The 3 cases of transfusion-transmitted malaria described here are of public health importance for several reasons. First, all 3 were caused by P. falciparum
, which may be associated with a potentially fatal outcome, particularly if there are delays in recognition and treatment.5,12,13
Cases 1 and 3 were diagnosed after approximately 6 and 3 weeks of symptoms respectively, whereas case 2 was diagnosed a few days after the onset of symptoms. In all cases, astute laboratory personnel made the diagnosis while examining blood smears ordered for reasons other than malaria. Fortunately, there were no fatalities among these cases. In the United States, transfusion-transmitted P. falciparum
malaria has resulted in 2 recent fatalities and has also been the most common cause of transfusion-transmitted malaria, responsible for 37% of 91 cases reviewed.5
Cases of imported P. falciparum
malaria are increasing in Canada as more Canadians travel to areas where malaria is endemic, and as drug-resistant P. falciparum
malaria continues to evolve and spread.14,15
Second, confirmed or suspected source donors met donor-screening criteria designed to prevent transfusion-transmitted malaria. The donors implicated in cases 2 and 3 were asymptomatic carriers of P. falciparum malaria. At the time of their donation, individuals with a history of malaria who had been symptom free for 3 years were allowed to donate blood. As a result of these 2 cases, the Bureau of Biologics and Radiopharmaceuticals, Health Canada, and the Canadian Red Cross changed the deferral criterion for past history of malaria. Beginning in July 1995, donors reporting a history of diagnosis or treatment of malaria at any time in the past were permanently deferred from donating components for direct transfusion. However, as illustrated by case 1, even with this added precautionary measure, complete prevention of transfusion-transmitted malaria may not be possible. In the absence of any practical donor-screening program for malaria, the management of the risk of malaria from blood transfusion is entirely based on the donor's answers to specific screening questions. Donors may give inaccurate information intentionally or unintentionally, or because they misunderstand the question posed, or because they are unaware or have forgotten that they previously have had malaria.
Third, it is unusual that P. falciparum persisted in the donors involved in these cases for over 3 years after they had left areas where malaria was endemic. In case 2, and probably in the other cases, there is a history of preceding inadequate therapy for malaria with chloroquine. Such noncurative therapy potentially may facilitate persistent infection, which increases the risk of transfusion-transmitted malaria in the population, particularly if infected individuals develop a semi-immune state (minimal symptoms but persistent parasitemia).
Finally, it is notable that transmission to the case 3 patient occurred through the transfusion of a platelet rather than a red-cell unit, and despite the apparently low level of parasitemia in the donor. This observation suggests that even a small number of infected red cells are sufficient to transmit malaria.
We are aware of one previous report of possible transfusion-transmitted malaria in a Canadian.16,17
Although this patient had been transfused an extremely long time (12 years) prior to the onset of P. falciparum
infection, making transfusion a very unlikely source, no other exposure could be identified. An alternative diagnosis of babesiosis could not be excluded.