Do leukoreduction filters passively reduce the transmission risk of human granulocytic anaplasmosis?

Background: Human granulocytic anaplasmosis, caused by Anaplasma phagocytophilum, poses an increasing public health risk in the United States. Since 2000, case reports have increased annually; 2782 cases were reported in 2013. Despite the increasing frequency of clinical cases, only eight cases of transfusion-transmitted anaplasmosis (TTA) have been reported. We investigated if current leukoreduction practices impact transfusion risk.

Study Design And Methods: Whole blood units (WBUs) with integral red blood cell (RBC) leukoreduction filters were collected and spiked with A. phagocytophilum-infected HL-60 cells equivalent to 0.01, 1, or 5% of total neutrophils. After 24 hours at 4°C WBUs were processed into plasma and RBCs, the latter subsequently leukoreduced (LR RBCs). To evaluate the removal of A. phagocytophilum by filtration, pre- and postfiltration samples were compared by culture and polymerase chain reaction (PCR).

Results: Compared to Day 0 or Day 1 positive controls, LR RBCs demonstrated reduced levels of A. phagocytophilum by culture and PCR. At 0.01% infection levels LR RBCs yielded no positive cultures and a log reduction of 2.5 by PCR. Similarly, at 1 and 5% infections levels, LR RBCs produced only 44% (4/9) and 56% (5/9) positive cultures, respectively. PCR results were comparable, 3.0 log reduction for 1% and 3.3 log reduction for 5% infection levels.

Conclusions: The recent increase in TTA suggests that A. phagocytophilum may represent an emerging blood safety issue. However, the current study indicates that the widespread practice of leukoreduction might passively reduce, but not eliminate, TTA risk. In the absence of viable testing or pathogen inactivation and/or reduction options, leukoreduction may offer some protection from transmission risk.