Viral Transmission

The current estimated risk of viral transmission by blood components is indicated in Table 4. The introduction of nucleic acid testing has substantially reduced the risks of HIV and HCV transmission to the present very low levels.

West Nile Virus (WNV) has recently emerged as a pathogen that can be transmitted by transfusion. This mosquito-borne virus is seasonal. When there are no recognized human cases in the donor population the risk is extremely small. Donor samples undergo a nucleic acid test for WNV. The sensitivity is currently unknown and there is likely a residual risk of transmission by transfusion when the disease is present in the donor population.

Other viruses known to be transmissible by transfusion include hepatitis D that can replicate only in the presence of HBV. EBV is rarely associated with clinical disease. Parvovirus B19 rarely causes significant disease but may be associated with chronic pure red cell aplasia in immunosuppressed persons. Cytomegalovirus positivity is present in up to 50% of Canadian donors. The risk of serious outcome from transfusion transmitted CMV infection is largely restricted to very low birthweight infants and immunocompromised hosts and CMV negative units are routinely supplied for these patients. The current practice of leukodepletion also reduces the risk of CMV transmission.

With regard to plasma protein (fractionation) products, the current risk of transmission of HIV, HCV, and HBV is almost zero. Similarly it appears to be negligible for non-enveloped viruses (parvovirus B19 and HAV).

The agent associated with Creutzfeld-Jakob disease is considered to be potentially transmissible by transfusion, but no such case has ever been documented. No test is currently available which would identify ‘at risk’ donors except that the small proportion with the genetic form can be identified by molecular genetic testing. Variant CJD has been reported to have been transmitted by blood transfusion in the U.K.

The possibility that other previously unrecognized pathogens may emerge in the blood supply needs to be considered.

Bacterial Contamination

Bacterial contamination may result from exposure to skin bacteria at the time of collection or to asymptomatic bacteremia in the donor (e.g. Yersinia enterocolitica). Note that the risk of such contamination is not reduced by the use of autologous blood.

According to Blajchman²¹ the overall incidence of contaminated cellular blood products is approximately 1:3,000. However, transfusion of contaminated units is not necessarily associated with clinically apparent morbidity. In a minority of instances contaminated units contain large enough numbers of virulent bacteria and endotoxins to cause significant morbidity and mortality. The incidence of severe septic episodes has been estimated at 1:50,000 platelet units transfused and 1:500,000 RBC units transfused. In the case of platelets, the incidence of bacterial comtamination has likely diminished significantly since the introduction of bacterial testing of all platelet units prior to issue.

Parasitic Contamination

The risk of malaria transmission has been estimated at 1 in 400,000 units in Canada. Other parasites which may rarely be transmitted by blood transfusion include Trypanosoma cruzi (Chagas’ disease), Toxoplasma gondii, Babesia microti, microfilarae and Leishmania donovani (Kala-azar).

Immunosuppression

An additional concern is that allogeneic transfusion may cause immunosuppression²⁰. A number of studies show a trend towards an increased rate of tumor recurrence and post-operative sepsis in recipients of allogeneic blood and this effect appears to be eliminated in those receiving only autologous blood.

Reporting of Adverse Effects

Any adverse effect resulting from a transfusion must be reported to Transfusion Medicine. In the case of infectious complications a traceback will be initiated, when appropriate, so that the donor can be identified and excluded from further donation.

Lookback

A lookback is undertaken when a donor subsequently tests positive for an infectious disease marker (e.g. anti-HCV). Previous donations from this donor are identified by CBS, and communicated to Transfusion Medicine so that the recipients can be identified and notified. The primary care physician will also be notified (if known).

Traceback

Traceback is undertaken when a transfusion recipient tests positive for an infectious disease marker. The blood components received are traced back to the donors. The donors are then tested and if a donor is found to be positive, this will result in additional lookback activity.