Although not contagious, TSEs are potentially infective, and in humans may present as sporadic, inherited, and acquired diseases. Human-to-human transmission of TSE is well documented and has occurred either through oral or mucocutaneous route of infection, as in kuru (11), or after medical and surgical procedures, as in iatrogenic CJD (12). Recently, animal-to-human transmission has also occurred. Epidemiological (13), experimental transmission (14), and biochemical PrP^Sc typing (8) have provided strong evidence that the single prion strain responsible for BSE has infected humans, causing variant CJD (vCJD), in addition to several animal species. In BSE and BSE-related disorders, including vCJD, the molecular typing of disease-associated PrP^Sc shows identical PrP fragment sizes and predominance of the high molecular mass glycoform both in natural hosts and in experimentally inoculated animals. To date, at variance with CJD in humans and scrapie in sheep, only a single strain and a single PrP^Sc type have been detected in BSE.

The spreading of the BSE agent across mammalian species barriers has aroused considerable concern for the following reasons: (i) the possible existence of new or previously unrecognized cattle TSE strains, potentially pathogenic for humans; and (ii) the occurrence of phenotypic variation of the BSE strain, with propagation of a new agent encoding distinctive molecular and biological properties.

In Italy, an active surveillance system on BSE in cattle was started in January 2001, and by August 2003 a total of 103 BSE cases had been diagnosed of 1,638,275 statutory tested brainstem samples. Confirmatory positive results have been obtained in all cases by immunohistochemical and Western immunoblot demonstration of disease-specific protease-resistant PrP^Sc.

Bovine PrP Gene Determination. Genomic DNA was isolated from frozen brain tissues by using a QIAamp DNA Mini Kit (Qiagen). PCR amplification of the PrP gene was performed in 50-μl reaction volumes containing 0.5-1 μg of genomic DNA, 25 mM Tris·HCl at pH 8.7, 200 μM each dNTP, 1.5 mM MgCl₂, 1 unit of Taq DNA polymerase, and 1 μM each primer, modified p78 (+) (5′-TAAGTGGGCATATGATGCTC-3′) and p9 (-) (5′-CTGGGATTCTCTCTGGTACT-3′), according to previously described procedures (16). Amplification reactions were performed in a Gene Amp PCR system 9700 (Applied Biosystems) for 41 cycles of 1 min at 94°C, 1.5 min at 56°C, and 1 min at 72°C. PrP polymorphisms were detected by DNA sequencing on both strands of the PCR products in an ABI 310 capillary system (Applied Biosystems). To determine the number of copies of the octapeptide repeats, PCR was carried out by using as primers modified p78 (+) and p60 (-) (5′-GATAGTAACGGTCCTCATAG-3′). PCR amplification products were examined in ethidium bromide-stained 3% agarose gels.

Neuropathology and PrP Immunohistochemistry. Histological sections obtained from each sampled specimen were deparaffinized, rehydrated, and stained with hematoxylin and eosin for evaluation of pathological changes; additional sections were stained with thioflavin-S. For the immunohistochemical study, after rehydration, sections were treated with 96% formic acid for 20 min at room temperature, followed by autoclaving at 121°C for 30 min. After rinsing, sections were incubated overnight at 4°C with anti-PrP monoclonal antibody F99/97.6.1 (17) diluted to 1/1,000. Subsequent antibody detection was carried out by using a biotinylated goat anti-mouse secondary antibody diluted to 1/200 for 20 min (Vector Laboratories, Burlingame, CA) at room temperature, followed by the avidin-biotin-peroxidase complex (Vectastain ABC kit, Vector Laboratories) according to manufacturer's protocol. Immunoreactivity was visualized by using 3,3′-diaminobenzidine as chromogen.

For electron microscopic study, formalin-fixed specimens of brain tissues were extensively washed in PBS, fixed in 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer, and postfixed with 2% osmium tetroxide for 2 h. After dehydration in graded acetone solutions, tissues were embedded in Spurr's resin. Subsequently, the sections were prepared for electron microscopy and observed with a Zeiss EM 109 electron microscope.

Neuropathology and Immunohistochemistry. Although the presence of early autolysis precluded an accurate pathological assessment in some brain areas, in all animals spongiosis was not consistently found in the brainstem, at the level of the obex or in more rostral areas. The frontal, parietal, and occipital cortices were apparently spared, and no vacuolation was detected in the olfactory bulb, piriform cortex, and hippocampus. Mild spongiform changes of the neuropil were observed only in two Friesian cattle at the level of the thalamus. However, after PrP immunohistochemistry, two groups of animals were readily distinguished, because of striking differences in patterns and topography of PrP deposition (Fig. 1). Group 1, comprising six Friesian and Bruna Alpina cattle, including the two cases with thalamic spongiosis, matched the typical phenotype of BSE, characterized by the occurrence of PrP deposits of granular type (in the neuronal cytoplasm or in gray matter neuropil), linear type (thick, thread-like profiles), and glial type, which confers a star-like appearance (Fig. 1 a, c, e, and g). By contrast, group 2, one each Piemontese and Bruna Alpina cattle 15 and 11 years old, respectively, was characterized by the presence of PrP-amyloid plaque-like deposits, kuru-like plaques, and granular extracellular and glial deposits (Fig. 1 b, d, f, and h). The kuru-like plaques appeared as dense unicentric (Fig. 2a), or less frequently multicentric, round structures up to 25 μm in diameter (Fig. 2b), with a pale core and a dark radial periphery. PrP-positive plaques were also fluorescent after thioflavin-S and were ultrastructurally composed of bundles of straight, unbranched fibrils with a diameter of ≈7 nm (Fig. 2c and d). The two groups of cattle also showed remarkable differences in brain regional distribution of PrP deposits. In group 1, large amounts of granular PrP deposits were observed in brainstem (Fig. 1a) and thalamus (Fig. 1c), whereas the lobus piriformis (Fig. 1e), the olfactory bulb (Fig. 1g), and cerebral cortexes were less involved. By contrast, the brainstem showed only a weak PrP positivity in group 2, and the dorsal motor nucleus of the vagus was unstained (Fig. 1b); PrP-amyloid plaques were seen in the thalamus (Fig. 1d), subcortical white matter and deeper layers of cerebral cortexes (Fig. 1f), and olfactory bulb (Fig. 1h). Finally, the molecular layer of the cerebellum exhibited PrP deposits of the glial type in group 1, whereas some amyloid plaques were observed in group 2.

Biochemical Characterization and Regional Distribution of PrP^Sc. Western immunoblots of proteinase K-treated brain homogenates, obtained from different cortical and subcortical regions of group 1 and group 2 animals, showed the presence of two distinct PrP^Sc types, which were distinguishable on the basis of the molecular mass of their unglycosylated fragments and the ratio of differently glycosylated forms. The typical molecular ”BSE signature,” characterized by overrepresentation of the high molecular mass glycoform, was detected in group 1 animals (Fig. 3a, odd lanes). In contrast, the Piemontese and Bruna Alpina cattle (group 2) showed a predominance of the low molecular mass glycoform and a protease-resistant fragment with a faster electrophoretic mobility (Fig. 3a, even lanes).

Previous pathological studies in clinically suspect cases of BSE in Great Britain have provided evidence for a uniform pattern in the severity and distribution of vacuolar lesions in affected animals, with medulla oblongata nuclei being the most involved (22). While confirming that the BSE epidemic has been sustained by a single agent, these studies have assessed the validity of statutory criteria for the diagnosis of BSE, which is currently based on both histopathological and immunobiochemical examination of the medulla. However, the prevailing involvement of cortical regions in the cattle with amyloid deposition suggests that postmortem brain sampling should not be limited to the obex. In addition, a careful analysis of PrP^Sc glycoform profiles at the confirmatory Western immunoblot may provide a molecular means of identifying atypical cases of bovine TSE.

Bovine Amyloidotic Spongiform Encephalopathy (BASE): A Second Bovine TSE. The present findings show that a previously undescribed pathological and immunohistochemical phenotype, associated with cattle TSE, is related to the presence of a PrP^Sc type with biochemical properties, including the gel mobility of the protease-resistant fragment and glycoform ratios, different from those encountered in cattle BSE. Brain deposition of this pathological isoform of cattle PrP correlates with the formation of PrP-amyloid plaques, as opposed to typical BSE cases. Although in several natural and experimental recipients of the BSE agent, including humans (13), neuropathological changes are characterized by the presence of PrP-positive amyloid deposits with surrounding vacuolation, cattle BSE is not associated with PrP-amyloid plaque formation. On the basis of the above features, we propose to name the disease described here BASE. Although observed in only two cattle, the BASE phenotype could be more common than expected. In previous studies, amyloid congophilic plaques were found in 1 of 20 BSE cases examined systematically for amyloid (23), and it was reported that focal cerebral amyloidosis is present in a small proportion of BSE cases (24). Although no biochemical analysis of PrP^Sc glycotype is available for these animals with ”atypical BSE phenotype,” our present results underscore the importance of performing a strain-typing in bovine TSE with amyloid deposition.

In sCJD, the neuropathological phenotype largely correlates with the molecular type of PrP^Sc and distinct polymorphic sites of PRNP (9, 19). This is in contrast with the situation in cattle, where different genotypes have been reported based on the variable numbers of octapeptide repeats in each allele, but no evidence for single-codon polymorphisms in the PrP gene has been established (25, 26). Because the present animals shared a similar genetic background and breed, differences in disease phenotypes between cattle with BSE and BASE can be tentatively related only to distinct PrP^Sc types or alternative routes of infection and spread of prion pathology. Accordingly, the lack of involvement of the motor dorsal nucleus of the vagus and the slight involvement of the brainstem in BASE, suggests a route for spreading of the agent other than the alimentary tract. Therefore, unless the BASE agent propagates throughout the olfactory pathway or other peripheral routes, it is possible that this disorder represents a sporadic form of cattle TSE, which would also explain the difference in ages between the two groups of affected animals.

Phenotypic Similarities Between BASE and sCJD. The transmissibility of CJD brains was initially demonstrated in primates (27), and classification of atypical cases as CJD was based on this property (28). To date, no systematic studies of strain typing in sCJD have been provided, and classification of different subtypes is based on clinical, neuropathological, and molecular features (the polymorphic PRNP codon 129 and the PrP^Sc glycotype) (8, 9, 15, 19). The importance of molecular PrP^Sc characterization in assessing the identity of TSE strains is underscored by several studies, showing that the stability of given disease-specific PrP^Sc types is maintained upon experimental propagation of sCJD, familial CJD, and vCJD isolates in transgenic PrP-humanized mice (8, 29). Similarly, biochemical properties of BSE- and vCJD-associated PrP^Sc molecules remain stable after passage to mice expressing bovine PrP (30). Recently, however, it has been reported that PrP-humanized mice inoculated with BSE tissues may also propagate a distinctive PrP^Sc type, with a ”monoglycosylated-dominant” pattern and electrophoretic mobility of the unglycosylated fragment slower than that of vCJD and BSE (31). Strikingly, this PrP^Sc type shares its molecular properties with the a PrP^Sc molecule found in classical sCJD. This observation is at variance with the PrP^Sc type found in M/V2 sCJD cases and in cattle BASE, showing a monoglycosylated-dominant pattern but faster electrophoretic mobility of the protease-resistant fragment as compared with BSE. In addition to molecular properties of PrP^Sc, BASE and M/V2 sCJD share a distinctive pattern of intracerebral PrP deposition, which occurs as plaque-like and amyloid-kuru plaques. Differences were, however, observed in the regional distribution of PrP^Sc. While in M/V2 sCJD cases the largest amounts of PrP^Sc were detected in the cerebellum, brainstem, and striatum, in cattle BASE these areas were less involved and the highest levels of PrP^Sc were recovered from the thalamus and olfactory regions.