Short Communication Progression of prion infectivity in asymptomatic cattle after oral bovine spongiform encephalopathy challenge Juan Carlos Espinosa, 1 Mo ´ nica Morales, 1 Joaquı ´n Castilla, 1 Mark Rogers 2 and Juan Marı ´a Torres 1 Correspondence Juan Marı ´a Torres jmtorres@inia.es 1 Centro de Investigacio ´ n en Sanidad (CISA-INIA), Valdeolmos, Madrid, Spain 2 School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland Received 16 October 2006 Accepted 22 December 2006 The presence of BSE prion infectivity in asymptomatic cattle and its tissue distribution are important concerns for both human and veterinary health and food safety. In this work, a collection of tissues from asymptomatic cattle challenged orally with BSE and culled at 20, 24, 27, 30 and 33 months have been used to inoculate intracerebrally BoPrP-Tg110 mice expressing bovine PrP to assess their infectivity. Results demonstrate that BSE infectivity in asymptomatic cattle is essentially restricted to the nervous system, Peyer’s patches and tonsils, as reported previously for terminally BSE-diseased cattle. BSE infectivity was detectable in Peyer’s patches and tonsils at all time points analysed, but infectivity in nervous tissues (brainstem and sciatic nerve) was only detectable after 27 months from inoculation. Infectivity in brainstem increased markedly at 33 months after inoculation. All other investigated tissues or fluids (spleen, skeletal muscle, blood and urine) revealed no detectable infectivity throughout the time course studied. Transmissible spongiform encephalopathies (TSEs) or prion diseases are associated with the accumulation of abnormal PrP Sc conformer in the brain and subsequent neurodegeneration (Prusiner, 2004). The mechanism of conversion of PrP C to PrP Sc is not well understood, but it may occur by direct interaction of PrP Sc with PrP C , promoting refolding of the latter to produce additional PrP Sc . The PrP Sc conformer can be recognized by its partial resistance to proteinase K treatment. Most TSEs are transmitted naturally by peripheral routes, either orally or transcutaneously. The mechanism(s) of spread from the periphery to the central nervous system (CNS) is an important issue. It is not clear how prions pass through the intestinal mucosa after oral uptake. M cells, which are portals for antigens and pathogens (Hathaway & Kraehenbuhl, 2000), may be involved in the transepithelial transport of prions (Heppner et al., 2001). Thus, the infec- tious agent may penetrate the mucosa through M cells and reach the Peyer’s patches. Although prion diseases are neurological disorders, critical events in their pathogenesis take place in restricted sites outside the nervous system, especially in peripheral lymph organs (Aucouturier et al., 2000). Bovine spongiform encephalopathy (BSE) was recognized as a cattle prion disease during the 1980s (Wilesmith et al., 1988) in the UK. Ingestion of foods contaminated with BSE is the likely cause of the new variant Creutzfeldt–Jakob disease in humans (Bruce et al., 1997; Hill et al., 1997). Several studies indicate that, to date, the BSE agent has been found only in the brain, spinal cord and retinal (eye) tissue of BSE-diseased cattle. Infectivity assessment in several tissues from orally inoculated cattle, using bioassays based on RIII mice (Wells et al., 1994, 1998), revealed BSE infectivity in the CNS, all brain regions, the spinal cord, the optic nerve, the retina (neuronal cells) and the facial and sciatic nerves, as well as in distal ileum and bone marrow. The skeletal muscles, spleen and other lymphatic tissues were shown to be free of detectable infectivity. More recently, Wells et al. (2005) showed infectivity in tonsil tissue from cattle killed 10 months after oral BSE challenge by intracerebral inoculation in cattle. These finding are in contrast to the spreading of the scrapie agent in infected sheep, mice and hamsters in tissues such as spleen, other lymphatic tissues, muscles etc., even during the preclinical stage (Bosque et al., 2002; Heggebo et al., 2003; Thomzig et al., 2003, 2004). In addition, PrP Sc can be found in the lymphoreticular system and is not restricted to the nervous system following oral inoculation of sheep and primates with the BSE agent (Bons et al., 1999; Jeffrey et al., 2001; Herzog et al., 2004; Andreoletti et al., 2006). Recently, experiments in transgenic mice overexpressing bovine PrP confirmed the essential restriction of infectivity to the nervous system in terminally BSE-diseased cattle (Buschmann & Groschup, 2005). The distribution of BSE infectivity in asymptomatic cattle incubating the disease and its progression through the silent period from inoculation to the appearance of clinical Journal of General Virology (2007), 88, 1379–1383 DOI 10.1099/vir.0.82647-0 0008-2647 G 2007 SGM Printed in Great Britain 1379