Influence of antibody-mediated immune suppression on clinical, viral, and immune responses to swine influenza infection.

Antibody-mediated immune suppression occurred when newborn pigs with naturally acquired passive antibody were exposed to seine influenza virus. Frequency and relative ease of recovery of virus from nasal secretions were inversely related to the concentration of specific passive antibody existing at time of exposure. Severe overt respiratory signs during the acute stages of the disease were observed only in pigs with low passive antibody concentrations. The concentration of passive antibody at the time of exposure determined the immune status of the pig during the convalescent stage of disease. Infection could occur in the presence of high passive antibody concentrations, but the pig was not immunologically stimulated. Reexposure after the decay of passive antibody produced primary immune respone, severe clinical reinfection, and recovery of virus from nasal secretions for a period of time similar to that seen in pigs having their first exposure. Infection of newborn pigs with low passive antibody concentrations led to immunologic priming. A second exposure to virus produced a secondary immune response, mild clinical disease, and shortened time during which virus was recovered from nasal secretions. The relevance of these studies for the practice of vaccination or infections of the dam before parturition so that the neonate will have specific passive immunity is discussed.