Virus infection and reproduction in humans involves complex interactions between the virus, cells of the host and the immune system. Virologists refer to cells that virus can enter as “permissive”. Both skin fibroblasts (form connective tissue) and epidermal keratinocytes (95% of skin cells) are highly permissive to the Zika Virus. When a mosquito injects saliva containing Zika Virus under the skin, the Zika can enter these cells and reproduce.
Entry into cells requires an interaction between the viral protein coat and factors present on the host cells. Known interactions occur between Zika Virus and cell proteins DC-SIGN, AXL & Tyro3. A larger set of cell proteins is potentially involved. Once inside the cell, a race between Zika Virus replication and host immune response ensues. Viruses can only cause disease if they replicate faster than the host immune response can destroy them. Zika triggers a host immune response indicating that it is detected by human cells. Variation in immune system response can lead to variation in symptoms experienced.
Skin cells are not the only cells permissive to Zika Virus. Zika is able to cross the placenta of pregnant mothers, infect and kill neural stem cells.* The neural stem cells replicate to produce brain tissue. If enough stem cells are killed, less brain tissue is produced and the brain will be smaller and result in microcephaly.
Initially, there was a correlation between Zika infection and microcephaly, but the underlying mechanism was obscure. We now have a mechanism by which Zika can cause microcephaly. There may be other effects of Zika virus that can contribute. Scientist are continuing investigations. However, available evidence strongly supports Zika as a cause of microcephaly. Zika is a disease that requires a serious response.
*Jonathan J. Miner, Michael S. Diamond. Understanding How Zika Virus Enters and Infects Neural Target Cells. Cell Stem Cell Volume 18, Issue 5, p559–560, 5 May 2016