Zika. Microcephaly. Aedes aegypti. Over the past year, these words have dominated headlines as the Zika virus has spread to 45 countries and territories, leaving a trail of deformed babies and neurological disorders in its wake.

One year after Zika’s arrival in the Americas, our understanding of the virus has grown by leaps and bounds but many questions remain. Diagnosing Zika also remains an imperfect art — leaving many Canadians anxious and confused, especially with summer vacations and the Rio Olympics looming.

Brendon and Nicole Healy from Orangeville had already booked a trip to the Dominican Republic for a friend’s wedding when Zika started making headlines. They were alarmed; the Healys, both 30, had recently decided to start a family.

The couple considered cancelling but couldn’t get their $3,800 back; Nicole was also a bridesmaid. So after much research and agonizing, they decided to go, applying mosquito repellent every day.

After returning home in late March, they visited their family doctors to get tested for Zika — but more than a month later, they still have +no answers. Nicole didn’t meet the criteria for Zika testing and Brendon’s doctor didn’t know the process for requesting one, forcing him to make three trips to the clinic to get his paperwork in order — only to find out he didn’t qualify either.

“I find this to be absolutely absurd,” Healy said. “I know that we shouldn’t have gone on our trip to begin with, but as responsible adults we wanted to be safe and have the necessary blood tests conducted … instead of this process easing our fears, it only made it worse.”

Dr. Vanessa Allen, Public Health Ontario’s chief microbiologist, is sympathetic and says the Healys are not alone. “For the first time in my life, I’ve gotten a lot of direct calls from patients,” she said. “They are terrified.”

But until a vaccine is available, the reality is that Zika will be a risk to people living in affected countries, as well as the millions of Canadians who visit them every year — and it’s not possible to test everyone.

“To universally offer the test is difficult right now,” said Dr. Matthew Gilmour, scientific director general of the Public Health Agency of Canada’s National Microbiology Lab in Winnipeg.

Currently, Canada’s criteria for Zika testing — which is publicly funded — say that people must have travelled to Zika-affected areas and developed symptoms (which the Healys did not). Pregnant women who meet both criteria are the highest priority, since Zika can cause serious birth defects.

Because Zika lingers in semen and can spread through sex, male travellers with previous symptoms can also get tested, according to the latest Canadian guidelines.

But Zika testing has little value for someone who is asymptomatic and unconcerned about pregnancy, Allen believes. Should limited resources be used to diagnose a virus that, for most people, will only last a few days without causing much problem?

“We need to focus on where we can make a difference,” Allen said.

This does leave would-be parents like the Healys in a tough spot. The fact that Zika can infect without causing symptoms is unsettling. Scientists are still developing a good test for detecting Zika in semen and nobody knows how long the virus can linger there; the longest documented case is 62 days.

But what many anxious couples don’t realize is that Zika tests can’t give them what they want most: a guarantee of zero risk.

Because Zika is so new, the diagnostics are still a work in progress, Allen said. Current tests can have false positives and they still can’t completely rule out Zika — even when results come back negative. With antibody tests, for example, “we don’t know how well it performs,” Allen said. “I have no idea if a negative test means that the person doesn’t have Zika.”

To understand why, it’s helpful to understand the tests themselves. There are two types; one is a molecular test, called PCR, which allows scientists to directly confirm Zika’s presence by detecting its genetic material. “PCR is really one of the best tests to confirm Zika infection,” Gilmour said.

The problem: the virus has to be physically present and Zika only circulates in blood for up to 10 days (and urine for a few days longer). So a negative result could simply mean the patient was tested too late, after the infection already cleared. “The window for which it works is usually very soon after someone has returned from travel,” Gilmour said.

The second test detects antibodies deployed by a person’s immune system to fight Zika; think of it as looking for footprints instead of the actual organism. But these tests are prone to false positives, sometimes detecting closely related viruses like dengue and yellow fever.

At Winnipeg’s national microbiology, positive results are backed up by a second test. This can take weeks but with this two-step verification process, Gilmour said he is “very confidant when we get a positive result.”

Negative results are a different story. “Confirming an absence of an infection is much more difficult,” Gilmour said.

There are multiple reasons why serology tests can be “falsely” negative. Maybe the person is immunosuppressed and their body failed to mount an antibody response. Perhaps the person just has a low amount of antibodies.

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Until better Zika tests are developed, public health officials are advising potentially-exposed couples to still take precautions, even when they get a negative test result.

“Imagine a scenario where we told someone that they don’t have an infection, so they went on to pursue pregnancy and in fact they did have an infection,” Gilmour said. “We don’t want to pass false information off to couples.”

Gilmour emphasized that people who don’t meet current criteria can still pursue testing if their reasons are justifiable and their doctors and provincial health labs are on board.

For the Healys, they are coming to terms with the fact that they will probably have to wait six months before trying for a baby, adhering to the strictest current guidelines for couples who may have been exposed.

If they knew in March what they know now, however, they likely would have cancelled their trip. And with Zika likely here to stay for some time, perhaps the most important message for Canadians is what Toronto infectious disease expert Dr. Isaac Bogoch tells his patients.

“If you really want a zero per cent risk of having a complication from Zika virus, the only way to do that is to not go.”

Zika testing in Canada

306 Zika specimens tested by PCR at Public Health Ontario

22 Positive samples tested in Ontario

3,734 Canadians tested since January

80 Canadians who have tested positive after travelling

1 Canadian infected through sex

5 Pregnant women in Canada who have tested positive

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