When Vertical Transmission Affects Only One Twin

Congenital babesia is rare—but this case proves it happens. The patient was born at 36 5/7 weeks by C-section. At five-weeks-old the newborn presented to the emergency department with pallor, increased lethargy and difficulty feeding. The newborn was “more difficult to arouse and very pale compared to her twin brother,” the mother reported. “She was feeding with a similar frequency, however, with a decreased duration from 20 minutes to 10 minutes due to fatigue.”

This case of Babesia transmitted from mother to baby is the first report describing asymmetric transplacental transmission in twins—one twin infected, the other not.

The Mother’s History: Congenital Babesia Risk Factors

The mother had one febrile illness during pregnancy, “occurring at approximately 23-24 weeks of gestation, which was associated with a maculopapular rash that resolved spontaneously,” according to Walker et al.

Throughout her pregnancy, the mother had made several trips to Cape Cod, Massachusetts, an area endemic for Lyme disease and tick-borne co-infections.

This maternal exposure history is critical. The febrile illness at 23-24 weeks gestation—accompanied by rash—likely represented acute Babesia infection. The timing matters: infection during second or third trimester creates opportunity for transplacental transmission as parasitemia develops.

Cape Cod represents high-risk geography. Babesia microti is endemic in Cape Cod, with some of the highest infection rates in the United States. Multiple trips during pregnancy increased tick exposure risk substantially.

Congenital Babesia: Clinical Presentation

At examination, the newborn was “febrile to 100.4°F rectally, noticeably pale, but vigorous with mild tachypnea and tachycardia into the 170s-180s beats per minute,” the authors wrote.

Blood work revealed anemia, thrombocytopenia and an elevated white count. Liver function tests were elevated, as well.

“Because of the profound hematologic abnormalities, a routine thin smear was obtained, which was significant for multiple intraerythrocytic ringed parasites consistent with Babesia microti,” the authors wrote.

The diagnosis was made visually—parasites visible inside red blood cells on blood smear. This represents relatively high parasitemia, as lower-level infections may not show visible parasites on routine microscopy.

The clinical presentation—pallor, feeding difficulty, lethargy at 5 weeks of age—reflects progressive hemolytic anemia as parasites destroyed red blood cells. The comparison to her twin brother provided the clinical clue that something was wrong.

Treatment for Congenital Babesia

The newborn was treated with a blood transfusion, intravenous atovaquone twice daily and azithromycin daily. Within 5 days, her Babesia parasites had cleared.

Her twin brother was negative for Babesia microti, while the mother’s test results revealed Babesia microti IgG of 1:160 and IgM of <1:10, with a negative PCR consistent with cleared infection. The rapid clearance—within 5 days—demonstrates that neonatal Babesia responds well to appropriate antimicrobial therapy when diagnosed early. The blood transfusion addressed acute anemia while antibiotics eliminated parasites. The mother's serology confirmed past infection: positive IgG antibodies indicate previous exposure, while negative IgM and negative PCR indicate the infection had cleared before delivery. This pattern is consistent with maternal infection during second trimester followed by immune response and parasite clearance.

Why Asymmetric Transmission Occurs

“Although case reports of congenital babesiosis exist, this is the first report describing asymmetric transplacental transmission in twins,” the authors wrote.

This raises fundamental questions about vertical transmission mechanisms. Both twins shared the same intrauterine environment. Both were exposed to maternal parasitemia at the same gestational age. Yet only one became infected.

Possible explanations include differences in placental anatomy and blood flow between twin placentas, timing of maternal parasitemia relative to placental development, stochastic factors in parasite crossing placental barrier, or differential immune responses between twins.

The asymmetric transmission pattern demonstrates that maternal Babesia infection does not inevitably cause fetal infection—even among twins sharing the same maternal parasite exposure.

Implications for Obstetric Care

Furthermore, they suggest that “Tick-borne diseases, such as babesiosis, should be considered a part of the differential for anemia, thrombocytopenia, and neutropenia in a febrile infant as they are increasing in geographic range due to climate change.”

This case adds to the growing evidence that Babesia can be transmitted during pregnancy—and that not all babies exposed will become infected.

For obstetric practice, this means pregnant women in endemic areas who develop febrile illness should be evaluated for tick-borne infections including Babesia. Maternal infection during pregnancy should prompt consideration of congenital transmission, with close monitoring of newborns for hematologic abnormalities.

The increasing geographic range of Babesia—driven by climate change expanding tick habitat—means obstetric providers in areas not historically endemic may encounter cases previously unseen in their practice.

Diagnostic Challenges in Neonatal Babesia

Neonatal Babesia is easily missed because symptoms are non-specific. Pallor, lethargy, and poor feeding occur in many neonatal conditions. Without considering tick-borne disease in the differential diagnosis, blood smear examination—the test that revealed parasites—might never be ordered.

This case was diagnosed because “profound hematologic abnormalities” prompted routine blood smear examination. But mild cases with less severe anemia might not trigger this testing, leaving congenital Babesia unrecognized.

The twin comparison provided diagnostic clarity. If this had been a singleton birth, the mother might have attributed pallor and feeding difficulty to normal newborn variation rather than recognizing it as pathologic.

Frequently Asked Questions

Can Babesia be passed from mother to baby?
Yes. Congenital babesia transmission can occur during pregnancy, though it’s rare. This case shows it can affect one twin and not the other, demonstrating that maternal infection doesn’t inevitably cause fetal infection.

What are the symptoms of congenital Babesia?
Infected newborns may present with pallor, lethargy, difficulty feeding, fever, anemia, and thrombocytopenia—often within the first weeks of life. This baby showed symptoms at 5 weeks old with profound anemia and visible parasites on blood smear.

How is congenital Babesia diagnosed?
A blood smear revealing parasites in red blood cells is diagnostic. PCR testing and antibody tests can confirm the infection. In this case, routine blood smear ordered due to profound hematologic abnormalities revealed multiple intraerythrocytic ringed parasites.

How is congenital Babesia treated?
Treatment includes atovaquone and azithromycin, often with blood transfusions for severe anemia. This newborn’s parasites cleared within 5 days of treatment, demonstrating that neonatal Babesia responds well to appropriate therapy.

Should pregnant women be tested for Babesia?
Women who live in or travel to endemic areas and develop febrile illness during pregnancy should be evaluated for tick-borne diseases including Babesia. This mother had febrile illness with rash at 23-24 weeks gestation after multiple trips to Cape Cod—both risk factors for Babesia.

Clinical Takeaway

This case report documents the first asymmetric transplacental transmission of Babesia microti in twins—one infected, one not—challenging assumptions about vertical transmission mechanisms. A mother who made multiple trips to Cape Cod, Massachusetts during pregnancy developed febrile illness with maculopapular rash at 23-24 weeks gestation. This likely represented acute Babesia infection during second trimester. At delivery, one twin appeared healthy while her sister developed progressive symptoms over the first 5 weeks of life: increasing pallor, lethargy, and difficulty feeding. At 5 weeks, she presented to emergency department febrile, noticeably pale compared to her twin brother, with tachypnea and tachycardia. Blood work revealed profound hematologic abnormalities: anemia, thrombocytopenia, elevated white count, and elevated liver function tests. Routine blood smear showed multiple intraerythrocytic ringed parasites consistent with Babesia microti—the diagnostic finding. Her twin brother tested negative for Babesia. The mother’s serology revealed IgG antibodies (1:160) indicating past infection, with negative IgM and negative PCR consistent with cleared infection. This pattern confirms maternal infection during pregnancy followed by immune response and parasite clearance before delivery. Treatment with blood transfusion and atovaquone plus azithromycin cleared parasites within 5 days. The asymmetric transmission raises fundamental questions. Both twins shared the same intrauterine environment and exposure to maternal parasitemia at the same gestational age. Yet only one became infected. This demonstrates that maternal Babesia infection doesn’t inevitably cause fetal infection—even among twins with identical maternal exposure. Possible mechanisms include differences in placental anatomy between twin placentas, timing of parasitemia relative to placental development, stochastic factors in parasite crossing placental barrier, or differential immune responses. The clinical implications are clear. Tick-borne diseases should be considered in the differential diagnosis for neonatal anemia, thrombocytopenia, and fever—particularly when maternal exposure history includes endemic area travel during pregnancy. Pregnant women who develop febrile illness in endemic regions should be evaluated for Babesia and other tick-borne infections. Congenital transmission should prompt monitoring of newborns for hematologic abnormalities. The increasing geographic range of Babesia driven by climate change means obstetric providers in previously non-endemic areas may encounter cases they’ve never seen before. Without considering tick-borne disease in neonatal differential diagnosis, congenital Babesia is easily missed because symptoms are non-specific.