Xenotransplantation and the potential risk of xenogeneic transmission of porcine viruses

The clinical success of allotransplantation and the shortage of donor organ s have led to a proposal for the use of animal organs as alternative therap eutic materials for humans. In that regard, swine are preferable to non-hum an primates as a source of donor organs. While applications for clinical tr ials for xenotransplantation have not yet been received in Canada, several trials have already been authorized in the United States. A major concern, however, is the potential for xenogeneic transmission of viruses from anima ls to humans via organ, tissue, or cellular transplantation or via ex vivo exposure of humans to porcine biologic materials. Xenotransplantation allow s viruses to bypass the normal immunological defense mechanisms of the reci pient. Furthermore, the use of immunosuppressive drugs following transplant ation may facilitate the xenogeneic transmission of zoonotic agents. Of por cine viruses, swine hepatitis E virus does not cause any clinical symptoms in the natural host but is a likely zoonotic agent that can infect humans a nd cause hepatitis. Porcine circovirus type 1 is prevalent in swine populat ions with no known association with clinical disease, while circovirus type 2 causes post-weaning multi-systemic wasting syndrome. Porcine endogenous retrovirus is integrated into the host chromosomes while porcine cytomegalo virus undergoes latent infection. Two additional porcine herpesviruses have recently been identified in swine and have been named porcine lymphotrophi c herpesviruses. These herpesviruses can potentially become reactivated in human recipients after xenotransplantation. All in all, there are a number of viruses in swine that are of primary concern to screen and eliminate fro m xenotransplantation protocols. Epidemiology and the current knowledge on xenogeneic risk of these viruses are discussed.