Background
The epidemiology of swine influenza A virus (swIAV) has undergone a change from causing mainly epidemic infections to continuous endemic circulation within herds. Several studies have traced the within-herd swIAV transmission pattern. However, studies with more frequent samplings of targeted age groups are needed to increase the current understanding of virus transmission dynamics. In addition, data on the efficacy of different intervention methods for reducing swIAV spread are highly relevant for disrupting endemic swIAV infections. Therefore, this study aimed to investigate the weekly influenza infection dynamics and evaluate subsequent interventions, including vaccination and management changes.

Method
Two batches of pigs in two sow herds (Herd 1 and 2) were sampled with nasal swabs and blood samples from one week of age to the end of the nursery period. In each batch, ten sows were included with each six piglets including three cross-fostered and three of the sows own piglets. Nasal swabs were tested for the presence of swIAV, and blood samples were tested for swIAV antibodies using ELISA and HI-test. In Herd 2, interventions including pre-farrowing vaccination, limited cross-fostering, litter-wise weaning and disinfection of tools and hands were implemented during a second round of sampling in two batches as described above.

Results
In both herds, an endemic presence of swIAV was observed, with Herd 1 having one circulating swIAV subtype (H1C.2.4N2G) circulating in the farrowing unit and Herd 2 having two swIAV strains (H1C.2.4N2G and H1N1pdm09) circulating both before and after weaning. Maternally derived swIAV antibodies were successfully transferred from sow to piglets, but despite high levels, swIAV infections was observed in the piglets down to one week of age and a lack of subsequent seroconversion was observed. The implementation of pre-farrowing vaccination increased the level of swIAV antibodies in sows and piglets but had no obvious effect on viral shedding and clinical disease.

Conclusions
The endemic state of swIAV infections in sow herds was clearly illustrated in this study along with the difficulties in controlling swIAV through vaccinations and management changes in an environment that favours virus circulation.