Xenozoonoses: the transmission of infectious agents from one species to another

4. Over sixty porcine infectious agents with a potential to cause disease in humans have been identified.(³⁸) Development of "clean" lines of source animals, with a certified health status, is under way.(³⁹) Control measures include the birth of pigs by hysterotomy (caesarean derived), carefully controlled environments and routine monitoring of pigs and their handlers. These steps appear to have excluded almost all known infectious agents of concern. However, it cannot be ruled out that an unknown porcine virus might exist which causes no pathology in pigs but which may cause disease in humans.

As is true for all other mammalian species, pigs have sequences in their DNA that encode retroviruses (PERV -Porcine Endogenous RetroViruses).(⁴⁰) Weiss and colleagues showed that pig retroviruses could infect human cells in vitro.(⁴¹) There are no satisfactory animal models to test the pathogenicity of these agents. The blood of 160 patients exposed to living pig tissues was studied for the presence of PERV. In 135 patients exposure was for only one hour or a little more. In a few of the remaining patients exposure was for longer periods, in one case for 460 days. None of the patients showed evidence of PERV infection, although pig cells containing retroviral sequences were found even several years after exposure to the pig tissue. It is a matter open to conjecture the extent to which one can take comfort from negative results in persons exposed for such short period of time, except for a few cases, and in any event to very few pig cells, as compared with the years of exposure that would presumably occur in an organ were it successfully transplanted into a human (⁴²). Certainly, the elimination from pigs of all PERV, which represents a continuing concern and hinders the move to clinical trials, will remain a challenge for years to come.