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

Following combined intraperitoneal and intracerebral injection with bovine spongiform encephalopathy (BSE) cow brain homogenate, SCID mice show a resistance to infection in comparison with immunocompetent CB20 mice. BSE occurred in only five out of 22 challenged SCID mice, with a mean incubation period of 573 days, whereas all the CB20 mice developed the disease with a mean incubation period of 456 days. In contrast, previous studies have shown that intracerebral infection of SCID mice with a mouse-passaged scrapie strain, ME7, produces 100% incidence of disease but no replication of infectivity in spleen. The results with BSE suggest that there is little or no direct infection of the CNS in interspecies transmissions, but that processing or replication of infectivity in peripheral lymphoid tissues may facilitate subsequent spread of infection to the CNS.