**8. Hepatitis B Virus (HBV)**

Acute infection with HBV can result in fulminant hepatic failure, whereas chronic HBV infection can lead to end-stage liver disease, including cirrhosis and hepatocellular carcinoma. Understanding of the natural history and basic biology of HBV has increased greatly in recent years. HBV infection is by far the most common chronic viral infection affecting the liver [119]. Reactivation of HBV replication in patients undergoing immunosuppressive therapy is well recognized and is a frequently reported complication of considerable clinical importance [120, 121]. HBV reactivation following immunosuppression is defined by an abrupt rise in HBV replication followed by laboratory signs of hepatocellular injury in "silent" HBV-infected individuals (HBsAg carriers). Reactivation can also occur at a lower rate in patients with "occult" HBV infections. The clinical presentation of reactivation is variable, ranging from an asymptomatic course to severe hepatitis, liver failure, and death. It is most frequently observed in patients with lymphoma treated with rituximab and corticosteroids, as well as in patients undergoing stem cell and bone marrow transplantation. Others risk groups include patients with solid tumors, subjects infected with HIV, organ transplant recipients, and those with autoimmune diseases [122, 123]. It is believed that about 12% of patients with malignancy have chronic HBV infection. In transplanted patients, infection can also reactivate after immuno‐ suppressive therapy. For these reasons, high-risk individuals should be identified and screened. Recommendation for screening for all three serologies, including HBcAb, HBsAg, and HBsAb in those planned for immunosuppression is available [124]. Despite advances in treatment of chronic HBV infection, liver transplantation remains the only hope for many HBVrelated end-stage liver disease patients. The high rate of HBV reinfection or recurrence after liver transplantation is probably due to enhanced virus replication resulting from immuno‐ suppression and other mechanisms. In the recent years, liver transplantation has shown encouraging results. The introduction of effective measures to prevent and treat reinfection or recurrence using strategies involving hepatitis B immune globulin (HBIG) and subsequently nucleos(t)ide analogues have significantly improved the outcome of liver transplantation [125, 126]. Overall HBsAg positive patients who are candidates for chemotherapy or treatment with biological agents, preemptive treatment with an antiviral agents such as lamivudine, and lately with the more potent tenofovir, entecavir, or adefovir, has become a standard of care, effec‐ tively preventing HBV reactivation. Patients with occult HBV should be monitored for alanine aminotransferase and HBV DNA (by real-time PCR) during the course of immunosuppression. Prompt administration of a potent antiviral agent upon diagnosis of reactivation may be lifesaving in such patients [122].
