Inclusion body hepatitis as a primary disease in commercial broiler chickens

Abstract

Inclusion body hepatitis (IBH) has been occurring as an economically important, emerging disease of broiler chickens in several countries. Historically, IBH has been identified as a secondary disease, often associated with common immunosuppressive diseases. However, few studies have identified IBH as a primary disease with no apparent association with immunosuppressive diseases. The objectives of this study were to develop an animal model of IBH in commercial broilers, to demonstrate vertical transmission of fowl adenoviruses (FAdVs) in broiler breeders and to control IBH in broilers by vaccinating their parents with an inactivated FAdV vaccine. In order to develop an animal model of IBH in commercial broilers, fourteen-day old broilers were inoculated intramuscularly with 1x104 – 1x107 CCID50 of either FAdV x11a-like virus, two strains of FAdV-8a (FAdV-8a strain TR-59 and FAdV-8a strain T8-A) or FAdV-11strain 1047. Four days following FAdV inoculation, 5% - 15% mortality was observed and dead birds showed histologic lesions of hemorrhagic necrotizing hepatitis. This animal model reproduced the clinical disease, and pathological lesions of IBH that have been described in commercial broilers. In order to demonstrate vertical transmission of the FAdV, 35-week-old broiler breeders were inoculated with 1x106 CCID50 of either FAdV x11a-like virus, FAdV-8a strain TR-59, FAdV-8a strain T8-A or FAdV-11 strain 1047. Eggs from infected breeders were collected and hatched seven days post-inoculation. Clinical signs or mortality were not observed in parents; however broiler progeny derived from broiler breeders inoculated with FAdV-8a strain T8-A had 30% IBH mortality by seven days of age. The hexon gene loop 1 sequence of the virus isolated from affected broiler progeny showed 100% identity to FAdV-8a strain T8-A. In order to demonstrate protection of broilers against IBH by vaccination of their parents, four groups of broiler breeders were vaccinated with either FAdV-8a strain T8-A (2x107 or 2x104 CCID50) formulated with 20% oil-in-water emulsion, or FAdV x11a-like virus (2x107 or 2x104 CCID50) formulated with 20% oil-in-water emulsion at the age of 12 and 15 weeks. The control group was inoculated with 20% oil-in-water emulsion. Broiler progeny were challenged with FAdV-8a strain T8-A to study the immunoprotective effect of the vaccine. Although, survival of broilers following FAdV-8a strain T8-A challenge was not significantly different among vaccinated and non-vaccinated groups (P>0.05), immunoprotective effect was enhanced by the increase dose of FAdV antigens (P>0.05). Further studies are necessary to improve the vaccine efficacy to protect broilers against IBH.

In conclusion, the results of this study support the hypothesis that IBH in broilers in Canada is a vertically-transmitted primary disease with no known immunosuppressive involvement. The results also demonstrated that inactivated antigens of FAdV are able to partially protect broilers against IBH by vaccinating their parents. Further studies with different formulations, and priming the immune system of broiler breeders with live FAdV prior to vaccination with inactivated FAdV vaccines are necessary to improve the efficacy of inactivated IBH vaccine.