Pathogenesis of bovine spongiform encephalopathy in sheep
L. J. M. van Keulen1 , M. E. W. Vromans1, C. H. Dolstra1, A. Bossers1 and F. G. van Zijderveld1
(1) Department of Bacteriology and TSE’s, Central Institute for Animal Disease Control (CIDC), Wageningen University and Research Centre, P.O. Box 2004, 8203 AA Lelystad, The Netherlands
Received: 16 August 2007 Accepted: 27 November 2007 Published online: 19 December 2007
Abstract The pathogenesis of bovine spongiform encephalopathy (BSE) in sheep was studied by immunohistochemical detection of scrapie-associated prion protein (PrPSc) in the gastrointestinal, lymphoid and neural tissues following oral inoculation with BSE brain homogenate. First accumulation of PrPSc was detected after 6 months in the tonsil and the ileal Peyer’s patches. At 9 months postinfection, PrPSc accumulation involved all gut-associated lymphoid tissues and lymph nodes as well as the spleen. At this time point, PrPSc accumulation in the peripheral neural tissues was first seen in the enteric nervous system of the caudal jejunum and ileum and in the coeliac-mesenteric ganglion. In the central nervous system, PrPSc was first detected in the dorsal motor nucleus of the nervus Vagus in the medulla oblongata and in the intermediolateral column in the spinal cord segments T7–L1. At subsequent time points, PrPSc was seen to spread within the lymphoid system to also involve all non-gut-associated lymphoid tissues. In the enteric nervous system, further spread of PrPSc involved the neural plexi along the entire gastrointestinal tract and in the CNS the complete neuraxis. These findings indicate a spread of the BSE agent in sheep from the enteric nervous system through parasympathetic and sympathetic nerves to the medulla oblongata and the spinal cord.
L. J. M. van Keulen Email: email@example.com
In this study, we investigated the pathogenesis of a BSE infection in sheep by following the accumulation of PrPSc in sheep killed at various time points during the incubation period of BSE. The first tissues to accumulate PrPSc were the lymphoid tissues of the GALT, followed by the GALT-draining lymph nodes and the spleen and, at a later stage, the non-GALT lymph nodes. The temporal spread and pattern of PrPSc deposition in the lymphoid tissues were similar to those described previously for natural scrapie [21, 22]. In the spleen of the BSE-infected sheep, however, a more abundant accumulation of PrPSc was seen in macrophages of the marginal zone. This probably reflects an active trapping and phagocytosis of PrPSc during a systemic circulation of the BSE agent. This is in agreement with the studies of Houston et al., who demonstrated the presence of infectivity in the blood of orally BSE-infected sheep, both in the preclinical and clinical phase, by the transmission of BSE to TSE-free recipients through whole-blood transfusion [12, 14]. The question remains which fraction of the blood contains BSE infectivity and thus whether the BSE agent is present in blood plasma or whether it is associated with a cellular component of the blood. Preliminary results from transfusion with blood fractions have shown infectivity in the buffy coat but could not exclude infectivity in plasma and/or red cell fractions .
The first peripheral nervous tissues to accumulate PrPSc in the pathogenesis of BSE in sheep were the post-ganglionic parasympathetic motor neurons in the submucosal and myenteric plexi of the ENS and the post-ganglionic sympathetic motor neurons in the coeliac-mesenteric ganglion. The first accumulations of PrPSc in the CNS were detected in the pre-ganglionic parasympathetic motor neurons of the DMNV in the brain stem and in the pre-ganglionic sympathetic motor neurons of the IMLC in the spinal cord. From these initial sites of PrPSc accumulation in the peripheral and central nervous system, PrPSc subsequently spread to involve the entire ENS and CNS. This pathogenetic route of the BSE agent from the ENS through the efferent motor fibers of the autonomic nervous system to the CNS is similar to the pathogenesis of natural scrapie in sheep . In scrapie- and BSE-infected sheep, the preceding infection of the Peyer’s patches could possibly play a role in facilitating the infection of the post-ganglionic motor neurons of the ENS. In cattle orally exposed to BSE, infectivity has also been found in the Peyer’s patches of the ileum [23, 24]. Preliminary observations on the pathogenesis of BSE in cattle indicate a similar spread of the BSE agent from the gut through the efferent autonomic nervous system to the CNS . However, the role of the ENS in the pathogenesis of BSE in cattle is still obscure as PrPSc was not found in the ENS during the early stages of a BSE infection and only sparsely in the myenteric plexus of cattle in the end stage of BSE .
The results of the present study can be compared with previous time course studies on BSE in sheep in which the same dose and the same BSE brain homogenate were used for oral inoculation of ARQ/ARQ Romney sheep. In these studies, infectivity was detected by bio-assay in RIII mice in the Peyer’s patches at 4 mpi . In contrast, at this time point, PrPSc was detected immunohistochemically only in the retropharyngeal lymph node and not in the Peyer’s patches [2, 15]. In our study of ARQ/ARQ Texel sheep, we detected sparse deposits of PrPSc at 6 mpi in both the tonsil and Peyer’s patches that were only detected after thorough screening of the entire tissue by examining sections every 50 µm until the blocks were exhausted. This might explain the difference with the immunohistochemical studies in the Romney sheep where PrPSc was only detected in the retropharyngeal lymph node, but it is in general agreement with the bio-assay, where infectivity was found in the Peyer’s patches at 4 mpi (but not in the tonsil). In the Texel sheep killed at 9 mpi, all GALT tissues, the spleen, ENS, IMLC and DMNV were positive for PrPSc, whereas in the Romney sheep at 10 mpi, PrPSc was only detected in the retropharyngeal lymph node, and infectivity was only detected in the spleen. This indicates a more rapid spread of the BSE infection in the Texel sheep compared to the Romney sheep, since both the dose and the inoculum were the same in the two studies. Because the age at inoculation and the PrP genotypes of the animals in both studies were also the same, the differences in the temporal spread of the BSE agent between the two breeds must be related to other genetic factors outside the PrP region, or to other unknown breed factors. In the Romney sheep, the BSE agent only became widespread at 16 mpi when PrPSc and infectivity were detected in a wide range of nervous tissues, lymphoreticular tissues and viscera including the liver. PrPSc in the liver was reported to be present within Kupffer cells at several stages of the BSE infection (M. Jeffrey, personal communication). In the Texel sheep however, no PrPSc was detected in Kupffer cells or hepatocytes at any stage during the BSE infection.
At present, active surveillance of TSE in sheep and goats has been increased in the EU (EU regulation 214/2005) because of the finding of a BSE-infected goat in France  and possibly in the UK (confirmation by strain typing is still pending) . Several rapid tests for TSEs in small ruminants are now available (EU regulation 260/2005), all of which use the medulla oblongata, and in particular, the obex region as source material for testing. In the present study, we detected PrPSc in the DMNV at the obex of BSE-infected sheep from 9 mpi. In a previous study of natural scrapie in sheep, PrPSc was detected in the DMNV in sheep from 10 months of age . Therefore, preclinically TSE-infected animals are likely to be identified through rapid testing as long as the DMNV region in the obex is excised and used for rapid testing. Further biochemical and/or immunohistochemical tests can then be used to differentiate BSE from scrapie infection, while final confirmation of the TSE strain will still require bio-assay in mice.
Acknowledgments We would like to thank Dr Martin Jeffrey for supplying the BSE brain homogenate. This work was supported by the EU (FAIR CT 98-7006) and the Dutch Ministry of Agriculture, Nature and Food Quality. Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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However, in the absence of comprehensive infectivity data to facilitate a QRA, it was concluded that Specified Risk Materials (SRM) removal alone was unlikely to be sufficient to eliminate the residual BSE risk to the consumer from a BSE-infected sheep carcass.
SEE FULL TEXT ;
SEE FULL OPINION 44 PAGES ;
Subject: OPINION, BSE RISK IN SHEEP, HOPING FOR THE BEST, PREPARING FOR THE WORST From: "Terry S. Singeltary Sr." <[log in to unmask]> Reply-To: Sustainable Agriculture Network Discussion Group <[log in to unmask]> Date: Fri, 16 Mar 2007 11:29:34 -0600
NOR-98 ATYPICAL SCRAPIE CASES USA
BSE BASE MAD COW TESTING TEXAS, USA, AND CANADA, A REVIEW OF SORTS
MADCOW USDA the untold story
CHRONIC WASTING DISEASE
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SEAC 99th meeting on Friday 14th December 2007
AS one of them _lay_ folks, one must only ponder ;
"WITH the Nor-98 now documented in five different states so far in the USA in 2007, and with the TWO atypical BSE H-BASE cases in Texas and Alabama, with both scrapie and CWD running rampant in the USA, IS there any concern from SEAC with the rise of sporadic CJD in the USA from ''UNKNOWN PHENOTYPE'', and what concerns if any, in relations to blood donations, surgery, optical, and dental, do you have with these unknown atypical phenotypes in both humans and animals in the USA ???"
"Does it concern SEAC, or is it of no concern to SEAC?"
"Should it concern USA animal and human health officials?"
----- Original Message ----- From: xxxxxxxxxx To: firstname.lastname@example.org Sent: Thursday, November 22, 2007 5:39 AM Subject: QUESTION FOR SEAC
Mr Terry S Singeltary Sr., Bacliff, Texas 77518 USA.
Dear Mr Singeltary,
"Thank you for your e-mail of yesterday with the question for SEAC. I can confirm that this will be asked at the meeting on your behalf and the question and answer will appear in the minutes of the meeting which will be published on the SEAC Internet site."
snip...see full text ;