Severely-combined immunodeficient (SCID) mice resist infection with bovine spongiform encephalopathy (BSE)

Following combined intraperitoneal and intracerebral injection with BSE, SCID mice show a resistance to infection in comparison with immunocompetent CB20 mice. Neurodegenerative disease occurred in only 5 out of 22 infected SCID mice, with a mean incubation period of 573 days, whereas neurodegenerative disease occurred in all the CB20 mice with a mean incubation period of 456 days. Infection of another immunocompetent strain of mice, RIII, produced disease in almost 100% of mice with a mean incubation period of 349 days. In contrast, i.c. infection of SCID mice with a mouse passaged scrapie strain, ME7, produces 100% incidence of neurodegenerative disease but with no replication of infectivity in spleen. These results support the hypothesis that, for infection to be established in a new species, processing of infectivity in peripheral lymphoid tissue is required before infection of the CNS can occur.