Abstract: 【Objective】 Duck hepatitis A virus genotype 3 (DHAV-3) was one of the primary pathogens causing acute lethal hepatitis in ducklings,with pathogenicity exhibiting significant age-dependent characteristics.This study aimed to elucidate the regulatory mechanisms of bile secretion pathway in ducklings of different ages infected with DHAV-3 and reveal the impact of host age factors on the viral pathogenic process.【Method】 Z2 strain Pekin ducks were used as the experimental subjects,divided into three experimental groups of 7,14 and 21 days of age (50/group),along with corresponding control groups (15/group).The ducklings in infected groups were administered 0.2 mL DHAV-3 (10^-8.83 ELD₅₀/0.2 mL) via intramuscular injection,while the ducklings in control groups were injected with an equivalent volume of PBS.The ducks were raised separately until 18 hours post-infection,at which point blood,liver and ileum samples were collected.The following parameters were analyzed:Plasma biochemical indices,viral load,total bile acid (TBA) content,and the expression of bile secretion pathway-related genes.【Result】 The viral load in liver of ducklings in all age groups was significantly higher than that in ileum (P＜0.05),and the viral load in the liver of 7-day-old ducklings was significantly higher than that of 14- and 21-day-old ducklings (P＜0.05).The activity of aspartate aminotransferase (AST) and the level of total cholesterol (CHOL) in the plasma of ducklings showed a significantly decrease with increasing age in control group (P＜0.05).After DHAV-3 infection,the CHOL levels in the plasma of 7-day-old ducklings in infected group were significantly reduced compared to control group (P＜0.05),while they were significantly increased at 14 and 21 days of age (P＜0.05),accompanied by a significant increase in immunoglobulin Y (IgY) levels (P＜0.05).Bile acid metabolism displayed tissue-specific regulation.In control group,TBA level in liver decreased with advancing age,while in ileum,it increased with increasing age.Following DHAV-3 infection,the TBA in liver and ileum of ducklings in the 7-day-old infected group were significantly decreased (P＜0.05).In the 14-day-old group,a reverse regulation pattern was observed,with liver TBA level increasing and ileal TBA level decreasing (P＜0.05).In the 21-day-old group,both liver and ileum TBA levels significantly increased (P＜0.05).The bile secretion pathway-related genes cholesterol-7α-hydroxylase (CYP7A1),ATP binding cassette transporter G5 (ABCG5) and ABCG8 were significantly upregulated with advancing age in liver of ducklings in control group (P＜0.05).After DHAV-3 infection,CYP7A1 gene transcription levels in the 14- and 21-day-old ducks were significantly decreased (P＜0.05),while ABCG5 and ABCG8 genes were significantly upregulated in ileum of the 7- and 14-day-old ducks (P＜0.05) and downregulated in liver at 14-day-old (P＜0.05).Farnesoid X receptor (FXR) gene expression in liver of ducklings in control group was significantly increased with advancing age (P＜0.05),while in ileum,it significantly decreased (P＜0.05).After infection,FXR gene expression in liver of 21-day-old ducklings and FXR gene expression in ileum of 14-day-old ducklings were significantly downregulated compared to control group (P＜0.05).【Conclusion】 This study described the age-dependent characteristics of host metabolism and immune response in Pekin ducks infected with DHAV-3,and for the first time,elucidated that DHAV-3 infection induced age-dependent pathological damage by interfering with the bile secretion pathway.These findings provided new insights into the metabolic-immune cross-regulation of age susceptibility in duck viral hepatitis and suggest that the bile acid negative feedback pathway mediated by nuclear receptors might be a key clue in exploring the age-dependent mechanisms of DHAV-3 infection.

Key words: duck; age; DHAV-3; gut-liver axis; bile acid