Discussion

Babesiosis is a worldwide emerging infection caused by intraerythrocytic protozoa transmitted most commonly through ticks but also from exposure to infected blood products through transfusion or organ transplantation. ^{Reference Vannier and Krause1–Reference Krause, Auwaerter and Bannuru3} The most common cause of babesiosis is B. microti with endemic areas in the northeastern and northern Midwestern United States and northeastern and southwestern China. ^{Reference Vannier and Krause1,Reference Krause, Auwaerter and Bannuru3} Disease severity ranges from asymptomatic infection in about a fifth of adults to severe disease requiring hospital admission. More than 1,000 patients a year, on average, were hospitalized for babesiosis in the United States from 2011 to 2016. ^{Reference Krause, Auwaerter and Bannuru3,Reference Bloch, Day and Krause4} Complications include cardiac, neurologic, pulmonary, renal, and hepatic impairment, as well as severe anemia, Disseminated intravascular coagulation (DIC), and shock. Fatality rates vary from 1% in the general population to 20% in highly immunocompromised hosts. ^{Reference Vannier and Krause1–Reference Bloch, Day and Krause4}

Needlestick and sharp injuries are well-known causes of bloodborne pathogen transmission to healthcare workers. Transmission risk varies substantially by pathogen type and blood quantity transmitted, ranging from 0.3% for HIV to 30% for hepatitis B in the United States. ⁵ Babesia are bloodborne pathogens that can be transmitted through red blood cell transfusion, platelet transfusion, and organ transplantation, with associated mortality rates as high as 20%. ^{Reference Herwaldt, Linden, Bosserman, Young, Olkowska and Wilson2}

The risk of transmitting Babesia through a needlestick or sharps injury is unknown because no such exposure has ever been reported. While no Babesia was noted on blood smear of the patient on the day of injury, B. microti PCR was positive and has a higher diagnostic sensitivity. ^{Reference Vannier and Krause1,Reference Krause, Auwaerter and Bannuru3} Relying on Babesia smears to determine transmission risk is subject to human error and a review of at least 200–300 fields under oil immersion should be performed to increase sensitivity, but is not always carried out. ^{Reference Krause, Auwaerter and Bannuru3} Positive PCR testing has been associated with persistent and relapsing infection during prolonged periods of negative blood smear but positive PCR results. ^{Reference Krause, Auwaerter and Bannuru3,Reference Rogers, Krause and Norris6} DNA is rapidly cleared from the bloodstream, so a positive PCR result indicates active infection. ^{Reference Malawista, Barthold and Persing7} As few as four B. microti-infected erythrocytes have been shown to infect immunodeficient NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice and as few as 30 have infected relatively immune intact DBA/2 J mice. ^{Reference Bakkour, Chafets and Wen8} These numbers are below the detection limit of standard blood smear and at or below the detection level of real-time PCR. ^{Reference Tonnetti, Townsend and Deisting9,Reference Meredith, Oakley and Kumar10} Nonetheless, the negative PCR results and long asymptomatic period in this healthcare worker are strong evidence that B. microti transmission did not occur.

Although needlestick and sharps injuries in health care providers treating babesiosis patients have not been reported, there is a theoretical risk of Babesia transmission. The incubation period for babesiosis ranges between 1–4 weeks after tick bite and 1 week to 6 months after blood transfusion. ^{Reference Herwaldt, Linden, Bosserman, Young, Olkowska and Wilson2} Although B. microti transmission was not demonstrated in this healthcare worker, we suggest that blood smear and PCR or nucleic acid testing (NAT) be offered to healthcare workers with blood exposure from a patient with B. microti infection. Nucleic acid testing quantitates ribosomal ribonucleic acid (RNA) and is used by the Red Cross to screen blood donations for Babesia with a detection level of as few as 1.4 Babesia parasites/mL. ^{Reference Tonnetti, Dodd, Foster and Stramer11,Reference Stanley, Stramer and Erickson12} We suggest testing at the time of exposure, anytime Babesia symptoms develop up to 6 months after exposure, and at 6 months post exposure. The bloodborne transmission potential of babesiosis should be taught to healthcare workers in endemic regions.