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Introduction
The liver contains great functional reserve capacity, which is important in domestic animals subjected to high production requirements. This organ adapts easily to different conditions by increasing the intensity of its functions. The liver has to cope with many challenges, including high energy level feed, the addition of chemotherapeutics, coccidiostats and other compounds, whereby metabolites must be maintained in equilibrium by hepatic homeostasis. Viral hepatitis in poultry is an important disease caused by a number of viruses including avian hepatitis E virus (avian HEV), duck hepatitis B virus (DHBV), duck hepatitis A virus (DHAV), duck hepatitis virus Types 2 and Type 3, fowl adenoviruses (FAdV), and turkey hepatitis virus (THV) ( Table 1 , Figure 1 ). DHBV or avihepadnaviruses, part of the family Hepadnaviridae, have been isolated from ducks, snow geese, white storks, grey herons, cranes, and parrots. DHBV evolved with the host as a non-cytopathic form without clinical signs and rarely progresses to chronicity. The outcome for DHBV infection varies by the host's ability to elicit an immune response and is dose and age dependent in ducks, thus mimicking the pathogenesis of human hepatitis B virus (HBV) infections and providing an excellent animal model for human HBV. DHAV is a picornavirus that causes a highly contagious virus infection in ducks with up to 100% flock mortality in ducklings less than six weeks of age, while older birds remain unaffected. Resulting high morbidity and mortality has an economic impact on intensive duck production farming. Duck hepatitis virus Types 2 and 3 are astroviruses, belonging to the family of Astroviridae, being similar phylogenetically to turkey astroviruses, and therefore has the potential for cross-species infections between strains. While these avian hepatitis viruses have the same target organ, the liver, they are very different in their biology and pathogenesis, as each belongs to a different family (Yugo et al., 2016). Therefore, understanding the common and unique features of these hepatitis viruses in poultry will aid the diagnosis and differential diagnosis of viral hepatitis in poultry and help develop more effective flock management and preventive and control measures. The focus of this review is on viral hepatitis infections in chicken and turkeys, with emphasis on HEV, FAdV and THV.
Table 1
Etiology of viral...