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. Because of public health concerns, some geographic areas have IAV monitoring in swine initiated. In Europe, active IAV surveillance includes demonstration IAV RNA in nasal swabs or oral fluids and/or demonstration of antibodies in serum (SER) samples; however, collecting appropriate numbers of individual pig samples can be costly and labor-intensive. The objective of this study was to compare the sensitivity and specificity of oral fluid (OF) and SER samples in detecting anti-IAV antibodies.

Twenty-seven commercial pig herds located in the UK were included in this study. Paired SER and OF samples were collected from all farms; 70.4% (19/27) farms were sampled on one occasion only and the remaining 29.6% (8/27) of the farms were visited at the time of weaning until around 12 weeks of age in approximately two week intervals. While OFs were collected at each time point, SER samples were collected during the last visit only. A commercial nucleoprotein (NP)-based blocking ELISA was used to test 244 OF and1004 SER samples from 123 pens each containing 20-450 pigs. At least two OF samples were selected per farm for detection of IAV matrix gene RNA by rRT-PCR (n = 92).

There was a strong positive correlation (r = 0.7584; P < 0.0001) between the average pen SER and the OF sample to negative (S/N) ratio. The number of pens classified as IAV seropositive with at least one positive SER or OF sample, was higher for SER than for OF (66.9% vs. 37.1%, P < 0.001). The overall agreement between qualitative ELISA data between paired samples was moderate (κ = 0.43) and increased with the increase of within-pen positive SER samples (P < 0.05). The highest agreement occurred for pens in which more than 60% of the SER samples were IAV antibody positive. While anti-IAV antibodies were detected in 62.9% (17/27) of the investigated farms, IAV RNA was only detected in 7.4% (2/27) of the farms.

Serological assays provide a number of benefits compared to molecular detection of IAV, the most important one being the ability to detect IAV exposure after active viral replication has ceased. Under the study conditions, SER samples provided a higher detection rate of IAV antibodies when compared to OFs. Collecting more than one OF sample in pens with more than 25 pigs should be considered in future to further elucidate the suitability of this sample type for IAV surveillance in herds with large pen sizes.

Disclosure of Interest: None Declared.