Abstract
Background:
Fowl adenovirus (FAdV) 8b causes huge economic losses in the poultry industry worldwide. Attenuated FAdV 8b could be useful in preventing FAdV infections globally and scale-up obstacles could be solved by bioreactor technology.
Aim:
This study was carried out to attenuate FAdV 8b isolate, propagate it in a bioreactor, molecularly characterize the passage isolates, and determine the immunogenicity, efficacy, and shedding of the virus of chickens.
Methods:
FAdV serotype 8b (UPM11142) isolate was passaged on chicken embryo liver cells (CEL) until attenuation and propagated in a bioreactor (UPM11142P20B1). Hexon and fibre genes of the isolates were sequenced and analysed. UPM11142P20B1 was administered to 116-day-old broiler chickens divided into 4 groups, A (control), B (non-booster), C (booster with UPM11142P20B1) and D (booster with inactivated UPM11142P5B1). Eight chickens from each group were challenged. Body weight (BW) and liver weight (LW), liver: body weight ratio (LBR), FAdV antibody titre, T lymphocyte sub-populations in the liver, spleen and thymus; and challenge virus load in the liver and shedding in cloaca were measured at weekly intervals.
Results:
The isolate caused typical cytopathic effects on CEL cells typical of FAdV. Novel molecular changes in the genes occurred which could be markers for FAdV 8b attenuation. BW, LW and LBR were similar among groups throughout the trial but the uninoculated control-challenged group (UCC) had significantly higher LBR than the inoculated and challenged groups at 35 dpi. Non-booster group had higher FAdV antibodies at all time points than the uninoculated control group (UCG); and the challenged booster groups had higher titre at 35 dpi than UCC. T lymphocytes increased at different time-points in the liver of inoculated chickens, and in the spleen and thymus as well, and was higher in the organs of inoculated challenged groups than the UCC. There was a significantly higher challenge virus load in the liver and cloaca of UCC chickens than in the non-booster chickens.
Conclusion:
UPM11142P20B1 was safe, efficacious, significantly reduced shedding and is recommended as a candidate vaccine in the prevention and control of FAdV 8b infections in broiler chickens.

Key words: Attenuation, Fowl adenovirus 8b, Humoral/cellular immunity, Vaccine, Virus shedding