When a newborn has been exposed to a parasite but looks completely well

Four studies from France, Italy and Spain published between August 2025 and July 2026 examine how doctors can still tell which exposed babies are infected with Toxoplasma, after key laboratory tests were taken off the market

Most babies born with toxoplasmosis look perfectly healthy, and the harm this parasite can do to the eyes and brain often appears only years later. Finding the infection early is what makes a year of treatment worth giving — yet two of the laboratory tests doctors relied on to find it were recently withdrawn from sale in Europe. Four new studies ask what still works.

Toxoplasma is a microscopic parasite carried by cats and found in soil, untreated water, unwashed vegetables and undercooked meat. Most adults who catch it never notice. The problem arises only when a woman catches it for the first time while pregnant, because the parasite can then cross the placenta and reach the baby. There is no rash and no fever to see at birth, which is why a baby whose mother caught the infection during pregnancy is sent for blood tests rather than simply watched: the tests look for something that has not shown itself yet.

Why this was a difficult problem long before any test was withdrawn

For decades, doctors have faced the same obstacle. A mother's antibodies — the proteins her immune system makes to fight an infection — cross the placenta into her baby before birth. A newborn's blood is therefore full of the mother's antibodies, and a positive test in the first days of life may simply show the mother's immune response, not the baby's own [1]. Those borrowed antibodies do not fade for six to twelve months, and waiting that long is not acceptable when early treatment is the whole point — so laboratories learned to look for the few kinds of antibody a mother cannot pass on, which means anything found must have been made by the baby.

Timing during pregnancy matters enormously, in a way that can seem backwards. The later in pregnancy a mother catches the infection, the more likely the baby is to catch it too — but the less likely the baby is to be seriously harmed. Infection early in pregnancy is uncommon but can be severe [2]. Babies who are infected and never treated do develop problems: eye scarring and neurological injury are well documented [3]. Worldwide, researchers have estimated that roughly 190,000 babies are born with the infection each year [4].

Countries have taken different paths in response. France and Italy test every pregnant woman monthly and, if she catches the infection, start treatment straight away and then follow the baby's blood tests carefully after birth [5]. The United States does not screen every pregnancy, relying instead on doctors noticing something and testing the newborn [6]. These two approaches grew from the same research, and the disagreement has lasted decades [7], partly because a large pooled analysis found only weak evidence that treating a mother during pregnancy prevents the baby from becoming infected at all, though it may reduce how badly the baby is affected if started early [8].

The screening approach relied on particular commercial test kits. Between 2023 and 2024, under changing European regulations, the most trusted test for one type of antibody was discontinued and the only two available tests for another type were withdrawn entirely [9]. Laboratories across Europe suddenly needed to know what they could still rely on.

What the four studies looked at

A hospital in Rennes, France, reviewed 1,171 blood samples from 483 babies born between 2010 and 2023 whose mothers had been infected during pregnancy, 56 of whom turned out to be infected. It asked how useful one of the withdrawn tests — for an antibody called IgA — had actually been [9].

A specialist centre in Pavia, Italy, followed 44 babies born between 2019 and 2022, 19 of them infected, and compared a more detailed laboratory technique called comparative Western blot against the two older antibody tests [10].

A team in Catalonia, Spain, examined 3,231 heel-prick cards — the small blood spots taken from every newborn for routine screening — collected between September 2022 and August 2023, plus 30 stored cards from babies already known to be infected. They asked whether these cards could be used to find infected babies [11].

And the TOXODIAG study (a new diagnostic approach for congenital toxoplasmosis), run across five Paris maternity hospitals, followed 70 pregnant women between 2018 and 2022 to test an entirely new idea: instead of looking for antibodies in the baby's blood, look for the immune cells that make them [12].

What they found

The Rennes team found that the withdrawn IgA test had never been pulling much weight. It correctly identified about 71% of infected babies, but a third of the babies who tested positive were not infected at all — in most of those cases the antibody had simply come from the mother and disappeared within a month. Out of 40 infected babies who tested positive at some point, the IgA test was the first test to give the answer in only two [9]. Losing it, in other words, costs very little.

The Pavia team found a clear winner. At birth, comparative Western blot identified about 89% of infected babies, compared with roughly 53% and 58% for the two older antibody tests — a difference large enough to be statistically meaningful [10]. It also gave no false positives at all among the uninfected babies tested at birth. When a result was uncertain, a different kind of test that measures the immune system's cellular response settled the question, a role it has been described as filling before [13].

The Catalonian result was the most clear-cut, and the most disappointing. Of 30 stored heel-prick cards from babies known to be infected, only one showed the antibody the test was looking for — even though 10 of 27 had shown it in an ordinary blood sample taken at birth [11]. The cards were excellent, however, at showing whether the mother had ever been exposed: that information was present in all 30, and across the 3,200 routine cards about 15.5% of mothers in Catalonia had been exposed, rising to about 30% among mothers born in South America. The researchers note the cards had been stored for months or years, which may have weakened them, but their central point stands: the heel-prick card is a good tool for understanding a population and a poor one for finding an individual infected baby. And there is a deeper objection — a test done after birth arrives too late to offer any treatment during pregnancy.

The Paris team's new idea did not work either, at least not yet. They could find the relevant immune cells in the mothers' blood, both when the infection was first detected and again at delivery, but not in any of the umbilical cord blood samples — not even from the babies who were infected [12]. They were candid: if the likely reasons cannot be solved, the approach is the wrong one.

What this means for families

If your baby has been exposed, the picture is more reassuring than the story of withdrawn tests might suggest. The tests that were lost were not the ones doing the important work. Where hospitals can perform comparative Western blot alongside a good antibody test, and where a sample from the mother is taken at the same time as the baby's, the ability to identify infected babies early is intact.

It also helps to know what to expect. You will be asked to bring your baby back repeatedly — monthly at first, then less often — for a year or so, and this is not a sign that something is wrong. It is how the borrowed maternal antibodies are watched until they disappear. A positive antibody result in the first days is very often your own immune system showing up in your baby's blood, which is why doctors will not act on a single result. And if treatment is started before the diagnosis is certain, the treatment itself can make the later tests fainter — so a quiet result after several weeks of medicine means less than a quiet result on day three. None of this is hidden; it is worth asking your team to explain where in that sequence your baby currently sits.

What researchers are working on next

Two convenient shortcuts have now been tested and closed off — reading the routine heel-prick card [11] and measuring the baby's own immune cells in cord blood [12]. Closing off a shortcut is genuinely useful, because it stops hospitals investing in something that would miss most cases. The Paris team did make one real technical advance along the way, showing that three separate laboratory reagents can be combined into one, which would make any future version of their test faster and cheaper [12].

The bigger unfinished work lies outside the countries where these studies were done. All were carried out where every pregnancy is screened and mothers treated quickly — and treating the mother, while good for the baby, makes the baby's own antibody response weaker and harder to detect, so the tests looked less sensitive here than they would elsewhere [3]. A large Spanish study found that screening during pregnancy identified nearly 93% of infected babies and that treating mothers improved how those babies did [14], which is the strongest argument for keeping screening where it exists. Where there is no screening programme at all, a single reliable newborn test would be transformative — and that test still does not exist. Studies from the Brazilian Amazon have documented how many babies are affected in such settings [15], and modelling suggests the burden across Africa may grow as populations and temperatures change [16]. Even the medicines are not fully standardised, which is why researchers reviewed the available treatment studies together in 2025 [17].

The answer these four studies give is a practical one: nothing essential was lost. Finding congenital toxoplasmosis still depends on testing mothers during pregnancy, testing mother and baby together after birth, and following the results patiently over the first year — and that path, though slow, still works.

For the other infections that can pass from mother to baby, see our articles on screening newborns for congenital cytomegalovirus and the gap in detecting congenital syphilis.

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