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Background Per- and polyfluoroalkyl substances (PFASs) are environmentally persistent and bioaccumulative chemicals. Immunomodulation is among the most concerning of toxic effects linked with PFAS exposure in mammalian models. However, no studies had yet shown this to be true in birds. Thus, we designed and conducted the first study to determine if PFASs could cause immunomodulation in birds. Secondly, we wanted to determine the effects on an avian host when exposed not only to immunomodulating chemicals, but also to a viral challenge. The aim, to determine if PFAS mediated immunmodulation functionally affects a pathogen challenge for a host. As innate immune system signalling pathways initiate crucial responses against a pathogen challenge, and are lesser studied than their adaptive counterparts, we focused on these pathways. To provide the first information on this, an in vitro experiment was designed and performed using chicken embryo fibroblasts exposed to perfluorooctane sulfonate (PFOS) (22 ppm) and immune markers characterised before and after being infected with gallid herpesvirus-2 (GaHV-2). Results The expression of two pro-inflammatory cytokines, namely interleukin 8 (IL-8) and tumor necrosis factor alpha (TNF-α), the nuclear factor ‘kappa-light-chain-enhancer’ of activated B-cells (NF-κB), and the anti-inflammatory cytokine interleukin 4 (IL-4) were investigated in various scenarios. These results showed that exposure to PFOS decreased immune gene expression in chicken fibroblasts from 36 h post-exposure. Next, it was shown that this decrease could be mitigated by infection with gallid herpesvirus-2, which increased gene expression back to the baseline/control levels. Conclusions Not only is this the first study to provide the expected evidence that PFOS has immunomodulatory potential in birds, it also provides unexpected data that virus infections can mitigate this negative effect. Thereby, further research, including in vivo and in situ studies, on the impact of PFOS on host-virus interactions is now warranted, as it has been overlooked and might contribute to our understanding of recent disease outbreaks in wildlife. The mechanisms by which gallid herpesvirus mitigates immunomodulation were beyond the scope of this study, but are now of interest for future study. Electronic supplementary material The online version of this article (10.1186/s12917-019-1953-2) contains supplementary material, which is available to authorized users.

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