This chapter discusses the enteric viral infections of pigs and strategies for the induction of mucosal immunity. Enteropathogenic viruses—such as transmissible gastroenteritis virus (TGEV) and rotavirus—replicate and induce lesions only in the gastrointestinal tract. The susceptible target cell is the villous enterocyte. Thus, active immunity against enteropathogenic viral infections depends on stimulation of local immune responses within the intestine. To date, only limited success has been achieved in the development of oral vaccines to prevent neonatal viral diarrheas, and commercial vaccines show limited efficacy in the field. Currently, only oral vaccines containing live replicating organisms have been highly effective in inducing mucosal immune responses, especially secretory, (S)IgA antibodies. The oral administration of soluble or killed antigens generally induces immunity of short duration or even systemic tolerance. Whether the problems encountered with oral administration of soluble protein antigens can be overcome by the use of improved mucosal adjuvants (such as muranyl dipeptide, immune stimulating complexes [ISCOMs], cholera or Escherichia coli enterotoxins, avridine, proteosomes, and cytokines] or new and novel delivery systems (liposomes, live recombinant vectors, microspheres, DNA plasmids, virus-like particles) requires further investigation, and specific examples are given in this chapter.
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