Why does Zika Virus cause birth defects?

The Zika virus, carried by infected mosquitos (Aedes aegypti), but can also be transmitted through intercourse and blood transfusions. A set of mild symptoms may present as fever, conjunctivitis, muscle and joint pain, and headaches, but asymptomatic cases are common. Of most concern is the dangers it poses on a fetus if the pregnant mother is infected.  

The pathway for the Zika virus to cross the placental barrier was closely studied by scientists from The Scripps Research Institute. Human umbilical endothelial cells were found to be much more susceptible to Zika than other viruses. 

Stages of Microcephaly severity- a defect from the zika virus

The virus uniquely exploits a cell surface molecule to act as a “secret passageway” directly to fetal circulation causing an increased probability of birth defects. The unique pattern of birth defects caused by the Zika virus is named congenital Zika syndrome. This may entail microcephaly, a condition where the circumference of a child’s head is smaller than normal due to abnormal (or lack of) development of the brain. Microcephaly can be caused if the mother abused alcohol or drugs or was infected with cytomegalovirus, rubella, or varicella during pregnancy. Congenital Zika syndrome can also lead to decreased brain tissue, damage to back of the eye, limited mobility of joints, excess muscle tone, damage to their eyes or brain causing a lack of visual development.  

Studies have shown that not all infected mothers had children with birth defects. During a Brazilian Zika outbreak in 2015, less than 10% of the children born from infection experiences birth defects. Yale scientists hypothesize that the chance of birth defects could be dependent on the mothers’ antibodies, and how her immune system reacts to the infection. Those who had children with birth defects were discovered to have specific antibodies that actually enhanced the transmission of the virus to the fetus.  

There is no treatment for microcephaly. The head will never return to a normal size so treatment is primarily concerned with decreasing other effects such as developmental delays and neurological disabilities. Physical, speech, and occupation therapy are essential in the child’s early life.  

There is currently no vaccine or treatment for the Zika virus. In order to avoid the dangers that come along with the Zika virus, pregnant women should avoid travel to areas where it is present. If it is essential to be in an area where an outbreak is present skin exposure should be limited and mosquito nets should be employed at night to avoid bites. 


The Scripps Research Institute. “Scientists find clue to why Zika, but not its close relatives, causes birth defects.” ScienceDaily. ScienceDaily, 6 February 2017.

Findings Shed New Light On Why Zika Causes Birth Defects In Some Pregnancies.” Yale School of Medicine, Yale School of Medicine, 16 Aug. 2019.




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