What the Science Tells Us So Far About Congenital Transmission of Borrelia, Babesia, and Bartonella
For decades, infections during pregnancy have been an important part of prenatal medicine.
Pregnant women are routinely screened for certain infections known to affect pregnancy or the developing baby. We also recognize that some infections acquired during pregnancy can cross the placenta and reach the fetus.
But there is another group of organisms that deserves more attention in this conversation: Borrelia, Babesia, and Bartonella.
These organisms are best known as vector-borne infections transmitted by ticks and other insect or animal vectors. But an important question has emerged from both human case reports and animal research:
Can these infections also be passed from an infected mother to her developing baby?
The evidence suggests that they can—at least under some circumstances.
What we don't yet know is how frequently this occurs, which pregnancies are most vulnerable, whether maternal treatment prevents transmission, and what congenital infection might mean for the long-term health of the child.
Those distinctions are important. Evidence that an organism can cross the placenta does not establish that congenital transmission is common—or that every exposed baby will become ill.
But the evidence that has accumulated deserves a closer look.
Borrelia: Evidence That Transplacental Transmission Can Occur
Borrelia burgdorferi and related species are the bacteria responsible for Lyme borreliosis.
The possibility of transmission during pregnancy isn't a new concern.
As far back as 1985, the CDC reported that transplacental transmission of B. burgdorferi had been documented in an untreated pregnant woman with Lyme disease. The infant had a congenital heart defect, although importantly, a causal relationship between the infection and the heart defect was not established.
The CDC continues to recognize that untreated Lyme disease during pregnancy can result in infection of the placenta and that spread from mother to fetus is possible, although considered rare. We also lack adequate studies evaluating the long-term developmental outcomes of children whose mothers had Lyme disease during pregnancy.
Other case reports have described Borrelia in placental or fetal tissues, but these reports are small and the methods used across studies have varied considerably.
A 2018 systematic review examined the worldwide literature on gestational Lyme disease. The investigators concluded that case reports provided limited evidence for transplacental transmission of B. burgdorferi but that the available evidence was insufficient to clearly characterize its frequency or consequences.
Interestingly, the review also found an association between treatment of Lyme disease during pregnancy and better pregnancy outcomes compared with untreated disease.
What Do Animal Studies Tell Us?
Animal research adds another layer of evidence, although results vary among species and experimental models.
Experimental infection of pregnant dogs has produced evidence consistent with intrauterine transmission of B. burgdorferi. Other animal models, particularly some rodent studies, have not consistently demonstrated transmission.
That variability is important.
Animal studies do not establish that congenital Borrelia transmission routinely occurs in humans. Instead, they help demonstrate whether maternal-fetal transmission is biologically possible and suggest that transmission may depend upon the organism, host, timing of infection and other factors.
Taken together with the human case literature, the evidence supports the possibility of transplacental Borrelia infection while leaving some very important questions unanswered.
How often does it happen?
Does the stage of maternal infection matter?
Does treatment reliably prevent transmission?
And if transmission occurs, what are the long-term consequences for the child?
We simply don't have adequate prospective human studies to answer those questions.
Babesia: The Clearest Evidence for Congenital Transmission
The evidence is considerably stronger for Babesia.
Babesia is a parasite that infects red blood cells, producing an illness with some similarities to malaria. Although tick transmission is the best-known route, congenital babesiosis has been documented in humans.
One particularly informative U.S. case involved a six-week-old infant hospitalized with fever, irritability and decreased feeding. The mother had been asymptomatic during pregnancy.
Investigators found evidence of Babesia antibodies in the baby's newborn heel-stick specimen and subsequently detected Babesia DNA in placental tissue, supporting transmission during pregnancy rather than a tick exposure occurring after birth.
Other reports have described congenital babesiosis presenting during the first weeks or months of life with findings such as fever, severe anemia, thrombocytopenia, jaundice, lethargy and hepatosplenomegaly.
More recently, investigators reported a particularly compelling case in which both mother and infant tested positive for Babesia microti using molecular methods. Genetic analysis identified an identical strain in mother and child, providing strong evidence for maternal transmission. The mother had been asymptomatic.
This demonstrates something important that applies to this entire discussion:
A mother does not necessarily have to be severely ill for maternal transmission to be considered.
Animal Studies Make the Babesia Case Even Stronger
Animal research provides particularly compelling evidence for vertical transmission of Babesia.
In one experimental study, a chronically Babesia gibsoni-infected female dog was bred with an uninfected male. She delivered four live puppies and one stillborn puppy. B. gibsoni DNA was identified in tissues from all five puppies, and the experimental design supported transplacental rather than nursing-related transmission.
Vertical transmission has also been demonstrated experimentally with B. microti in mice.
In one study, researchers detected congenital B. microti infection by PCR in 96% of pups born to females with post-acute or chronic infection.
Another mouse study found vertical transmission in approximately 63% of offspring from chronically infected females and demonstrated B. microti DNA in embryos, pups and placental tissues.
Researchers have also demonstrated vertical transmission in the white-footed mouse, Peromyscus leucopus—an important natural reservoir host for B. microti—suggesting that maternal transmission may even contribute to maintenance of the organism within natural animal populations.
For Babesia, therefore, congenital transmission isn't merely theoretical.
It has been demonstrated in humans as well as multiple animal models.
What remains unclear is how frequently it occurs in human pregnancies.
Bartonella: An Emerging but Incomplete Story
The evidence surrounding Bartonella is less developed than the Babesia literature, but there are intriguing findings in both humans and animals.
Bartonella species are unusual bacteria capable of establishing persistent infection in blood and vascular-associated tissues.
Evidence From Human Cases
A 2010 report described a family in which molecular testing identified Bartonella henselae and/or Bartonella vinsonii subsp. berkhoffii in multiple family members. Based on the clinical history and molecular findings, the investigators concluded that the results supported perinatal transmission of Bartonella species within the family.
Another report involved Bartonella bacilliformis, the organism responsible for Carrion's disease.
A 22-day-old infant in Peru developed fever, jaundice, severe anemia, gastrointestinal bleeding and other complications. B. bacilliformis was identified and confirmed by PCR. The infant had been born in an area where the disease wasn't endemic and had not traveled to an endemic region, while his mother came from an endemic region and had experienced illness during pregnancy consistent with possible Bartonella infection. The investigators concluded that the findings supported vertical transmission.
An additional report described an infant born prematurely to a mother with B. bacilliformis infection. A blood culture obtained from the baby approximately 90 minutes after birth was positive for B. bacilliformis, providing another intriguing observation supporting maternal-fetal transmission.
These are important observations—but they remain case reports rather than large prospective studies.
We therefore cannot use them to determine how frequently Bartonella is transmitted during human pregnancy.
Animal Research Adds Biological Plausibility
The animal literature strengthens the biological case.
Researchers have isolated Bartonella species from placental tissues, embryos and neonatal offspring of naturally infected rodents. Experimental mouse studies have also demonstrated transplacental transmission under certain conditions.
Studies of naturally infected voles have likewise found evidence of Bartonella infection in embryos from infected mothers, although transmission rates vary considerably among species and studies.
That variability is itself informative.
Vertical transmission may depend upon the Bartonella species, host species, maternal bacterial burden, timing of infection, immune response and other biological factors.
For humans, however, we remain at an early stage of understanding.
Transmission Does Not Automatically Mean Disease
This may be the most important distinction in this entire discussion.
There are actually several different questions:
Can the organism reach the placenta?
Can it cross from mother to fetus?
Can it persist in the infant?
And if it persists, does it cause illness?
Those are not the same question.
A microorganism could potentially be transmitted without producing recognizable disease. Another infant might develop an acute illness. Whether some congenitally acquired infections could persist and contribute to health problems later in childhood is a much larger question—and one that has simply not been adequately studied.
This is particularly important when evaluating families in which both mother and child test positive for the same organism.
Finding Borrelia, Babesia or Bartonella in both does not prove congenital transmission. Mother and child could potentially have acquired the same infection independently through environmental or vector exposure.
The strongest evidence comes from studies that establish maternal infection during pregnancy and then examine the placenta, cord blood, neonatal blood or tissues using validated methods.
Why Don't We Know More?
One reason may be surprisingly simple:
We generally aren't looking.
If an infection isn't routinely considered during pregnancy, mothers aren't systematically tested for it.
Placentas and cord blood aren't routinely examined for the organism.
Babies aren't prospectively tested and followed.
That creates a difficult scientific problem.
If we don't look for congenital transmission, we collect very little data about congenital transmission. And without adequate data, we can't determine how frequently it actually occurs.
The solution isn't to assume these infections are commonly transmitted from mother to baby.
It's to study the question properly.
What Research Is Needed?
The next step should be well-designed prospective studies, particularly in regions where these infections are common.
Researchers could identify women with well-documented Borrelia, Babesia or Bartonella infection before or during pregnancy, characterize whether infection appears active, document treatment, and follow both mother and child.
Studies could examine appropriate combinations of maternal blood, placenta, cord blood and neonatal specimens using validated molecular, culture, microscopy, antigen and serologic techniques.
Children could then be followed longitudinally.
This would allow us to answer the questions that case reports cannot:
How often does transmission occur?
Which species are most likely to be transmitted?
Does the timing of maternal infection matter?
Does chronic or asymptomatic infection carry risk?
Does appropriate treatment during pregnancy reduce transmission?
Do transmitted infections persist?
And most importantly, what are the short- and long-term consequences for the child?
Where I Hope the Science Leads
We already routinely screen pregnant women for infections when evidence demonstrates that identifying and treating them improves outcomes for mothers or babies.
I believe Borrelia, Babesia and Bartonella deserve the same level of rigorous investigation.
The current evidence is not sufficient to recommend universal prenatal screening for all three infections. We still need prospective studies establishing prevalence, determining which testing methods are reliable during pregnancy, clarifying the consequences of maternal infection, and demonstrating whether screening and treatment improve outcomes.
But absence of sufficient evidence for universal screening is not the same thing as evidence that congenital transmission doesn't occur.
For Babesia, congenital transmission has already been documented in humans and demonstrated experimentally in animals.
For Borrelia, human evidence supports the possibility of transplacental transmission, although its frequency and clinical significance remain uncertain.
For Bartonella, animal studies demonstrate biological plausibility and human case reports provide evidence suggesting that maternal transmission can occur, but the evidence base remains limited.
That should be a reason for more research—not dismissal of the question.
My hope is that carefully designed studies will eventually give us enough information to determine who should be tested, which tests should be used, when treatment is appropriate, and whether Borrelia, Babesia and Bartonella should someday be added to the infections for which women are routinely screened during pregnancy.
Until then, the most scientifically responsible conclusion is also the simplest:
There is evidence that congenital transmission can occur. What we urgently need to understand is how often it happens—and what it means for the health of the child.
References
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- Centers for Disease Control and Prevention. Lyme Disease and Cases Occurring During Pregnancy—United States. MMWR Morb Mortal Wkly Rep. 1985.
- Centers for Disease Control and Prevention. How Lyme Disease Spreads: Pregnancy and Lyme Disease. CDC.
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- Tufts DM, et al. Vertical transmission: a vector-independent transmission pathway of Babesia microti in the natural reservoir host Peromyscus leucopus. 2021.
- Breitschwerdt EB, Maggi RG, Farmer P, Mascarelli PE. Molecular evidence of perinatal transmission of Bartonella vinsonii subsp. berkhoffii and Bartonella henselae to a child. J Clin Microbiol. 2010;48(6):2289–2293.
- Tuya XL, Escalante-Kanashiro R, Tinco-Valdez C, et al. Possible vertical transmission of Bartonella bacilliformis in Peru. Am J Trop Med Hyg. 2015;92(1):126–128.
- Kosoy MY, Regnery RL, Tzianabos T, et al. Isolation of Bartonella spp. from embryos and neonates of naturally infected rodents. J Wildl Dis. 1998;34(2):305–309.
- Bartonella infections in three species of Microtus: prevalence and genetic diversity, vertical transmission and the effect of concurrent Babesia microti infection on its success. Parasites & Vectors. 2018.