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.
Clinical studies have already shown that infection of pregnant women by the Asian Zika virus strain especially during the first and second trimester of pregnancy leads to significantly higher foetal anomalies. Now, laboratory studies using blood samples taken from pregnant women in their first, second and third trimester of pregnancy and non-pregnant women have confirmed that foetuses are more vulnerable to the Asian strain during the first and second trimester.
A study published 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, which may contribute to adverse pregnancy outcomes. The experimental evidence also strongly suggests that the early stage of pregnancy is a time of “high susceptibility” to 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% sequence similarity), the number 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 cells to become M2-macrophages that suppresses the immune system.
During pregnancy, particularly during early and mid-stages, the body naturally suppresses the immunes 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, as the experimental evidence by the authors show. 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 hypothesize that the expression of these genes may be induced during the first and second trimester “specifically” when infected by the Asian strain. While elevated expression of ADAMTS9 gene is implicated with pre-term birth and small birth weight, higher expression of the other gene causes foetal–growth restriction, which is commonly seen in children born to mothers infected by Zika virus.
The African strain was seen to be causing more inflammation as seen in Japanese encephalitis. Overt inflammation during early- and mid-stages of pregnancy can result in pre-term birth. “This suggests the possibility of complicated pregnancy outcomes regardless of African or Asian Zika virus strain infection,” the authors write.