Virology Group, International Centre for Genetic Engineering and Biotechnology, New Delhi-110067, India.
Viral hepatitis is the cause of much morbidity and mortality worldwide. Five viruses have been identified as the etiologic agents for various forms of viral hepatitis. These are the hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis E virus (HEV) and the delta agent (HDV). Though all of these pathogens target the liver as the primary site of viral replication, they belong to different viral families and show distinct biology and pathogenesis.
The HAV, HCV and HEV are positive sense single-stranded RNA viruses belonging to the families Picornaviridae, Flaviviridae and an unclassified group, respectively. The HBV has a partially double-stranded DNA genome that replicates through a RNA intermediate and belongs to the Hepadna viridae family. The delta agent is a viroid that requires HBV helper functions to complete its replicative cycle. The transmission and pathogenesis of these viruses is also distinct. HAV and HEV are transmitted primarily by the enteric route, though contaminated food and water, and cause an acute self-limiting infection. In contrast, HBV and HCV are transmitted through blood products and sexually, and causes chronic infection in 20–80% of acutely infected persons. The delta agent depends upon HBV and therefore it only causes coinfection with HBV or superinfection of HBV carriers. Thus, the transmission dynamics of these viruses in the population are quite different.
Research on hepatitis viruses is hampered by the general lack of good in vitro culture systems and small animal models of disease. Vaccines and specific antiviral agents are few and far between. Current research is focused on these aspects as well as the basic biology of these important human pathogens.
