Using oral fluids samples for indirect influenza A virus surveillance in farmed UK pigs

Influenza A virus (IAV) is economically important in pig production and has broad public health implications. In Europe, active IAV surveillance includes demonstration of antigen in nasal swabs and/or demonstration of antibodies in serum (SER) samples; however, collecting appropriate numbers of individual pig samples can be costly and labour-intensive. The objective of this study was to compare the probability of detecting IAV antibody positive populations using SER versus oral fluid (OF) samples. Paired pen samples, one OF and 5-14 SER samples, were collected cross-sectional or longitudinally. A commercial nucleoprotein (NP)-based blocking ELISA was used to test 244 OF and 1004 SER samples from 123 pens each containing 20-540 pigs located in 27 UK herds. Overall, the IAV antibody detection rate was higher in SER samples compared to OFs under the study conditions. Pig age had a significant effect on the probability of detecting positive pens. For 3-9-week-old pigs the probability of detecting IAV antibody positive samples in a pen with 95% confidence intervals was 40% (23-60) for OF and 61% (0.37-0.80) for SER (P = 0.04), for 10-14-week-old pigs it was 19% (8-40) for OF and 93% (0.71-0.99) for SER (P < 0.01), and for 18-20-week-old pigs it was 67% (41-85) for OF and 81% (0.63-0.91) for SER (P = 0.05). Collecting more than one OF sample in pens with more than 25 less than 18-week-old pigs should be further investigated in the future to elucidate the suitability of OF for IAV surveillance in herds with large pen sizes.