Characterization of posttransfusion Plasmodium falciparum infection in semi-immune nonparasitemic patients

Transfusion-transmitted malaria (TTM) has not been studied with molecular means in hyperendemic areas where it is assumed to occur frequently. The African Investigation of the Mirasol System (AIMS) trial provided the opportunity to study TTM from standard whole blood (WB) units.

The Plasmodium genome in transfused WB units and patient samples both before transfusion and 1, 3, 7, and 28 days after transfusion was screened and quantified using real-time polymerase chain reaction. Parasitemic samples were confirmed, three alleles were sequenced, and the percentage homology was determined between paired WB units and patient samples. Anti-Plasmodium titers were quantified by serial dilution. Clinical symptoms and microscopic detection of malaria were monitored.

Microscopy was negative below 3 × 10(6) genome copies/mL. Thirty-seven patients who were nonparasitemic before transfusion were exposed to parasitemic WB. The amount of Plasmodium genome load transfused ranged between 0.1 × 10(6) and 2.0 × 10(9) copies. The parasite load received through transfusion by 13 patients with TTM was higher than that received by patients without TTM (p = 0.01). Patients with a single parasitemic posttransfusion sample were not considered to have TTM. Four elements critical to predict outcome emerged: parasite load, patient anti-Plasmodium titer pretransfusion, percentage clearance of parasites at Day 1, and level of anti-Plasmodium humoral immune response. Four patients with TTM became parasite-free at Day 28 (effective control), four patients with TTM maintained relatively stable levels of parasitemia (uncertain control), and five patients reached high levels of parasitemia at Day 28 posttransfusion, indicating ineffective control of malarial infection by semi-immune individuals.

TTM in endemic areas is relatively frequent (13 of 112 donations; 11.6%) and, although largely controlled by semi-immunity in recipient patients, may require antimalarial treatment.