Evidence of possible vertical transmission of duck hepatitis A virus type 1 in ducks

Duck hepatitis A virus (DHAV) causes a highly contagious and acute disease in ducklings younger than 3 weeks of age and spreads rapidly by horizontal transmission to all susceptible ducklings in the flock. To date, there is no evidence of vertical transmission of DHAV-1. In a previous study, we identified a novel DHAV type 1 (DHAV-1) isolate that could infect adult ducks and induce laying drop. In this study, 30 non-embryonated duck eggs and 60 17-day-old embryos were collected from three breeding duck flocks with egg drop syndrome caused by DHAV-1 in China, and 30 17-day-old embryos were randomly selected from the 60 embryos and allowed to hatch. DHAV-1 RNA was detected by RT-PCR in 10 of 30 non-embryonated eggs, 9 of 30 17-day-old embryos, 5 of 7 dead embryos and 5 of 23 newly hatched ducklings. Overall, 29 of 90 (32.2%) eggs and embryos were positive for DHAV-1. Three DHAV-1 strains were isolated from the dead duck embryos of the three breeding duck flocks, respectively. Pathogenicity studies showed that the three DHAV-1 isolates had median embryo lethal doses but were highly pathogenic to healthy ducklings. Compared with the DHAV reference strains, there were two specific amino acid mutation sites (F169 and S220) in VP1 of the three isolates. To the best of our knowledge, this is the first report that DHAV-1 is isolated from duck embryos. The findings provide evidence of possible vertical transmission of DHAV-1 from breeding ducks to ducklings.