At this time, the Zika virus is affecting the most people in hot, humid tropical and subtropical locations near coastlines and people who live in temperate zones but got infected during travel closer to the equator. More than 2 billion people live in tropical and subtropical zones as does the Aedes aegypti mosquito, the main carrier for the virus. It is well understood that the Aedes mosquito is the primary source of infection but little is known how the virus travels across the maternal-fetal barrier that otherwise protects a fetus from outside threats during gestation.
Risk on a Global Scale
Of the 2 billion people cohabiting with the Aedes mosquito, approximately 4 million are expected to become infected with the virus during 2016. Most people won’t be harmed by exposure to the virus but the damage done to a fetus is severe and carries life-long consequences so challenging it is predicted some local public health services will be overburdened to the point of collapse.
The Maternal-Fetal Barrier
The ability of the virus to go from mother’s bloodstream to the fetal brain has been a mystery to medical scientists around the world but a new study brings new insights on how the Zika virus affects pregnancy. The researchers studied tissue from the placentas of three women free of the virus to see what changes occurred once the virus was introduced.
The placenta is an organ vital to all human beings but, even so, very little is understood about how it works. It is the only organ that forms in adulthood but, since it does so during pregnancy, there has been very little study of it. With a pregnancy and new human life dependent upon the health of the placenta, it has been almost impossible to study the placenta in depth. The researchers used placental tissue naturally expelled shortly after childbirth for their study.
Viral Effect on Placenta
The research revealed two types of placental cells that seem to be important for the survival and spread of the virus:
- Hofbauer cells– Usually work with the immune system to keep pathogens (foreign bodies such as a virus or bacterium that causes disease) away from the fetus and amniotic sac.
- Fibroblasts– These building blocks of the placenta give the organ its shape and structure.
The researchers discovered high concentrations of the virus in these cells. The Hofbauer cells were described as “physically housing” the virus and it is suggested these immune-system cells may actually transport the virus to the fetal brain.
It remains unknown how the virus crosses the maternal-fetal barrier to reach the placenta but theories include:
- Piggybacking– The virus might attach itself to antibodies that go from mother to fetus to keep the fetus healthy in normal circumstances.
- Viral size– The virus might be so small it can push its way, undetected, between the cells of the membrane that separates mother and fetus. The cells of this membrane are packed extremely closely together so that nothing more than nutrients and oxygen can pass through.
At this time, it is believed the extent of fetal damage is linked to which trimester the mother becomes infected. Long-term consequences remain unknown at this time, however, since the link between the Zika virus and microcephaly in Brazil first became apparent only in the summer of 2015. The researchers indicate more study of how the virus affects the placenta may be the key to understanding more about the mechanisms of disease.
Sources:
Boeuf, Phillipe, et al. "The global threat of Zika virus to pregnancy: epidemiology, clinical perspectives, mechanisms, and impact." PMC. BMC Medicine / BioMed Central, Aug. 2016. US National Library of Medicine / National Institutes of Health. Web. 19 Aug. 2016.
Hemphill, Sandy. "Zika Virus: How It Destroys Fetal Brains, Which US Cities Most Vulnerable." babyMed. BabyMed.com, Mar. 2016. Web. 19 Aug. 2016.
Jurado, Kellie Ann, et al. "Zika virus productively infects primary human placenta-specific macrophages." JCI Insight / American Society for Clinical Investigation (2016). Web. 19 Aug. 2016.
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By Sandy Hemphill, Contributing Writer, BabyMed