Placental malaria is a localized infection of the placenta that occurs during pregnancy. It is a complication of malaria where parasites, particularly Plasmodium falciparum, accumulate in the placental tissue. The infection poses a health risk to both the mother and the developing fetus, representing a public health issue in regions where malaria is common. Each year, an estimated 125 million pregnancies are at risk globally.
The Mechanism of Placental Sequestration
The development of placental malaria is a process of sequestration, where red blood cells infected with the Plasmodium falciparum parasite accumulate in the placenta. These infected erythrocytes express a protein on their surface known as P. falciparum erythrocyte membrane protein 1 (PfEMP1). A variant of this protein, VAR2CSA, functions as a biological adhesive.
This VAR2CSA protein allows the infected red blood cells to bind to a sugar molecule called chondroitin sulfate A (CSA), which is present in the intervillous spaces of the placenta. This is the area where nutrient and gas exchange between mother and fetus occurs. This binding enables the parasite to anchor itself within the placenta, avoiding clearance by the spleen and allowing it to multiply undisturbed.
This mechanism explains why the condition is most frequent and severe in women experiencing their first pregnancy, often referred to as primigravidae. These individuals have not yet developed the specific antibodies needed to fight off the CSA-binding parasites. Subsequent pregnancies see a lower risk because the mother has acquired immunity from previous exposure.
Health Consequences for Mother and Fetus
The sequestration of parasites in the placenta triggers an inflammatory response and obstructs blood flow, leading to health consequences for both mother and fetus. For the pregnant woman, the infection increases the risk of developing severe maternal anemia because the parasites destroy red blood cells. This condition also makes the mother more susceptible to progressing to severe malaria, which can be life-threatening.
For the developing fetus, the accumulation of infected red blood cells and inflammation impair the placenta’s function. This disrupts the transfer of oxygen and nutrients from the mother to the fetus. This placental insufficiency can lead to several adverse outcomes:
- Intrauterine growth restriction (IUGR), where the baby does not grow at a normal rate.
- Low birth weight (LBW), a predictor of infant mortality.
- Preterm delivery, where the baby is born before 37 weeks of gestation.
- Stillbirth or death of the newborn in severe instances.
- Congenital malaria, if the infection is transmitted to the baby.
Diagnosis and Clinical Presentation
A challenge in managing placental malaria is that many infected women show no symptoms, or only mild ones like fever and fatigue that can be mistaken for normal pregnancy discomforts. This means many infections go undetected and untreated.
The standard method for diagnosing malaria is a microscopic examination of a peripheral blood smear, but this method has limitations for placental malaria. Because the parasites are sequestered within the placenta, they may be at undetectable levels in the mother’s circulating blood. Therefore, a negative blood smear does not rule out a placental infection.
A definitive diagnosis can be made after delivery through a histological examination of placental tissue, which involves looking for parasites and pigment deposits. While this confirms the infection, it is not a practical approach for management during the pregnancy. This diagnostic challenge underscores the importance of preventive measures.
Prevention and Treatment Strategies
Given the diagnostic difficulties and risks involved, the focus for managing placental malaria is on prevention. The World Health Organization (WHO) recommends a two-pronged approach for pregnant women in malaria-endemic areas. The first strategy is the consistent use of insecticide-treated nets (ITNs) to prevent mosquito bites, as the mosquitoes that transmit malaria are most active at night.
The second preventive measure is Intermittent Preventive Treatment in pregnancy (IPTp). This involves administering a therapeutic course of an antimalarial drug at routine antenatal care visits. The most common medication for this is sulfadoxine-pyrimethamine (SP), given to pregnant women starting in the second trimester to clear existing parasites and prevent new infections.
When a pregnant woman is suspected of having malaria, prompt treatment is necessary. The choice of antimalarial medication is guided by the trimester of pregnancy and local patterns of drug resistance. Certain antimalarials are avoided during the first trimester due to potential risks to the developing fetus.