Local and systemic immune response in pigs during subclinical and clinical swine influenza infection

• We examined the immune response during subclinical and clinical swine influenza infection.
• Significantly higher titres of specific antibodies were found in IT-inoculated pigs.
• Changes of haematological parameters were observed only in IT-inoculated pigs.
• SIV induces local cytokine responses during both acute and subclinical infection.
• We hypothesise that viral load and the cytokines play a role in the induction of lung lesions.

Local and systemic immune responses in pigs intranasally (IN) and intratracheally (IT) inoculated with swine influenza virus (SIV) were studied. No clinical signs were observed in IN-inoculated pigs, while IT-inoculated pigs developed typical signs of influenza. Significantly higher titres of specific antibodies and changes of haematological parameters were found only in IT-inoculated pigs. Because positive correlations between viral titre, local cytokine concentration, and lung pathology have been observed, we hypothesise that both viral load and the local secretion of cytokines play a role in the induction of lung lesions. It could be that a higher replication of SIV stimulates immune cells to secrete higher amounts of cytokines. The results of the present study indicate that pathogenesis of SIV is dependent on both, the damage caused to the lung parenchyma directly by virus, and the effects on the cells of the host's immune system.