analytic tools; V.B., O.A., and H.L. analyzed data;and H.L. wrote the paper.A.L.D. and V.B. contributed equally to this work.To whom correspondence should be addressed.Hubert Laude,
OF COURSE, the USDA et al once was concerned for human health from typical scrapie, and rightly so, typical scrapie transmits to primates by there NON-FORCED ORAL CONSUMPTION ;
ALSO, The agent responsible for French iatrogenic growth hormone-linked CJD taken as a control is very different from vCJD but is similar to that found in one case of sporadic CJD and one sheep scrapie isolate.
INTERNATIONAL FORUM FOR TRANSMISSIBLE ANIMAL DISEASES AND FOOD SAFETY
a non-profit Swiss Foundation(May 16, 2007)TAFS1
Position Paper on Atypical scrapie and Atypical BSE
Although most atypical cases occur singly in flocks, there are some instances where two affected sheep have been identified in flocks. This may indicate that natural transmission may occur, or that the sheep were infected from a common alternative source(22, 29). Possible indications of an association with the feeding of vitamins and mineral feed supplements were detected in Norway, but remain to be proven(22).
Atypical BSE may arise spontaneously in a small proportion of cattle. The existence of sporadic CJD in humans has led to postulation that disease could arise spontaneously in any animal, but this is still not proven to happen. Despitethe small numbers of atypical BSE detected so far, in some countries the numbers are too great to suggest that they all arise spontaneously, if it were assumed that such a phenomenon occurred at the same frequency as sporadic CJD in humans.