Reports of the increasing problems stemming from the mosquito-borne Zika virus in Brazil and South and Central America have dominated headlines in recent months. High numbers of typically rare side effects, microcephaly and brain damage in newborn infants and temporary paralysis from Guillan-Barré syndrome, are being seen in the region.
Last week’s report from the US Centers for Disease Control and Prevention published in the New England Journal of Medicine (link is external) confirmed the strong suspicions that the Zika virus is behind the rise in cases of microcephaly in infants in South America. Microcephaly normally occurs at a rate of 1 in 5,000 to 1 in 10,000 births. But since the Zika outbreak across the Americas this rate has swollen to almost 1 in 100 births. While microcephaly is the most obvious side effect in newborn babies, others are born with normal appearance but may be blind or deaf, they may develop behavioural and learning difficulties in childhood or mental disabilities later in life, as a result of Zika attacking in utero.
No one knows whether Zika, like other flaviviruses- dengue and West Nile, is here to stay, presenting a perennial risk for pregnant women or whether this outbreak will die down with the development of herd immunity within the population of Latin America and the Caribbean. Currently no treatments exist for Zika infection and though vaccines are in development none of these have yet reached clinical trials. With no prevention and no cure available, all that can be offered to future parents worried over the potential fate of their unborn children is a clear diagnosis. Only there is no widely available specific assay for Zika diagnosis.
The current go-to Zika test is based on RT-PCR. While it can specifically determine the presence of the virus, tests must be carried out within 5-7 days of being infected to be able to identify the virus. Within a week of infection the Zika virus moves out of a patient’s circulation and its levels tail off, closing the diagnostic window of the assay. Although the specificity of this assay is good, in practice by the time a patient makes it to the clinic for testing, viral levels have decreased and are beyond the limit for detection.
Within the past few weeks the Centres for Disease Control and Prevention have authorised a MAC-ELISA, for emergency use by qualified laboratories within the United States, for the purpose of Zika testing. This ELISA measures the level of anti-Zika IgM antibodies within a patient. IgM antibodies are the initial antibodies produced in response to an infection. They appear in the blood beginning 4-5 days post- infection and remain in circulation for approximately 12 weeks. The diagnostic window for identification of Zika infected individuals is increased by this test, but issues remain.
If tested too early, before the generation of anti-Zika IgM antibodies in the patient, or too late, towards the end or after the 12 week period, the antibodies might not be detected giving a false negative result. False positives are also possible using this Zika test. The Zika ELISAs a
re not totally specific for the Zika virus, they cross-react with other flaviviruses, like dengue, so if a patient has been infected with a different virus this might be read as Zika. This makes the interpretation of these tests difficult and requires skilled scientists, making rapid point-of-care diagnostics impossible.
Inconclusive results from a Zika test, such as the MAC-ELISA, require further analysis. This can be achieved using a plaque-reduction neutralisation test (PRNT), where a patient’s serum is mixed with the virus and technicians watch over several days to see if the virus can infect the cell culture. If it can’t then the virus has been neutralised by antibodies in the patient’s serum, indicating prior exposure to Zika. The PRNT is Zika-specific and offers clinicians and epidemiologists the ability to determine if a baby has been exposed to Zika in utero many months before. The problem is that this Zika test requires live virus and appropriate safety conditions, so few labs are able to offer it. This has led to a current backlog of many thousands of people across the Americas who are at risk for infection yet still months later are awaiting their Zika test results.
What is needed now is a specific and rapid point-of-care diagnostic for Zika testing. This could offer peace of mind to non-infected individuals and help to direct infected patients, both pregnant and trying to conceive. Researchers are working at breakneck speed to develop affinity reagents and diagnostic assays that can deliver these results, but the long development times of many affinity reagents means that months will be needed before we can even determine if they can help.