Development of Early Humoral and Cell-Mediated Immunity in Piglets with Experimentally Induced Subclinical Swine Influenza

Development of early immune response in piglets with subclinical swine influenza was investigated. Fourteen, seronegative piglets were used. Ten of them were infected intranasally with swine influenza virus (SIV) H1N1 subtype. Temperature and clinical signs were assessed daily. Leukocyte proportions and concentrations were analysed on a haematology analyser. Antibodies against SIVs were measured by haemagglutination inhibition assay. To measure influenza-specific cell-mediated immunity (CMI), the proliferation assay was performed. The real time reverse transcription PCR method was used for detection of SIV. No relevant respiratory or systemic clinical signs were observed. The presence of SIV RNA in nasal swabs from all infected piglets was confirmed between 2 and 5 dpi. The overall number of leukocytes did not differ during the study. The number of medium-sized cells (MID) was significantly higher on 2 and 4 dpi, as compared to day 0 level. The percentage of lymphocytes decreased from 74% on day 0 to 67.06% on 4 dpi, while the percentage of MID significantly increased at the same time. In control pigs no significant changes were observed. All infected pigs exhibited specific antibodies between 7 and 10 dpi. Specific CMI was observed before specific antibodies were present. Results of our research indicate that kinetics of the humoral and CMI response during subclinical infection is similar to that observed in clinical form of the disease.