Babesiosis in the United States, 2011–2015: The First Five Years of National Surveillance

Summary

• The disease: babesiosis is caused by Babesia parasites, spread in nature by the bite of an infected tick. It can be life-threatening, especially for people without a spleen, with weakened immune systems, or who are elderly.
• The system: babesiosis became nationally notifiable in January 2011. States where it is reportable voluntarily notify CDC through the National Notifiable Diseases Surveillance System (NNDSS) and send extra details on a case report form. It was reportable in 22 states in 2011 and 33 in 2015.
• The results: 7,612 cases in 2011–2015, 94.5% of them in seven states with well-known tickborne transmission. Maine and New Hampshire reported rising numbers, suggesting transmission may be spreading. About half of patients were hospitalized.
• What to do: in areas where babesiosis is common, avoid tick-infested areas, use repellent on skin and clothing, check the whole body for ticks after being outdoors, and remove ticks promptly with fine-tipped tweezers.

The disease

Babesia are protozoan parasites that live inside red blood cells. Infection may cause no symptoms, or flu-like illness — fever, chills, headache, body aches, fatigue — with hemolytic anemia and thrombocytopenia; multiorgan failure and other life-threatening complications can occur.

• Diagnosis: parasites or their DNA confirm infection; serologic tests support it. For acute illness, providers should request manual light-microscope examination of blood smears. Babesia can be hard to tell from malaria parasites or from artifacts, so a reference laboratory may be needed, and smears cannot identify the species.
• Treatment: atovaquone plus azithromycin, or clindamycin plus quinine, for at least 7–10 days; clindamycin plus quinine is standard for severe cases.
• Species and spread: most U.S. cases are caused by B. microti, carried by Ixodes scapularis ticks in the Northeast and upper Midwest in spring and summer. B. duncani has caused cases in the West, and B. divergens-like parasites elsewhere. Babesia can also spread through blood transfusion or from mother to baby.
• History: the first documented U.S. case was in 1966, in a Californian without a spleen; the first shown to be B. microti was in 1969, on Nantucket Island, Massachusetts.

Cases, year by year

Incidence ranged from 0.6 per 100,000 (2012) to 0.9 (2015). 82.5% of cases were confirmed and 17.5% probable. The 11 states that began surveillance during the period reported only 21 cases.

Reported cases by year, 2011–2015. Chart: CDC.

Where

Cases came from residents of 27 states, but 7,194 (94.5%) were in seven states:

• Maine (152) and New Hampshire (149) were the only others above 100 — and both rose: Maine from 9 cases in 2011 to 55 in 2015, New Hampshire from 13 to 53. Of their cases with travel data, 43.5% involved travel to one of the seven states in the 8 weeks before illness.
• The remaining 117 cases came from 18 states; at least 26.5% of those patients had traveled to the seven states.

Cases by patient's county of residence, 33 states, 2015. Map: CDC.

Who, and when

• Median age 63 (range under 1 to 99); 57.9% were 60 or older, and the most cases each year were among people in their 60s.
• 65.9% were male; of those with data, 89.1% were white and 90.7% non-Hispanic.
• In every year, more than 70% of patients fell ill in June–August.

Cases by age group and year. Chart: CDC.

Cases by month of symptom onset and year. Chart: CDC.

Laboratory findings

Of 6,399 patients with results, 83.5% tested positive by blood smear, PCR or (in one case) animal inoculation; 16.5% were positive only by serology, which counts as supportive rather than confirmatory. Species were reported for 37.7% of cases — all but three B. microti. The three were attributed to B. duncani by serology, in residents of Maryland and Connecticut, where local B. duncani transmission has not been documented.

Illness, hospital and treatment

• Symptoms: fever (84.2%), chills (69.5%) and thrombocytopenia (68.8%) were most common.
• Complications: 36.7% of 630 patients with data had at least one — most often kidney problems, liver compromise and acute respiratory distress — rising to 46.9% of those 60 and older.
• Hospital: 46.9% were hospitalized overnight or longer, from 16.0% of those aged 10–19 to 72.6% of those 80 and older; the median stay was 4 days (1–63). Patients without a spleen were hospitalized far more often (84.1% vs 46.1%).
• Treatment: of 2,728 patients with named drugs, 83.0% got at least one recommended drug and 71.1% of those a recommended combination. Doxycycline, not recommended for babesiosis, was listed for 48.4% — probably often given for other tickborne infections, though some mistaken prescribing cannot be ruled out.
• Deaths: 46 were reported — 7 attributed to babesiosis, 4 not, and 35 unknown.

Hospitalization by length of stay. Chart: CDC.

Hospitalization by age group. Chart: CDC.

How people were infected

• 45.5% of 3,173 patients with data recalled a tick bite in the 8 weeks before illness; of those who didn't, 23.1% reported outdoor exposure. The most common activities were gardening or yard work, hiking and camping.
• 51 cases were transfusion associated. Of those with known onset dates, 70.3% fell ill in September–May, unlike most patients.
• Of 15 babies under 1, four were infected by transfusion and one before birth.

Slow reporting

The median time from the first date linked to a case to CDC's first report was 85 days, and to receipt of the detailed form, 369 days. CDC is working on electronic submission to make these data more timely, uniform and complete.

What it means

Rising counts may reflect real increases in some places, such as Maine and New Hampshire, or changes in testing and awareness. Telling local infections from travel-related ones is key to learning whether B. microti's range is growing. In May 2019, the Food and Drug Administration recommended year-round molecular testing of blood donations in 14 states in the East and upper Midwest and the District of Columbia, but transfusion cases can still happen, anywhere, so health departments should keep asking about transfusions.

Limitations: babesiosis is likely underdiagnosed and underreported (reporting is passive and not required everywhere); some cases may be misclassified (four did not meet the definitions, without changing the conclusions); and much data was incomplete.

Advice: the nymph of the tick that carries B. microti is about the size of a poppy seed and easy to miss, so full-body tick checks matter — above all for people without a spleen or who are elderly. Clinicians should consider babesiosis in unexplained flu-like illness, hemolytic anemia or thrombocytopenia, especially after possible tick exposure or a transfusion.

Sources

Based on Gray EB, Herwaldt BL, "Babesiosis Surveillance — United States, 2011–2015," MMWR Surveillance Summaries, volume 68, Centers for Disease Control and Prevention; a work of the United States government in the public domain.