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How Viruses Like Zika Can Penetrate the Womb

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How Viruses Like Zika Can Penetrate the Womb

Aug 04, 2016

The placenta seems like the perfect organ to take care of a fetus. Letting in everything good and keeping out everything bad. But the placenta isn’t perfect. OBGYN Dr. Kirtly Parker Jones explores how the womb works and how infections like the Zika virus can sneak past its defenses to affect the growing fetus.

Episode Transcript

Dr. Jones: If the womb is such a safe place, how does the Zika virus get in? This is Dr. Kirtly Parker Jones from Obstetrics and Gynecology at University of Utah Health Care and this is The Scope.

Announcer: Covering all aspects of women's health. This is "The Seven Domains of Women's Health" with Dr. Kirtly Jones on The Scope.

Dr. Jones: When I was a second-year medical student, I absorbed the idea that the womb was a perfectly safe place. I use the word absorbed because I don't think they taught that to me, I think I just figured that out. The placenta was the perfect mother, serving and protecting the fetus. Serving in that the placenta actively pumps some nutrients like to the developing fetus like oxygen and glucose. Protecting in that it keeps out large molecules and only smaller molecules can get in. Well, this was a rather romantic idea, but I was a rather romantic medical student.

By the time I was a third-year medical student, I was taught about congenital infections, infections in the child that start in the womb. Infections such as toxoplasmosis from uncooked pork and kitty litter, syphilis, well, from you know how. These aren't just molecules, these are whole organisms. They get through the placenta and into the fetus. We also learned about viruses such as chicken pox and rubella infecting the fetus. But in the dazed mind of a totally engaged but overextended third-year medical student, I never really questioned how they did that.

If the womb is a perfect mother protecting the fetus, how do those things get in? Well, first, it's important to remember that viruses are really sneaky. Here's a little bit on virus biology. They're just a little packet of DNA wrapped in a membrane. They don't have the ability to reproduce themselves. They get into cells and hijack the cell's energy and DNA and proteins and make the cell make more virus. The interesting part is that viruses are choosy. They have markers on their covering that attach to specific proteins on certain kinds of cells. Some viruses like the respiratory tract but not the skin. Some viruses like white cells, like the HIV virus.

The Zika virus, which has already found cells in the infected mother to turn into Zika factories, and the virus spreads throughout the mother's body. The virus then may specifically bind to the lining of the placenta, the amnion. Then the virus turns the amniotic membrane into a Zika virus factory. Then the Zika virus hatches out of the amnion cells into the fluid around the fetus and the fetus gets infected. Specifically, the virus likes nerve cells; it likes fetal brain cells. It binds the fetal brain cell, turns it into a Zika factory and then in the process it kills the brain cell.

There is another mechanism by which the Zika virus might get into the womb and that is through the leaky placental cells that are made early in development in the first trimester. The virus then seems to specifically attach to the stem cells of the embryonic brain. Destroying these cells, which have been infected by the virus, may explain why the findings of microcephaly, small brain, in some of the babies that were infected in pregnancy. The virus can get in at any time of pregnancy, but it's particularly successful and potentially damaging in the first trimester.

Researchers also found that different strains of the Zika virus were more infectious, more successful at hijacking placental cells than others. The earlier African strain was less infectious than the current South American strain, which is much more infectious and much more likely to infect the fetus. In fact, it seems it's not just spread through infection by mosquitoes, but it's spread sexually from men to women and now, we think, from women to men. There's the possibility that it might be spread from other body fluids as well.

Actually, it is remarkable that the placenta and the membranes can protect the fetus from as many viruses as it does. Only a few viruses that we know about specifically attack placental cells and then go on to infect and harm the fetus. The chicken pox virus, the measles virus and the cytomegalovirus are examples and we have vaccines for some of these. Understanding the mechanisms by which viruses can infect placental cells can help us develop anti-viral agents and other approaches to treatment.

Viruses are particularly successful and rapidly changing in their molecular appearance, meaning the way their face looks to different cells. So it may be difficult to make a vaccine. Or because the Zika virus belongs to a family of viruses that include Dengue yellow fever and we have a vaccine for some of those, we might be able to make a vaccine for all three or maybe not.

In the meantime, it's important to remember that a womb isn't a completely protected place. And the placenta isn't a perfect mother, letting in only the good in keeping out the bad. We need to remember to offer protection to pregnant women and women who might become pregnant from viruses, drugs and molecules in air and water pollution that can get through the placenta and affect the fetus. Thanks for joining us on The Scope.

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