[Viruses and unconventional transmissible agents: update on transmission via blood ]

The blood borne viruses must be separated into major and minor agents. Major viruses transmissible by blood transfusion are human immunodeficiency virus (HIV) and hepatitis B and C viruses (respectively HBV, HCV). The prevalence of virological markers in French blood donors has been continuously decreasing since the implementation of serological screening methods as soon as they were available. In 1998, the prevalences (per 10,000 donations) were 0.17 for antibody to HIV, 0.08 for antibody to human T-cell leukemia virus (HTLV), 2.23 for hepatitis B surface antigen (HBs Ag) and 2.52 for antibody to HCV. The values are, of course, higher in new donors when compared to regular donors: approximately five-fold for HIV, 50-fold for HCV and 300-fold for HBs Ag. The remaining major questions concern the residual risk due to infectious donations which could escape the preventive measures. It seems evident that the major risk is imputable mainly to donations collected during the window period. During the 1996-1998 period, the residual risk for HIV was 1 out of 1,350,000 donations, 0 for HTLV, 1 out of 375,000 for HCV, and 1 out of 220,000 for HBV. A few cases of "immunosilent" patients have been reported. They remain exceptional. The first data collected after the implementation of nucleic acid technology (NAT) confirm the very low residual risk. The recent introduction of leukodepletion probably brought an important contribution to diminishing the risk of transmission of leucotropic viruses such as cytomegalovirus (CMV), Epstein-Barr virus (EBV), human herpesviruses-6, -7 and -8, and HTLV. If the purification process of plasma-derived medicinal products including inactivation procedures makes it possible to be confident with the elimination of infectivity due to enveloped viruses, the detection of nucleic acid sequences derived from naked viruses in plasma pools such as parvovirus B19 or hepatitis A virus (HAV), and/or the introduction of a nanofiltration step during the purification process, when possible, may greatly contribute to their safety. The emergence of a new form of the Creutzfeldt-Jakob disease (nvCJD) introduces a new series of questions about the safety of blood products that, although the risks appear limited, are not yet resolved.