What Is Congenital Toxoplasmosis?
Toxoplasmosis is an illness caused by the single-celled parasite, Toxoplasma gondii . While this parasite is found worldwide and can infect nearly all warm-blooded animals, its life cycle is uniquely dependent on the cat family. Although a large portion of the global population may carry the parasite, a healthy immune system typically keeps it in a dormant, symptom-free state.
Humans can become infected by eating undercooked meat containing parasite cysts or by accidentally ingesting parasite eggs from contaminated soil, water, or cat feces. While these infections are usually mild in healthy individuals, the situation changes dramatically if a woman acquires the parasite for the first time during pregnancy. In this case, the infection can be passed to her fetus, causing a condition known as congenital toxoplasmosis.
The Main Cause: Primary Infection During Pregnancy
Congenital toxoplasmosis is almost always the result of a primary infection—when an expectant mother contracts T. gondii for the first time shortly before or during her pregnancy. While her own immune system can typically control the parasite, often without any noticeable symptoms, it may still cross the placenta and infect the developing fetus.
The Paradox of Timing and Risk
The link between when the infection occurs and the potential outcome for the baby is crucial. A paradoxical relationship exists between the risk of transmission and the risk of severe harm.
- Risk of Transmission: The likelihood of the parasite passing from mother to fetus is lowest in the first trimester and steadily increases, peaking in the third trimester. This is because the placenta becomes more permeable over time.
- Risk of Severity: Conversely, the potential for severe, long-term damage to the fetus is highest if the infection occurs in the first trimester and decreases as the pregnancy progresses.
Impact by Trimester
The consequences of a congenital infection are directly tied to when the mother was infected.
- First Trimester: During this critical period of organ formation, the fetus is most vulnerable. Although transmission is less likely, an infection can have devastating outcomes, including miscarriage or significant, lifelong damage to the brain and eyes.
- Second Trimester: As the risk of transmission rises, an infection can still cause serious problems. These may include neurological conditions or an eye inflammation called retinochoroiditis, which can permanently impair vision.
- Third Trimester: Transmission is most likely in the final months. Because the fetus is nearly fully developed, many babies are born with a subclinical infection, meaning they appear healthy at birth. However, they remain at high risk for developing complications years later.
Diagnosing a New Infection
Most new toxoplasmosis infections are asymptomatic or cause only mild, flu-like symptoms, such as fatigue or swollen lymph nodes. Therefore, the infection can easily go undetected. To confirm a new infection, doctors use blood tests to look for specific antibodies the immune system creates to fight the parasite. A diagnosis is confirmed if a person who previously tested negative now tests positive, or if their antibody levels rise sharply over a few weeks.
A Less Common Cause: Reactivation of a Prior Infection
For individuals who were infected with toxoplasmosis before becoming pregnant, the risk of passing it to their baby is extremely low. A healthy immune system creates lifelong defenses that keep the parasite dormant. However, in rare circumstances, this prior infection can reactivate and pose a threat.
This risk primarily applies to pregnant individuals with a severely weakened immune system, particularly the part responsible for keeping chronic infections in check. This can be caused by conditions like advanced HIV or the use of powerful immunosuppressant drugs for organ transplants or autoimmune diseases. In these cases, the body’s defenses may fail, allowing the dormant parasite to reactivate, multiply, and potentially cross the placenta.
An even rarer possibility is reinfection with a completely different strain of the T. gondii parasite. If a person encounters a genetically distinct type that their immune system does not fully recognize, it could trigger a new active infection, posing a risk similar to a primary one. This is considered an exceptional event.
Infant Outcomes and Delayed Symptoms
A significant challenge with congenital toxoplasmosis is that most infected infants, especially those infected late in pregnancy, appear perfectly healthy at birth. This is known as a subclinical infection. While this is encouraging, the parasite remains dormant in the baby’s body as tissue cysts, typically in the brain, muscles, and eyes, creating a risk for serious health problems that may not surface for years.
Because of these risks, long-term medical follow-up is essential for any infant diagnosed with congenital toxoplasmosis, even if they show no symptoms at birth.
- Delayed Vision Problems: Dormant parasite cysts in the eyes can reactivate years later, often during the teen years. This inflammation can cause scarring and permanent vision loss, making regular eye exams crucial.
- Neurological Issues: Children born with subclinical infections are at risk for developing more subtle neurological or developmental challenges. These may appear later as learning disabilities or cognitive delays.
- Importance of Monitoring: Ongoing care from specialists, particularly an ophthalmologist, is vital to screen for emerging issues. Early detection and management of late-onset complications are key to preserving a child’s vision and long-term health.
