Babesiosis
2025 Case Definition

Background

In the U.S., most cases of babesiosis are caused by Babesia microti, transmitted through the bite of the Ixodes scapularis (blacklegged) tick. In the northeastern and upper midwestern U.S., B. microti circulates between ticks and animal reservoir hosts,³ a cycle shared by the etiologic agents of Lyme disease (Borrelia burgdorferi) and anaplasmosis (Anaplasma phagocytophilum). Very rarely, U.S. cases caused by other Babesia species have been described, such as B. duncani (transmitted by Dermacentor albipictus) and B. divergens (possibly transmitted by I. dentatus).^4-7 Babesiosis typically presents with nonspecific symptoms (e.g., fever, chills, sweats, headache, myalgia, malaise, and fatigue) within one month of a tick bite. Additionally, laboratory anomalies such as hemolytic anemia, thrombocytopenia, and elevated levels of liver enzymes are often noted.^{1, 8}

Cases of babesiosis increased significantly between 2011 and 2019, with some northeastern states experiencing several fold increases in incidence rates.⁹ The disease is increasingly identified in non-endemic areas as tick populations expand, following a similar pattern to Lyme disease^{7, 8} although at a slower pace for babesiosis.¹⁰ This pattern of disease emergence following vector expansion is well described^{11, 12} and would seem to predict continued expansion in the future.^{13, 14} Babesiosis cases are also likely underreported, as non-specific symptoms and a relatively high proportion of asymptomatic infections can make diagnosis difficult.⁷

Due in part to the ability of people to maintain low levels of parasitemia while asymptomatic, transfusion-transmitted babesiosis has been the most frequently reported parasitic disease associated with transfusions in the U.S.^{15, 16} In 2019, FDA issued guidance¹⁷ directing blood donor screening for Babesia spp. where vectorborne transmission was known to occur (14 states and the District of Columbia). Blood collection agencies in those jurisdictions-initiated screening in 2020 using the FDA-approved nucleic acid test that detects multiple Babesia spp. (B. microti, B. duncani, B. divergens, and B. venatorum). Although the number of reported transfusion-associated cases of babesiosis decreased after the implementation of donor screening,^{2, 18} transfusion-associated babesiosis is not restricted to regions where tickborne babesiosis is endemic. Because of donor travel and the importation of blood products, blood-borne transmission in the U.S. has been reported outside of Babesia-endemic regions,¹⁹ underscoring the importance of having thorough and complete nationwide surveillance to monitor trends.

Clinical Criteria

• Objective: fever as reported by patient or healthcare provider, anemia, or thrombocytopenia
• Subjective: chills, sweats, headache, myalgia, or arthalgia

Laboratory Criteria

Confirmatory Laboratory Evidence:

• Identification of intraerythrocytic Babesia organisms by light microscopy in a Giemsa, Wright, or Wright-Giemsa-stained blood smear; OR
• Detection of Babesia spp. DNA in a whole blood specimen through nucleic acid testing such as polymerase chain reaction (PCR) assay, nucleic acid amplification test (NAAT), or genomic sequencing that amplifies a specific target, in a sample taken within 60 days of illness onset; OR
• Serological evidence of a four-fold change^{^} in IgG-specific antibody titer to B. microti antigen by indirect immunofluorescence assay (IFA) in paired serum samples (one taken within two weeks of illness onset and a second taken two to ten weeks after acute specimen collection).^{^^}

Presumptive Laboratory Evidence:

• Serologic evidence** of an elevated IgG*** or total antibody reactive to B. microti antigen by IFA at a titer ≥1:256 in a sample taken within 60 days of illness onset.

Supportive Laboratory Evidence:

• Serologic evidence** of an elevated IgG*** or total antibody reactive to B. divergens antigen by IFA at a titer ≥1:256; OR
• Serologic evidence** of an elevated IgG*** or total antibody reactive to B. duncani antigen by IFA at a titer ≥1:512.

^ A four-fold change in titer is equivalent to a change of two dilutions (e.g., 1:64 to 1:256).

^^A four-fold rise in titer should not be excluded as confirmatory laboratory criteria if the acute and convalescent specimens are collected within two weeks of one another.

* Note: The categorical labels used here to stratify laboratory evidence are intended to support the standardization of case classifications for public health surveillance. The categorical labels should not be used to interpret the utility or validity of any laboratory test methodology.

** Antibodies can be indicative of active or previously resolved infections, so it is recommended that laboratory results be evaluated in conjunction with information on symptoms and exposure whenever possible. If symptom information is available, specimens meeting supportive laboratory criteria should be collected within 60 days of illness onset.

*** While a single IgG serologic test is adequate for surveillance purposes, molecular testing or blood smear are recommended for clinical diagnosis, especially in cases where species other than B. microti are suspected.

Criteria to Distinguish a New Case from an Existing Case

A new case is one that has not been previously enumerated within the same calendar year (January through December).^‡

‡ Using calendar year allows case counting which more closely correspond with the seasonality of babesiosis than using a number of months between case reports. 

Suspect

• Meets supportive laboratory evidence.

Probable

• Meets presumptive laboratory evidence AND meets at least one of the objective clinical criteria.

Confirmed

• Meets confirmatory laboratory evidence criteria AND at least one of the objective or subjective clinical criteria.

• Babesiosis ( Babesia spp.) | 2011 Case Definition