Babesiosis is a tickborne protozoan infection of red blood cells which can also be transmitted by blood transfusion.  In the U.S., babesiosis causes about half of fatal infections caused by blood transfusions.

Babesia microti/CDC
Babesia microti/CDC

Transfusion-transmitted babesiosis (TTB) is a threat to our blood supply since many blood centers do not screen donated blood for Babesia microti (B. microti).  The FDA recommends but does not require such screening, as it does for HIV, hepatitis C and other blood-borne pathogens.

More than 225 TTB cases have been reported in the U.S. in 22 states since 1980, with 28 deaths.  

Even these reports probably underestimate TTB risk since babesiosis is both under-diagnosed and under-reported.

A 2016 study provides estimates of the risk of TTB after unscreened blood transfusions. 89,153 units of donated blood were tested for B. microti antibodies and DNA between 2012 and 2014 in the endemic states of Connecticut, Massachusetts, Minnesota, Wisconsin (in which most U.S. babesiosis cases occur).

0.38% of blood units were positive for B. microti antibodies, B. microti DNA or both and were not transfused.  No TTB cases were seen in patients who received screened blood, but the study authors also attempted to estimate the risk of TTB after transfusion of unscreened blood.

To do this, the authors used the total number of blood transfusion in the endemic states listed above, along with Maine and New Hampshire.  There were no cases of TTB after transfusion of 75,331 units of screened blood but one TTB infection after transfusion from the 18,074 units of blood not included in their study which had not been screened for B. microti.

Screening donated blood for B. microti antibodies and DNA would probably eliminate the risk of TTB, but many blood centers omit these tests.  The added expense for blood centers and the lack of an FDA requirement may help explain this. Apparently,  many blood centers now have trouble covering the costs of blood that they then sell to hospitals.  An FDA requirement for B. microti screening of all donated blood as well as financial and policy changes will probably be needed to insure the sustainability and safety of the U.S. blood supply.

Author: Brian Carty, MD, MSPH