Two different types of unusual BSE have been described in the literature (Figure 1). First, the L-type was named for the low molecular mass of the unglycosylated isoform of the PrPSc in Western blots and with amyloid plaque deposition in the brain, differing from classical BSE cases which lack amyloid plaque deposition; these cases also display a wider distribution of PrPSc throughout the brain. The L-type is also referred to as "bovine amyloidotic spongiform encephalopathy" or BASE. This isoform was first reported in Italy1. Other cases have been detected in Japan2, Germany3, and Belgium4, Denmark5 and Poland6.

(Figure 1)

Representation of Western blots of the two major types of unusual/atypical BSE. M.W., molecular weight in kilodaltons; L type, "low molecular" pattern; H type, "high molecular" pattern.5

The H-type was named for the high molecular mass of the unglycosylated isoform of the PrPSc in Western blots. Not much is known about the distribution of PrPSc in the brain of H-type cases, but it appears that the staining intensity for PrPSc is less intense when compared to classical BSE cases. The H-type was first reported in France7. Additional cases have been found in Germany3, the Netherlands8, Sweden8, Switzerland9, Canada3 and the United Kingdom10. The two cases of BSE from animals born and raised in the United States display characteristics of H-type BSE cases11 and were discovered through the active surveillance program for classical BSE; the animals did not enter the food chain.

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