Molecular epidemiology of fowl adenoviruses in Greece

Outbreaks of inclusion body hepatitis (IBH) and adenoviral gizzard erosion have been anecdotally reported in Greece since approximately 2011. However, a relevant increase in clinical outbreaks compatible with IBH has been described since 2014. Unfortunately, with limited exceptions, only serological assays were performed, and involved strains were not properly characterized. In the present study, 35 outbreaks were investigated in the period between July 2017 and February 2018 in Greece. In addition to clinical and histopathological diagnosis, fowl adenovirus (FAdV) presence was investigated by PCR and sequencing. Thirty-four out of 35 samples tested FAdV positive. Twenty-nine (85.29%) and 5 (14.71%) strains were classified as FAdV-E and FAdV-D, respectively. Fowl adenovirus-E strains were genetically homogeneous and formed an independent cluster of Greek-only sequences, including the sole previously available sequence, suggesting the prolonged circulation of this species in Greece. On the contrary, FAdV-D strains were more heterogeneous and closely related to strains sampled in other European countries, testifying the occurrence of multiple introduction events. The evaluation of phylogenetic relationships, geographic clustering, age of infection, and origin of the broiler breeder flocks suggests that both vertical and horizontal transmission are important in FAdV epidemiology in Greece and highlights the limited efficacy of currently implemented control measures. Of note, a significantly higher mortality was observed in precociously infected flocks, likely because of the higher susceptibility of younger animals. This evidence stresses the need of preventing vertical and/or early infection to limit the economic impact of adenovirus-induced diseases.