Occult hepatitis B virus infection: a case of reactivation in a patient receiving immunosuppressive treatment for allogeneic bone marrow transplantation.

The presence of hepatitis B virus (HBV) genome in HBsAg-negative subjects is known as occult HBV infection1. This particular form of hepatitis, already recognised in the 1980s, has been confirmed and studied using molecular biology techniques. In fact, occult infection is usually associated with the presence of anti-HBc and anti-HBs, but given the relatively high percentage (approximately 20%) of subjects who are negative for all markers1, the introduction of a test to detect HBV DNA was fundamental. Occult infection is related in some cases to mutant viruses that are not detectable by the commonly used tests; it has also been observed that the reactivation of HBV related to variants of the viral genome often has an unfavourable clinical prognosis2,3. Much more frequently, however, an occult infection is associated with strong suppression of viral replication, which is responsible both for the negativity for HBsAg and the undetectable or very low levels of HBV DNA in the serum, although this latter can be found in liver tissue4,5. An occult infection can have a important impact in various different clinical settings, including transmission through blood transfusion or organ transplants6,7 and reactivation following immunosuppressive therapy. Indeed, it has been shown that the host's immune response, co-infections (e.g. with hepatitis C virus) and epigenetic factors all play significant roles in occult infection8. We present the case of a Georgian child negative for HbsAg who, after receiving an allogeneic bone marrow transplant and immunosuppressive therapy, was found to be positive for HBV. We then describe the investigations conducted in order to determine whether this was due to a new infection or reactivation of an occult infection.