BSE transmission studies with particular reference to blood.

Tissue infectivity in BSE has been comprehensively investigated in cattle with natural BSE and during the incubation period in an experimental pathogenesis study in which cattle were challenged orally with infected cattle brain from natural cases. In natural cases of BSE in cattle, infectivity has been found only in the CNS, (the brain, the spinal cord and retina). No infectivity has been found in about 50 other tissues including bone marrow, clotted blood, buffy coat, serum or foetal calf serum. In the pathogenesis study in which clinical disease was first detected at 35 months post-infection (39 months of age), infectivity has not been found in blood or any assayed component of blood. Experimental parenteral challenge of cattle and mice in three separate experiments with (i) a pool of five brains, (ii) a pool of five spleens and (iii) a pool of lymph nodes from five cattle is incomplete. However, whereas the brain has transmitted disease to both species (in cattle even when diluted about one million times) neither the spleen pool nor the lymph node pool has transmitted disease to either, although the cattle study is incomplete. These experiments have also shown that cattle can detect about 1000 times less infectivity/g than can mice. No infectivity has ever been detected in the blood or any component of blood in natural scrapie of sheep and goats, natural BSE of cattle or experimental BSE of cattle.