Laboratory Testing Recommendations for Zika Virus
Current Testing Recommendations as of February 4, 2016:
These guidelines and recommendations will be updated as new information becomes available.
What types of tests are available to detect Zika virus infection?
There are currently two testing methods available for detection of Zika virus:
- Detection of Zika virus genetic material: This test (called PCR) looks for evidence of the genetic material (RNA) of Zika virus. The test is most effective when looking in blood (ideal within 5 days of start of symptoms, although within 7 days is acceptable) or in urine (within 10 days of start of symptoms). The major limitation of this test is the short time after the start of Zika virus associated illness during which it is positive.
- Detection of antibodies against Zika virus (serology): This test looks for the human body's reaction against the virus in the form of antibodies. Antibodies begin to form approximately one week after start of symptoms. The major advantage of a serology test is that it remains positive for several months or longer after infection. The major limitations are that it is slow to perform and prone to cross-react with other viruses related to Zika virus. A positive result may in fact represent a previous exposure to another virus or past vaccination (e.g. to Yellow fever). This makes interpreting the result difficult.
Who can get tested for the Zika virus?
If you are worried about having been potentially infected with Zika virus during travel to areas at risk, it is recommended that you contact your health care provider. The decision regarding who and when to test is complex, because interpretation of the test results (particularly the serology test, see above) are not straightforward at this time. Below are some common traveller scenarios.
Travellers with active symptoms of Zika virus infection
If the symptoms began less than ten days before seeking medical attention, the best test is Zika virus PCR. A blood and/or urine sample may be requested depending on how long ago the symptoms began. Because the symptoms for Zika virus infections are similar to other diseases transmitted by mosquitoes in the Americas, your doctor will likely also order tests for other infectious diseases such as dengue. This testing recommendation applies to all travellers with active symptoms, including pregnant women.
Travellers with a history of travel to a Zika virus area but no history of symptoms
We know that 75-80% of Zika virus infections occur without symptoms. For these travellers, there is likely no additional benefit to test for evidence of past asymptomatic infection. Should you have questions about this scenario and whether a test would be warranted, you should discuss this with your health care provider. For pregnant patients with a history of travel and no symptoms, see below.
Special Considerations for Testing of Pregnant Women for Zika Virus
Zika virus infection in pregnancy is also a self-limited disease that lasts a few days up to two weeks. However, there is concern that there may be a risk of microcephaly (abnormally small head size) or abnormal brain development in the fetus. It is unknown at this time whether the timing of the infection in pregnancy (i.e. early versus late) or the severity of the symptoms in the mother have any impact on the chance of the fetus being impacted. Please see below for specific pregnancy-related scenarios. Early consultation with your health care provider regarding questions of Zika virus in pregnancy is recommended for travellers.
Pregnant travellers with active symptoms suggestive of Zika virus infection
Pregnant travellers to areas at risk who develop symptoms of Zika virus infection should seek medical attention. If the symptoms began less than ten days before seeking medical attention, the best test is Zika virus PCR. Your health care provider should discuss with the local laboratory which test to send, as a blood and/or urine sample may be requested depending on how long ago the symptoms began. Your doctor will likely also order tests for other infectious diseases that are transmitted by mosquitoes in the Americas, such as dengue.
Pregnant travellers with a history of active symptoms compatible with Zika virus but whose symptoms started more than ten days ago
In this context, the Zika virus PCR will likely be negative. Serology for Zika virus would be the most appropriate test. At this time, based on our existing knowledge, pregnant women who developed active symptoms of Zika virus are expected to be most at risk for impact on their fetus. As a result, testing should be offered and consideration given to consulting with an infectious diseases specialist or maternal-fetal medicine specialist.
Pregnant Travellers with a History of Travel to an Area at Risk for Zika Virus But no History of Symptoms
Testing is possible if discussion has occurred between the patient and their health care provider, and they are aware and accepting of the test limitations and that testing may take weeks to complete.
This scenario is the most difficult to navigate in terms of testing. It is estimated that approximately 75-80% of individuals who are infected by the Zika virus never develop symptoms. However, the risk of asymptomatic infection in pregnancy to the fetus has not been characterized. In the absence of symptoms, our best Zika virus test (PCR) is unlikely to detect the virus. As a result, serology is the test to consider in this scenario. The use of serology as a screening tool in these instances must be carefully considered, including because we do not yet know how well the serology test will perform for these patients. Furthermore, specific challenges with the test will be the fact that cross-reactions with other related viruses (such as dengue) are common, making the final interpretation of the test difficult.
Women with a history of travel to a Zika virus area who did not develop symptoms of Zika virus but are concerned about their risk of infection, should consult with their health care provider to discuss the testing available as well as its limitations, including the significant questions around the meaning of positive and negative test results. Information around country of travel, mosquito bite exposure, past vaccination history (eg. Yellow fever) and previous travel history to areas where dengue infections are common should be discussed. Difficulties in test interpretation further compound when considering the health of both the mother and their baby.
How does Zika virus testing occur in Canada?
Currently, Zika virus PCR is performed at the National Microbiology Laboratory and some provincial laboratories. Serology is also offered through the National Microbiology Laboratory. Health care providers should familiarize themselves with these testing recommendations and those recommendations produced by their provincial public health laboratory.
What are the current limitations and risks of testing for the Zika virus?
A positive PCR test for Zika virus genetic material signifies an acute infection. As the infection is self-limiting and generally mild, this will self-resolve.
A negative PCR test for Zika virus genetic material may mean that there was no infection or that the individual was infected but at the time a patient's sample was collected the virus was no longer present in a bodily fluid such as blood or urine. Serology would be the next step in this case.
A positive test for Zika virus by serology is harder to interpret or may require more extensive testing due to the risk of cross-reaction with other viruses related to Zika virus. Your health care provider should help interpret the meaning of this result.
A negative test for Zika virus by serology signifies either that the test was done too early in the disease course (no antibodies have been made and are not present in the collected sample), or that there was no infection. A negative serology test performed one to two months following return from travel would indicate that there was no infection, but this again is best interpreted with your health care provider.
When can I expect a test result?
PCR results are usually available within a week of sample receipt and are directly reported to the submitting laboratory.
Serology testing involves different techniques to determine if the patient has Zika specific antibody and so the time taken to obtain a result is much longer than a PCR test. The initial tests will take at least a week to complete, and it may take several weeks before all testing is completed.
What should I expect if I am diagnosed with the Zika virus infection?
There currently exists no specific antiviral therapy for the treatment of Zika virus infection. Treatment is supportive with antipyretics (acetaminophen in pregnancy), hydration and rest. Symptomatic disease typically lasts for up to 7 days. Urgent medical care is recommended for any symptoms associated with Guillain-Barré syndrome, and treating health care providers should be made aware of recent travel to area with Zika virus circulation and/or symptoms of Zika virus infection.
If Zika virus infection is confirmed in the setting of pregnancy, a referral to a Maternal Fetal Medicine Specialist and Infectious Disease Specialist should be made.
Who can help?
The Public Health Agency of Canada works with its national and international partners, including the World Health Organization, to monitor and share information on the detection, surveillance and management of cases during outbreaks. The Agency will continue to assess the risks on an ongoing basis by working with our partners, sharing information and advice on best practices as the situation evolves and support enhanced awareness for Canadian travelers.
The Public Health Agency's National Microbiology Laboratory is able to detect the virus, and is in a position to support Provinces and Territories to test specimens as the need arises. Further testing methods and research is underway to support the needs of this outbreak.
Health care providers, infectious disease experts and clients create an important connection in the consultation and decision making process on testing for the Zika virus.
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