Blood supplies in the US and other countries are becoming increasingly infected with Babesia, a tickborne parasite of red blood cells.

A number of medical device and diagnostic companies are working on developing tests for Babesia for donor-screening purposes, as there is an urgent need for a screening test and prevention strategies.

Medical epidemiologists at the US Centers for Disease Control and Prevention (CDC; Atlanta, GA, USA) have tracked case-patients who were transfused during 1979–2009 and had post transfusion babesiosis diagnosed by 2010, without reported evidence that another transmission route was more likely than transfusion. Implicated donors had laboratory evidence of infection by immunofluorescence antibody tests (IFA), blood films, or polymerase chain reaction (PCR) assays.

The scientists found 159 Babesia microti transfusion-associated cases and donors were implicated for 136 (86%). The case-patients' median age was 65 years with a range from less than a year old to 94 years. Most cases were associated with red blood cell components, but four were linked to whole blood–derived platelets. Cases occurred in all four seasons, and in 22 of the 31 years studied. However, 77% of the cases were reported during 2000-2009. Cases occurred in 19 US states, yet 138 cases were in the seven main B. microti–endemic states. In addition, three cases due to B. duncani were documented in western states. Among the 117 linked donors, the B. microti IFA-test results were known; 24 donors (20%) had parasitologically confirmed infection by blood films; and 20 donors had positive PCR results.

Babesiosis is a parasitic disease, very similar to malaria, and is caused by Babesia, a protozoan parasite of the blood. Babesia parasites are believed to be the second most common blood parasites of mammals, after trypanosomes. Although much less common in humans than other mammals, experts say reported human cases have risen over the last decade mainly because of better medical awareness. Babesiosis is a potentially life-threatening complication of blood transfusion, but it is treatable. An infected individual has a risk of multi-organ failure, and even death. Patients without a spleen, individuals with weakened immune systems, and the elderly are more vulnerable to complications.

Barbara Herwaldt, MD, MPH, from the CDC and lead author of the study, said, "We want clinicians to become more aware of babesiosis, including the small possibility of transmission via blood transfusion. If a patient develops unexplained fever or hemolytic anemia after a transfusion, babesiosis should be considered as a possible cause, regardless of the season or US region." Most cases in Europe have involved splenectomized patients and 50% of European cases were fatal. The study was published September 5, 2011 in the Annals of Internal Medicine.

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