Comparison of individual, group and environmental sampling strategies to conduct influenza surveillance in pigs

Background: Influenza A virus (IAV) is an important pathogen in pigs that affects productivity and has important public health implications because of its zoonotic nature. Surveillance is central to the control of influenza, however, detection of IAV infections can be challenging in endemically infected herds with low prevalence of infection. Methods: In groups of suckling (18-21 days of age) and growing (35-45 days of age) pigs, we compared various sampling approaches to detect, isolate and sequence IAV using individual (nasal swabs, nasal wipes and oropharyngeal swabs), group (oral fluids, surface wipes and sow udder skin wipes) and environmental (airborne particles deposited on surfaces and air samples) sampling approaches. All samples were tested by IAV rRT-PCR and a subset was used for virus isolation and direct sequencing. Results: In general, environmental and group samples resulted in higher odd ratios (range = 3.87-16.5, p-value < 0.05) of detecting a positive sample by rRT-PCR compared to individual pooled samples, except for oropharyngeal swabs (OR = 8.07, p-value < 0.05). In contrast, individual samples were most likely to yield a viral isolate by cell culture. Oropharyngeal swabs in suckling pigs (78.4%), and nasal swabs (47.6%) or nasal wipes (45%) in growing pigs, and udder wipes in lactating sows (75%) were the preferred samples to obtain an isolate. Conclusions: Our findings indicate that group and environmental sampling strategies should be considered in influenza surveillance programs in particular if the goal is just to detect infection. This study provides new information on sampling approaches to conduct effective influenza surveillance in pigs and identifies udder wipes from lactating sows as a novel sample type that offers a convenient, cheap and sensitive manner to monitor IAV in litters prior to weaning.