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More about "Atypical" BSE

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Mike

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KEY WORDS: Bovine spongiform encephalopathy, BSE, Western blot, atypical BSE.

ABSTRACT

For many years, researchers believed that only one bovine spongiform encephalopathy (BSE) strain existed, in contrast to the many different scrapie strains found. However, only very recently reports emerged about unconventional BSE strains seen in Italy, France, and Japan. The present case describes an atypical strain of BSE in Belgium in a 64-month-old East-Flemish cow with an electrophoretic profile and other features similar to those described in Japan.

INTRODUCTION

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are a group of fatal neurodegenerative diseases including sheep and goat scrapie, bovine spongiform encephalopathy (BSE), and Creutzfeldt-Jakob disease (CJD) in humans. They are characterized by the accumulation of an abnormal protein, called PrPsc, which is formed post-translationally from the normal isoform (PrPc).1,2 At present, the agent causing TSEs is still incompletely characterized, although PrPsc is believed to be its major if not unique constituent.3

Research in mice showed the existence of different scrapie strains.4,5 Scrapie strain discrimination is currently based on biologic typing in a panel of inbred mice, using incubation time and brain pathology scoring as criteria.6 However, no large-scale studies of the molecular features of PrPsc have been reported for bovine BSE to date. Till now, the BSE strain seemed to maintain constant biologic and molecular properties even after experimental or accidental passages into different species, such as mice, humans, primates, and sheep.7–10 However, very recently, variant forms of BSE have been reported in Japan, Italy, and France.11-13 These forms were characterized by atypical histopathologic, immunohistochemical, or biochemical phenotypes. The present case is the description of the first atypical BSE case in Belgium.



MATERIALS AND METHODS

Since January 2001, all cattle older than 30 months are tested for TSE via a rapid test (TeSeE-kit, Bio-Rad, Nazareth, Belgium) after EC regulation 999/2001.14,15 Samples positive according to the enzyme-linked immunosorbent assay (ELISA) screening are further subjected to scrapie-associated fibrils (SAF), histopathology, immunohistochemistry, and Western blot (WB) testing16,17 at the National Reference Laboratory (NRL).

RESULTS

A positive ELISA sample from a 64-month-old East-Flemish cow or Belgian white and red (Figure 1) was presented at the NRL for confirmation. The animal was reported healthy before slaughter. The optical density (OD) titers at the local laboratory were 2.324 and 2.116.16 The OD titers at the NRL were 0.953 and 0.708 (sample taken at the contralateral side of the first sampling side of the obex region). The histopathology of the obex, pons, and midbrain showed no spongiform changes; immunohistochemistry of the brainstem revealed no signal of PrPsc accumulation typical for BSE; and SAF was negative. However, WB analysis (Bovine WB, Bio-Rad, France; antibodies 12F10 and SAF60) of the same homogenate that was prepared from the obex region for ELISA revealed a small amount of PrPsc with an electrophoretic profile different from that of typical BSE-associated PrPsc.18,19 The band on the gel of the non-glycosylated form of PrPsc of the present case clearly showed a lower migration pattern compared with that of a typical BSE case (Figure 2).

DISCUSSION

For many years, researchers assumed that only one BSE strain existed.7–10 Only in the past months, reports of atypical BSE cases were announced.11–13 The Japanese case11 describes a very young bull (23 months) characterized by the absence of spongiform changes and PrPsc deposits immunohistochemically. The WB analysis revealed an electrophoretic profile different from that of typical BSE, characterized by low content of the di-glycosylated molecular form of PrPsc and a faster migration of the nonglycosylated form of PrPsc. In Italy,12 two BSE affected cattle with a previously unrecognized neuropathologic profile and PrPsc type were seen. These cases were determined using a different staining pattern on immunohistochemistry, a difference in size and glycoform ratio of PrPsc on immunoblot and a difference in regional distribution of lesions. The two cases in France13 showed variant molecular features with a different PrPsc electrophoretic profile from other BSE cases, mainly characterized by a higher molecular mass of the nonglycosylated PrPsc. The present case shows the most similarities (ie, identical electrophoretic profile, only ELISA and WB positive and histopathology and immunohistochemistry negative) with the Japanese case,11 although the cow in the Japanese case was only 23 months old, and the cow in this case was 64 months old.

The fact that these strains were detected worldwide and in several breeds suggest that there is no local or breed-dependent feature involved. It could be that the WB techniques have become more specific within the past year in the detection of minor differences in di-, mono-, and nonglycosylated molecular forms of PrPsc. Infection of cattle by scrapie could also be considered since scrapie can be transmitted by direct contact between animals or through environmental contamination.13

In conclusion, this Belgian case should be added to the list of atypical BSE strains only very recently detected worldwide and may contribute to further research studies about epidemiologic significance. Current continued research on BSE would appear to reveal different BSE strains in analogy with the different scrapie strains.

ACKNOWLEDGMENTS

The authors wish to thank Rita Geeroms, Patrick Van Muylem, Stephanie Durand, Raphaël Foubert and Amina Chama for their technical assistance. Mario Vanpoucke is acknowledged for providing references.

REFERENCES

1. Oesch B, Westaway D, Walchii M, et al: A cellular gene encodes PrP 27–30 protein. Cell 40:735–746, 1985.
 

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