Public health officials around the world are warning about the Zika virus, which in Brazil and French Polynesia, has been linked to microcephaly, a condition where babies are born with unusually small heads. The spread of Zika in nearly every subtropical or tropical country, via mosquitoes or male-to-female sexual transmission, is leading to increased concerns about possible exposure to pregnant women and their male partners while travelling abroad.

In early February, public health officials in Canada released guidelines suggesting all pregnant women who have travelled to countries affected by Zika be tested for the virus. While the priority is focused on pregnant woman, officials are recommending that all recent travellers who present with symptoms of Zika be tested. Symptoms include: Fever, rash, joint pain, eye pain (conjunctivitis), muscle pain and headache.

The following are answers to important questions surrounding Zika testing.

Zika detection tests

Two different types of tests for Zika detection are now being rolled out:

• Serology: Test looks at the body’s reaction against the virus in the form of antibodies, which begin to form approximately one week after the start of symptoms. A serology test is better suited to detecting prior exposure to Zika because it remains positive for several months or longer after infection. A major limitation is that it is prone to cross-react with other viruses related to Zika.

• Polymerase Chain Reaction (PCR): Looks for evidence of the genetic material of Zika virus. Test is most effective when looking in blood (ideal within five days of start of symptoms) or urine (within 10 days of start of symptoms). A major limitation is that the test can only detect the virus within two weeks of the person being infected.

How do you get tested for Zika?

The tests can be ordered free of charge through any physician, whether it’s a family doctor or at a walk-in clinic. However, not everyone is eligible for testing, and the ordering physician must complete a form to justify testing.

Experience with these tests is limited as the knowledge of the disease is emerging. Therefore, the federal recommendations for testing are likely to change over time.

Who should get tested?

Anyone with symptoms of Zika illness following travel to areas at risk should contact their healthcare provider, who can then decide whether Zika testing is needed. Pregnant women with or without symptoms should do the same.

Zika is spreading from mosquitoes to humans in at least 33 countries, according to the World Health Organization. It has now been encountered in nearly every subtropical or tropical destination, including Brazil, Colombia, Mexico, Dominican Republic, Panama, Jamaica and Costa Rica.

The following is a breakdown of scenarios in which Zika testing could be considered:

1. Pregnant traveller with symptoms

A pregnant woman who presents symptoms of Zika virus during or within two weeks of returning from a Zika-affected destination is a top priority group for Zika testing. Zika virus in a pregnant woman only lasts up to two weeks, however, there is concern that it may cause increased risk of microcephaly (abnormally small head) or other growth defects in a fetus.

If a pregnant woman tests positive for Zika virus, careful follow-up with an obstetrician is recommended. Further testing of amniotic fluid for the genetic material of Zika might be undertaken in these circumstances. However, there is no current medical intervention for Zika if the virus is detected in a pregnant woman. The risk of microcephaly following infection acquired in pregnancy is not zero. However, it is not as high as initially estimated based on more favourable data from Colombia.

If the symptoms began less than 10 days before seeking medical attention, the best test is PCR. Blood and or urine may be requested depending on how long ago the symptoms began.

If a pregnant traveller has a history of Zika-suggestive symptoms that started more than 10 days ago, an antibody (serology) test would be the most appropriate test method.

2. Pregnant traveller but no symptoms

Testing is possible but patients are encouraged to discuss options with their health care provider.

It is estimated that 75-80 per cent of individuals who are infected by Zika never develop symptoms. The serology test is likely in this scenario, but there are challenges. For example, cross-reactions with other related viruses such as dengue are common, making the final interpretation of the test difficult.

A negative test more than two months after travel to an affected area may not detect prior exposure to the virus. Conversely, a negative test administered within two months of travel would indicate that the virus is not present in the body during that time interval.

A positive test may be a false positive, and needs confirmation with further testing. Those without symptoms who have a true positive test for Zika still may not have the same risk of giving birth to a child with microcephaly as a pregnant woman who has symptoms.

It is currently not known what the percentage of Zika infections in pregnancy lead to microcephaly, however the risk appears to be smaller than initial estimates. Thus, a positive test for Zika in pregnant women may not be as concerning as once feared.

3. Non-pregnant traveller was in a Zika-affected area has no symptoms, or has since recovered

In either of the above scenarios, testing is not recommended, as a positive test does not lead to any intervention or counselling. However, if dealing with a male whose spouse is pregnant, the CDC recommends certain precautions relating to sexual intercourse for the duration of the pregnancy.

4. A male traveller to a Zika-affected area has no symptoms, but he and his partner are trying to conceive

Rare reports of sexual transmission from male-to-female are another twist in the evolving story on Zika.

Testing is not available for men in this situation. However, some medical experts say that a male who travelled to an affected area may wish to abstain from conceiving in the two months after travel.

In an ideal world, anyone could be tested, as a negative result within two months of returning from a Zika-affected area might provide reassurance.

Keep in mind that a positive test comes with uncertainties. Researchers do not know if a male who tests positive, and who potentially transmits the virus sexually can give rise to microcephaly if the infection is acquired at the time of conception.

We also do not know how long or how easily a male with recent infection remains a potential source of sexual transmission, or if infection acquired in pregnancy from a mosquito bite carries the same risk of microcephaly as infection acquired through sexual means outside an endemic area.

Dr. Neil Rau is a Medical Microbiologist; Assistant Professor, at the University of Toronto; medical director of Infection Prevention and Control with Halton Healthcare, and a consultant for CTV News as the Infectious Diseases Specialist