After decades of dormancy, Zika virus disease (Zika) is estimated to have arrived in Brazil in 2013 . Since then, the disease has been confirmed in 49 countries and territories worldwide, including 38 within the Americas [2,3]. Zika’s rapid spread and linked neurological effects have resulted in an ongoing epidemic and an urgent need for effective diagnostic and treatment strategies.
What is Zika?
“Zika” refers to Zika virus disease, which is caused by the Zika virus (ZIKV). ZIKV is most often transmitted to humans through the bite of an infected Aedes mosquito, but it can also be spread by infected individuals through sexual contact, blood transfusions, and from mother to fetus during pregnancy. Common Zika symptoms include fever, rash, and joint pain—these are usually mild, resulting in many carriers who are unaware of their infection. In pregnant women, ZIKV infection is strongly linked to microcephaly (abnormally small head) and severe brain defects in their resulting newborns. Due to the increasing number of these cases in particular, the World Health Organization (WHO) declared Zika a Public Health Emergency of International Concern (PHEIC) in February, 2016 .
A model for in utero transmission of ZIKV infection
Recent research at Washington University (St. Louis, MO, USA) has provided insight into the link between ZIKV infection and pregnancy-associated microcephaly. Using ZEN Double-Quenched Probes for RT-qPCR, scientists found high levels of ZIKV in the placentas of pregnant mice and in the brains of their infected fetuses. Further histological examination showed that the high levels of ZIKV were strongly associated with apoptosis and vascular damage in these tissues, leading to fetal abnormalities similar (but not identical) to those seen in humans . While numerous other studies have suggested a correlation between ZIKV infection and fetal abnormalities, this is the first to establish causality.
Progress in diagnostics and treatment
While no vaccine or antiviral therapy currently exists for Zika, significant advances are being made with the help of tools from IDT. Following are several examples.
RT-qPCR for ZIKV viral load quantification
Researchers at the University of Leuven (Leuven, Belgium) administered potential ZIKV inhibitors to infected cell cultures and performed RT-qPCR to quantify resulting RNA levels. In their assays, they used ZEN probes to increase assay sensitivity and reduce background fluorescence. The scientists describe a viral polymerase inhibitor, 7-deaza-2′-C-methyladenosine, which efficiently stops ZIKV replication in vitro and delays disease progression in mice .
Researchers at the University of North Carolina (Chapel Hill, NC, USA) investigated ZIKV pathogenesis in vivo by infecting healthy mice, as well as those lacking major components of the antiviral response. Following ZIKV administration, the researchers performed RT-qPCR with ZEN Probes to quantify resulting ZIKV RNA levels in various organs. Mice producing limited amounts of interferon α/β, and mice lacking the interferon receptor, were seen to be highly susceptible to ZIKV infection. These groups showed high viral loads in the brain, spinal cord, and testes, thus demonstrating their potential to be suitable animal models for future Zika/ZIKV studies .
Field-ready ZIKV detection using paper-based toehold sensors
In conjunction with the Wyss Institute for Biologically Inspired Engineering (Boston, MA, USA), collaborators at the University of Toronto (Toronto, Canada) and Arizona State University (Tempe, AZ, USA) have designed a rapid, low-cost diagnostic platform that overcomes many of the practical challenges facing field-ready ZIKV detection tools. Based on principles from NASBA (nucleic acid sequence-based amplification) and CRISPR-Cas9, their approach uses Ultramer® Oligonucleotides as templates for the construction of paper-based toehold switch sensors. They demonstrated these molecular sensors to be highly sensitive, capable of detecting ZIKV RNA in samples with extremely low viral loads .
Universal control RNA for comparison of RT-qPCR assay performance
A group of researchers across Germany, France, and the Netherlands evaluated the standardization and diagnostic performance of 6 RT-qPCR assays, which are widely used for ZIKV detection. Using gBlocks® Gene Fragments, they constructed a “universal control RNA (ucRNA)” containing the target regions for each of the 6 distinct assays. The ucRNA allowed the researchers to quantitatively evaluate assay performance, and in doing so, they found that some assays showed limited sensitivity to low viral loads, while others were incompatible with new ZIKV outbreak strains. This finding suggests the need for more advanced, highly accurate diagnostics .