Experimental production of congenital persistent swine fever infections: II. Effect on functions of the immune system

Twelve pregnant sows were infected at various stages of gestation with a low-virulent field strain of swine fever (SF) virus. The sows developed neutralizing antibody in serum, colostrum and milk.Only one pig had antibody to SF virus at birth. Twenty-three congenitally infected pigs developed a persistent viraemia. In the plasma samples of these pigs, neither antibodies nor virus-antibody complexes were detected after the disappearance of maternal antibodies.The persistently infected pigs showed a normal antibody response against sheep red blood cells, except in the terminal stage of disease. These observations indicate immunological tolerance to SF virus in these pigs. Immunological tolerance was also induced in pigs which had been infected in utero after the onset of immune competence.The lymphocyte response to phytohaemagglutinin seemed to be slightly depressed in the persistently infected pigs, whereas the response to pokeweed mitogen was comparable with that of control pigs.A clear, cell-mediated immune response to SF virus could not be demonstrated by the lymphocyte stimulation test.Three pigs that were born uninfected did not produce neutralizing antibody following natural exposure from in-contacts, whereas the littermates did. These pigs appeared to be sensitized when challenged with virulent SF virus.