Conventional light microscopy has been the standard for diagnosis of babesiosis. Thick and thin blood films are typically prepared using anticoagulated venous blood. Ideally, the specimens should be collected before the initiation of therapy and smears should be prepared within 1 hour after collection. Larger amounts of blood are used during the preparation of thick films than in the preparation of thin films, thus increasing the possibility of detecting light infections. It has been suggested that thick films have limited use for the diagnosis of B. microti infections, due to the small size of the organism. However, some clinicians have found that thick films are essential for diagnosing infections with low levels of parasitemia, particularly in immunosuppressed individuals. The morphological characteristics of parasites are best observed in thin films because they are fixed and RBC morphology is retained, permitting extracellular (extraerythrocytic) organisms to be visualized. To optimize detection and identification, both thick and thin films should be prepared. Since the degree of infection may vary, a single negative set of blood films does not rule out infection, and specimens should be collected every 6 to 8 hours for up to 3 days to increase the likelihood of detecting organisms (Westblade et al. 2017). While PCR has been shown to be more sensitive than blood film examination for patients during and following anti-Babesia therapy, the role of nucleic acid-based technologies in therapeutic management should be approached with caution. Parasite DNA may be detectable for long periods (weeks to months), even after organisms are no longer evident on blood films, which may reflect detection of DNA from nonviable organisms rather than active infection (Wang et al. 2015).

Diagnostics in Wisconsin (Stein et al. 2017)

In the state of Wisconsin, babesiosis became officially reportable in 2001. In 2001, a confirmed case of babesiosis was defined as “the occurrence of fever, anemia, or thrombocytopenia with confirmatory lab findings” (identification of Babesia organisms by blood smear or fourfold increase in B. microti IgG antibodies). During 2001-2015, major changes occurred in babesiosis surveillance, which may have affected reported incidence rates. The first was expansion of the definition of confirmatory laboratory evidence for babesiosis to include PCR, and the second was initiation of automatically-generated electronic laboratory reports. Before 2007, peripheral blood smear exams were most commonly used to provide confirmatory laboratory evidence. Blood smear exams have a substantially lower sensitivity of detection of parasites than PCR. The inclusion of the more sensitive PCR assay, combined with increased use of these tests by providers, likely contributed to an increase in babesiosis diagnoses. However, the number and distribution of actual cases is also increasing due to expansion of the deer tick population (caused by global warming, habitat fragmentation, etc).


It has been proposed that co-infection with B. burgdorferi increases B. microti transmission, and it is thought to increase disease severity and duration in rodent and human hosts (Diuk-Wasser et al. 2014). Additionally, as of 2017, B. microti was the leading cause of blood transfusion-transmitted infections reported to the FDA and the leading infectious cause of transfusion-related deaths (Westblade et al. 2017). In 2018, a screening assay for blood donors was approved by the FDA (FDA 2018).


Stein et al. 2017 (“Babesiosis Surveillance”): https://www.cdc.gov/mmwr/volumes/66/wr/mm6626a2.htm

Diuk-Wasser et al. 2014 (“Monitoring Human Babesiosis Emergence through Vector Surveillance New England, USA”): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3901474/

Westblade et al. 2017 (“Babesia microti: from Mice to Ticks to an Increasing Number of Highly Susceptible Humans”): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5625376/

Wang et al. 2015 (“Utilization of a real-time PCR assay for diagnosis of Babesia microti infection in clinical practice”): https://www.sciencedirect.com/science/article/pii/S1877959X15000382?via%3Dihub

FDA 2018 (“FDA approves first tests to screen for tickborne parasite in whole blood and plasma to protect the U.S. blood supply”): https://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm599782.htm