In 1980 and 1982, respectively human T-Lymphotropic virus type 1 (HTLV-1) and type 2 (HTLV-2) were the first retroviruses identified in human beings. HTLV-1 is associated with adult T cell leukemia/lymphoma (ATL) and HTLV-associated myelopathy/Tropical Spastic Paraparesis (HAM/TSP). These viruses can be transmitted vertically (from mother to child), mainly by breast feeding; by sexual relationships and parenteral drug delivery (intravenous drug users and transfusion of blood and blood components). In endemic areas, vertical and sexual transmission has been the principal manner of dissemination of HTLV-1 infection. However, blood transfusion seems to have an important role in introducing HTLV in non-endemic populations. The most efficient way of transmission of HTLV-1 is through cell components of contaminated blood. In the past, this occurred chiefly through blood transfusions not tested for HTLV-1. An efficiency of transfusion transmission of 60% was described in the first reports of Japanese research. Thereafter, extremes of 13% to 80% were described in retrospectives studies performed in the USA. Such variations in the efficiency of transmission by transfusions were influenced by parameters such as: blood product type, time spent from collection of the cell components until its transfusion and proviral load of the donor. It is estimated that about 4 to 8% of receptors of HTLV infected cell units can develop HAM/TSP, with ATL being rare in these receptors. Look-back is the term used in hemovigilance for a program that notifies blood transfusion receptors of the risks involved in exposure to infectious agents due to a preceding transfusion. "Targeted look-back"is the program used to identify receptors of blood units donated by specific individuals that subsequently have been identified as infected by a specific agent (for example HTLV). This involves identification of previous blood component units transfused. The receptors that are alive and located are notified of the possible risk of being infected, usually by their physician. Laboratorial tests are performed to check the receptor serostatus. Many look-back studies accomplished in endemic areas of HTLV promoted the improvement of universal screening tests of blood donors. During the last 20 years, screening tests for HTLV-1/2 were implemented in many countries worldwide. This important public health measure excluded seropositive individuals from the donor pool and has resulted in a reduction of the infection rate among blood component receptors, thereby decreasing the HTLV infection rates in the general population.
HTLV; hemovigilance; look-back; transfusion; bloodborne infections
