Isolation, molecular and biological characterization of Fowl Adenovirus-D serotype 2 from infected commercial broilers in Egypt during 2024

Fowl adenovirus (FAdV) is a significant pathogen in poultry causing considerable economic losses worldwide, particularly in broiler farms. This study aimed to detect, isolate, and characterize FAdVs from suspected outbreaks in commercial broiler flocks in Beheira and Menoufia governorates, Egypt, during 2024. Fifteen broiler flocks exhibiting gross lesions of inclusion body hepatitis (IBH) were sampled. Cloacal swabs and liver tissues were collected and processed for virus detection using polymerase chain reaction (PCR) targeting the hexon gene. Positive samples were further subjected to virus isolation in chicken embryo liver (CEL) cells and propagation in specific pathogen-free embryonated chicken eggs (SPF-ECEs). Sequence and phylogenetic analyses were performed to determine genotype (serotype) and assess genetic relationships. Results confirmed the presence of FAdV in 9 out of 15 flocks. Two representative isolates, Chicken-Egypt/Behera/2024 (GenBank accession No. PV243151) and Chicken-Egypt/Mounofia/2024 (GenBank accession No. PV243152), were identified as FAdV serotype 2 (species D) and showed 99.1–99.2% sequence identity with Chinese strains. Phylogenetic analysis clustered them closely with global FAdV-D strains. Typical cytopathic effects observed in CEL cell cultures, and embryonic lesions supported their pathogenicity. The virus titers were determined as 106.1 TCID₅₀/0.1 mL and 102.2 EID₅₀. These findings highlight the persistence of FAdV serotype 2 strains in Egypt and underscore the need for continuous monitoring and vaccine efficacy evaluations to improve its control measures.