Above: Head of a child with microcephaly (left), compared to a child with a typical head size (right) (US Centers for Disease Control and Prevention Public Domain, Wikimedia Commons
Travel advisories are warning pregnant women not to travel to countries affected by the Zika virus (ZIKV). This is because ZIKV has been linked to birth defects, especially microcephaly.
ZIKV is mainly transmitted by the Aedes mosquito. It can also be transmitted through sexual contact with an infected partner. Pregnant women who become infected with ZIKV may pass the virus onto their unborn babies.
What is microcephaly?
Sometimes, babies of ZIKV-infected mothers develop a condition known as microcephaly. Microcephaly occurs when a baby’s brain does not fully develop in the womb. As a result, these babies are born with smaller-than-normal heads.
When these babies grow up, they may suffer developmental delays, learning disabilities, or problems with movement, hearing or vision. Microcephaly has been around for a while and has many different causes. However, the 2015-2016 ZIKV outbreak has been linked to a large increase in the number of cases.
Did you know? To diagnose microcephaly, doctors measure the circumference of a baby’s head. Then, they compare it to the standard measure for the baby’s age and weight.
How does a mosquito bite lead to microcephaly?
Let’s follow the path from a mosquito bite to microcephaly. Zika is a virus. In order to reproduce, it must infect living cells. Once it enters cells of the host (the individual bitten by the mosquito), the virus makes copies of itself. It can then infect other cells.
Not all viruses attack all cells. Some target specific cells, a fact that often determines the effects of the virus. For example, HIV, the virus that causes AIDS, infects and kills immune cells called T-cells. Hepatitis A targets liver cells.
A cell-specific viruses will have proteins on its surface that will only bind with matching receptors on the surface of specific cells. It’s like a key and lock. When a virus comes in contact with a receptor that fits, it binds to the cell. Once attached, the virus can enter the cell.
ZIKV seems to be especially good at infecting fetal brain cells. But in order to get to the fetus, the virus must pass through the placenta. That’s the organ that connects a developing baby to its mother. Usually, the placenta acts as a barrier against foreign substances. However, researchers have found that ZIKV may have figured out a way to get through it! It seems that ZIKV can infect immune cells in the placenta without actually killing them. Instead, it uses them to get access to the fetus.
Did you know? Microcephaly can occur when the bones that form the skull fuse together too soon. When this happens, there’s not enough room left for the brain to grow.
When it makes it to the womb, ZIKV appears to infect a group of cells called neural progenitor cells. These are the cells responsible for building brain tissue. In one study, researchers infected several different types of cells with ZIKV. Then, they waited to see how well the virus would spread. Within three days, most of the progenitor cells were infected with ZIKV! Not only that, but ZIKV appeared to be killing some of the cells. The other cell groups were also infected with ZIKV, but not to the same extent.
So ZIKV targets progenitor cells. But how does it cause microcephaly? Recently, a group of scientists tried tried to answer this question using mice as a model species. They infected pregnant mice with ZIKV and studied the baby mice once they were born. The babies showed signs of microcephaly, including thinner layers of brain tissue and decreased numbers of brain cells.
In fact, there are currently multiple theories to explain the link between ZIKV and microcephaly. According to one leading theory, once ZIKV has infected progenitor cells, it activates enzymes. These enzymes cause cells to die, and the brain does not fully develop.
A baby’s skull is very soft, so as the brain grows it pushes on the skull. This creates pressure and forces the skull to form around it. However, in babies with underdeveloped brains, there is no pressure to stimulate growth. This results in the skull being smaller than normal.
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So what can be done about the Zika crisis? For now, with no real treatment or cure, the best approach is prevention. If you know someone who is pregnant, recommend that she and her partner avoid travelling to Zika-affected countries!
About the Zika virus:
About scientific research on Zika and microcephaly: