Viral infections transmitted by blood and its products.

Modern transfusion practice is associated with an increased risk of transmitting viral agents because of the changing nature of the patients and of the therapeutic blood products. More immunosuppressed patients are receiving blood released faster and with more elaborate blood components. In addition to the classically recognized importance of hepatitis B virus (itself disseminated most efficiently by contamination of products derived from large pools of plasma containing many donations) other agents are assuming increasing importance. They frequently display one or more of the predisposing characteristics of prolonged viraemia, inapparent infections and a carrier or latent state. Some of these infections like cytomegalovirus and the human T-cell leukaemia virus are transmitted only by the cellular component of blood. Others like B and non-A, non-B hepatitis and the putative agent(s) of the newly recognized acquired immune deficiency syndrome can also be transmitted in the plasma or its products. Not all the agents transmitted cause severe illness, however; human parvovirus appears to cause no clinical illness when transmitted by transfusion and infections with non-A, non-B hepatitis are largely detected only by elevations in transaminase levels. Screening tests for the presence of these agents in donor blood or for evidence of infection by them in donors continue to be studied. Other approaches, related in particular to the selection of donors, are becoming increasingly important where serological screening tests are not available.