By Stan Deresinski, MD, FACP, FIDSA

Clinical Professor of Medicine, Stanford University

Dr. Deresinski reports no financial relationships relevant to this field of study.

SYNOPSIS: The number of cases reported to the CDC from 2011-2015 has increased and there is concern about expansion of the areas in which it is transmitted.

SOURCE: Gray EB, Herwaldt BL. Babesiosis Surveillance — United States, 2011-2015. MMWR Surveill Summ 2019;68:1-11.

Babesiosis has been a nationally notifiable condition since 2011, with 33 states reporting cases to the Centers for Disease Control and Prevention (CDC) as of 2015. During that interval, 7,612 cases were reported. The annual reports ranged from 909 in 2012 to 2,074 in 2015, with annual population-adjusted incidence rates ranging from 0.6 to 0.9 per 100,000 persons in those same years, respectively. (See Figure 1.) In 2015, 87.1% of cases were confirmed and 12.9% were probable.

Figure 1: Number* of Reported Cases of Babesiosis, by Year — United States, 2011-2015

Babesiosis chart

* A total of 7,612 cases of babesiosis were reported (n = 1,126 for 2011, n = 909 for 2012, n = 1,761 for 2013,
n = 1,742 for 2014, and n = 2,074 for 2015).

Year as reported by the health department.

Source: Centers for Disease Control and Prevention

Although 27 states reported cases from 2011-2015, 94.5% of these were reported by seven states, including five from the northeastern United States (New York, Massachusetts, Connecticut, New Jersey, and Rhode Island) and two from the Upper Midwest (Wisconsin and Minnesota). (See Figure 2.) The only other states reporting more than 100 cases were Maine and New Hampshire, where cases increased over the years of the study. Overall, > 70% of cases had onset during June to August.

Figure 2: Reported Cases of Babesiosis, by Patient’s County of Residence* — 33 States, 2015

Babesiosis map

* N = 2,070; county of residence was known for all but four (< 1%) of the 2,074 total patients. Each dot represents one case; dots were placed within the patient’s county of residence.

† Year as reported by the health department.

Source: Centers for Disease Control and Prevention

Among the 6,399 cases for which data were available, 5,343 (83.5%) had a positive blood smear while 1,056 (16.5%) were diagnosed only by serological testing, a result considered insufficient for classification as confirmation. Molecular or serologic testing allowing species level identification was available for 2,867 cases, and all but three identified Babesia microti as the etiology. Those three were identified only by serological testing as Babesia duncani.

Most patients had fever and/or chills, while approximately two-thirds were thrombocytopenic. Approximately half of patients were hospitalized. At least one complication occurred in 231 of the 630 (36.7%) patients for whom the information was available, with renal insufficiency the most frequent complication, followed by hepatic injury and respiratory distress or failure. Approximately one-half of patients were hospitalized, with asplenia a strong risk factor. Eighty-three percent of patients for whom information was available received treatment with atovaquone, azithromycin, clindamycin, or quinine — alone or in combination. Seventy-one percent of patients received guideline-compliant combination therapy with either atovaquone plus azithromycin or clindamycin plus quinine. Almost half of patients received doxycycline. Forty-six patients died, but not all deaths were due to babesiosis.

Among the 3,173 patients for whom the information was available, 1,443 (45.5%) reported a tick bite in the eight weeks prior to the onset of symptoms or diagnosis, while 23.1% of the remainder reported prior outdoor activities. Fifty-one patients had transfusion-related babesiosis.


Most cases in this series were diagnosed by examination of a blood smear. The frequency of nucleic acid amplification test use is increasing. In contrast to morphologic examination, nucleic acid amplification tests allow distinction between the dominant species, B. microti, and the infrequently identified B. duncani. Babesia must be distinguished from plasmodia on blood smear. In patients with low-level parasitemia, repeated examination of blood smears may be necessary to detect the organism.

An important consideration in dealing with a patient with suspected or known babesiosis is the coinfection with other pathogens also transmitted by Ixodes scapularis ticks. In the United States, these include Borrelia burgdorferi, Anaplasma phagocytophilum, Borrelia miyamotoi, Borrelia mayonii, Powassan virus, and an Ehrlichia species. It is likely that concern about one or more of these accounted for the fact that almost 50% of patients with babesiosis reported to the CDC received doxycycline. As with other tick-borne diseases, there is concern that the areas in which transmission occurs may be expanding, as evidenced by the increasing number of cases in Maine and New Hampshire. The main defense against infection in areas of transmission is avoiding tick-infested areas, applying tick repellent to both skin and clothing, undergoing full-body inspection for ticks after outdoor activities, and removing attached ticks with fine-tipped tweezers as soon as possible.