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Experimental transmission of the bovine spongiform encephalopathy (BSE) agent has been successfully reported in pigs inoculated via three simultaneous distinct routes (intracerebral, intraperitoneal and intravenous). Sheep derived BSE (Sh-BSE) is transmitted more efficiently than the original cattle-BSE isolate in a transgenic mouse model expressing porcine prion protein. However, the neuropathology and distribution of Sh-BSE in pigs as natural hosts, and susceptibility to this agent, is unknown. In the present study, seven pigs were intracerebrally inoculated with Sh-BSE prions. One pig was euthanized for analysis in the preclinical disease stage. The remaining six pigs developed neurological signs and histopathology revealed severe spongiform changes accompanied by astrogliosis and microgliosis throughout the central nervous system. Intracellular and neuropil-associated pathological prion protein (PrPSc) deposition was consistently observed in different brain sections and corroborated by Western blot. PrPSc was detected by immunohistochemistry and enzyme immunoassay in the following tissues in at least one animal: lymphoid tissues, peripheral nerves, gastrointestinal tract, skeletal muscle, adrenal gland and pancreas. PrPSc deposition was revealed by immunohistochemistry alone in the retina, optic nerve and kidney. These results demonstrate the efficient transmission of Sh-BSE in pigs and show for the first time that in this species propagation of bovine PrPSc in a wide range of peripheral tissues is possible. These results provide important insight into the distribution and detection of prions in non-ruminant animals.

S U M M A R YWe assessed three different visualization systems used routinely in research and diagnosis of transmissable spongiform encephalopathies (TSEs) to demonstrate whether the methodology applied to immunohistochemical (IHC) examination may alter the results concerning detection of prion protein (PrP sc ) in the lymphoreticular system (LRS): avidin-biotin-peroxidase (Vectastain ABC kit; Vector), Envision (DAKO), and catalyzed signal amplification (CSA; DAKO). The study aimed to determine which of these showed the highest sensitivity, with the hope of providing an accurate tool for pathogenesis and preclinical diagnosis research in TSEs. Histological sections from palatine tonsils, spleen, GALT (ileum and ileocecal valve), and lymph nodes from sheep belonging to a Spanish scrapie-positive flock were processed by IHC using L42 as primary antibody. As substrate chromogen, diaminobenzidine was used, and all slides were subjectively assessed by light microscopy. A further study using an image analyzer software system was carried out to confirm that the conclusion provided by microscopic examination was objective. The CSA system showed the highest sensitivity in all cases, increasing both variables assessed: the number of follicles that were PrP sc -positive was detected as well as the intensity of immunostaining in each of them.

Maedi-visna, a multisystemic disease of adult sheep, was first described in Spain in 1984. To get an idea of the seroprevalence of the disease locally and to estimate the number of seropositive animals with lesions, samples of blood, lungs and mammary glands were taken from 124 randomly selected sheep killed in the main slaughterhouse of Zaragoza. In the agar gel immunodiffusion test, 74 (59.7 per cent) of the sheep were positive and 50 were negative. Among the 74 seropositive animals, 19 (25.6 per cent) had no lesions in any organ, 12 (16.2 per cent) had lesions in the lungs only, 15 (20.2 per cent) had lesions in the mammary glands and 28 (37.8 per cent) had lesions in both organs. In the lungs hyperplasia of lymphoid follicles was more evident than an interstitial infiltrate but in the mammary glands this relationship was not observed. Even when the lesions occurred in both organs, they did not show the expected proportion in terms of either type or severity. Among the 50 seronegative sheep, eight (16 per cent) showed maedi-like lesions, formed exclusively by the hyperplasia of lymphoid follicles.