Coinfection of goose parvovirus (GPV) and duck circovirus (DuCV) occurs commonly in field cases of short beak and dwarfism syndrome (SBDS). However, whether there is synergism between the two viruses in replication and pathogenicity remains undetermined. We established a coinfection model of GPV and DuCV in Cherry Valley ducks. Tissue samples were examined histopathologically. The viral loads in tissues were detected by qPCR, and the distribution of the virus in tissues was detected by immunohistochemistry (IHC). Coinfection of GPV and DuCV significantly inhibited growth and development of ducks, and caused atrophy and pallor of the immune organs and necrosis of the liver. GPV and DuCV synergistically amplified pathogenicity in coinfected ducks. In the early stage of infection, viral loads of both pathogens in coinfected ducks were significantly lower than those in monoinfected ducks (P < 0.05). With the development of the infection process, GPV and DuCV loads in coinfected ducks were significantly higher than those in monoinfected ducks (P < 0.05). Extended viral distribution in the liver, kidney, duodenum, spleen, and bursa of Fabricius was consistent with the viral load increases in GPV and DuCV coinfected ducks. These results indicate that GPV and DuCV synergistically potentiate their replication and pathogenicity in coinfected ducks.

Repeated incursions of highly pathogenic avian influenza virus (HPAIV) H5 subtype of Gs/GD lineage pose a serious threat to poultry worldwide. We provide a detailed analysis of the spatio-temporal spread and genetic characteristics of HPAIV Gs/GD H5N8 from the 2019/20 epidemic in Poland. Samples from poultry and free-living birds were tested by real-time RT-PCR. Whole genome sequences from 24 (out of 35) outbreaks were generated and genetic relatedness was established. The clinical status of birds and possible pathways of spread were analysed based on the information provided by veterinary inspections combined with the results of phylogenetic studies. Between 31 December 2019 and 31 March 2020, 35 outbreaks in commercial and backyard poultry holdings and 1 case in a wild bird were confirmed in nine provinces of Poland. Most of the outbreaks were detected in meat turkeys and ducks. All characterised viruses were closely related and belonged to a previously unrecognised genotype of HPAIV H5N8 clade 2.3.4.4b. Wild birds and human activity were identified as the major modes of HPAIV spread. The unprecedentedly late introduction of the HPAI virus urges for re-evaluation of current risk assessments. Continuous vigilance, strengthening biosecurity and intensifying surveillance in wild birds are needed to better manage the risk of HPAI occurrence in the future.