Clinical studies have already shown that infection of pregnant women by the Asian Zika virus strain, especially during the first and second trimesters, leads to significantly higher foetal anomalies. Now, laboratory studies using blood samples taken from pregnant women in their first, second and third trimesters of pregnancy have confirmed that foetuses are more vulnerable to the Asian strain during the first and second trimester.
A study published on August 21 in the journal, Nature Microbiology, has also found that infection by the Asian strain leads to expression of two genes that are associated with pregnancy-associated complications. The experimental evidence also strongly suggests that the early stage of pregnancy is a time of “high susceptibility” to the Zika infection.
Suan-Sin Foo from the University of Southern California, Los Angeles, and others tested whole blood samples of pregnant and non-pregnant women using both the African and Asian Zika virus strains. While the two strains share closely similarity (90%), the presence of African virus in certain blood cells was much higher than the Asian strain. However, the Asian strain showed greater ability to trigger immune suppression, which allows the virus to replicate and even possibly sneak into the womb and cause more foetal damage.
Particular white blood cells called CD14 are the primary target cells for Zika infection. The CD14 cells turn into another kind of white blood cells (macrophages) that swallow bacteria and viruses and keep the body healthy. But the researchers found that the Asian Zika virus strain was pushing the CD14 cell to become a kind of macrophage that suppresses the immune system.
During pregnancy, the body naturally suppresses the immune system to protect the growing foetus from being rejected by the womb. The reduced immunity makes pregnant women highly susceptible to Zika infection during this period. After infection, the Asian strain further promotes immune suppression.
Also, there is higher level of expression of two genes — ADAMTS9 and FN1 — that are detrimental to pregnancy. The authors hypothesise that the expression of these genes may be induced during the first and second trimesters “specifically” when infected by the Asian strain. While elevated expression of the ADAMTS9 gene is implicated with preterm birth and small birth weight, higher expression of the other gene causes foetal-growth restriction.
