A nurse practitioner gives a pregnant patient insecticide and information about mosquito protection in Miami last summer. (Lynne Sladky/AP)

Federal health officials are changing their testing recommendations for pregnant women who may be exposed to the Zika virus through travel or sex or because of where they live.

In updated guidance released Monday, the Centers for Disease Control and Prevention is no longer recommending routine testing for pregnant women without any Zika symptoms but who may have been put at risk because they have traveled to a region where Zika is circulating.

The new guidance should not be seen as a sign that Zika infections are any less dangerous for pregnant women, experts said. Instead, the revised recommendations reflect the limitations of the most commonly used blood test for the virus. In recent months, CDC has seen a growing number of false-positive test results from states.

As the number of Zika cases declines in the Americas and as scientists learn more about the disease, “the likelihood of false-positive test results increases,” according to CDC's latest report.

Yet officials emphasized that pregnant women with possible Zika exposure and symptoms should continue to be tested as soon as possible after those begin. Symptoms include fever, rash, headache, joint pain and red eyes.

This blood test looks for a type of antibody that a person’s immune system produces to fight the infection. Scientists have now learned that those antibodies can stay in the blood for more than 12 weeks. As a result, the test cannot always reliably tell whether a Zika infection took place before or during a current pregnancy.

The change in recommendations is raising concern for some obstetricians, who worry that infections will get missed because of the de-emphasis on routine testing for some asymptomatic pregnant women.

Many people infected with Zika won’t have symptoms or will only have mild symptoms. But a pregnant woman who is infected but has no symptoms can pass Zika to her developing fetus and cause an array of serious birth defects, including microcephaly, hearing and vision problems, swallowing difficulties and joints with limited range of motion.

“By not testing an asymptomatic pregnant patient, we are keeping her from getting the increased surveillance she would get if test results are positive and taking away the mother’s options if fetal abnormalities are found, since they won’t be found until after birth,” said Rita Driggers, medical director of maternal and fetal medicine division at Sibley Memorial Hospital in Washington.

At the same time, the new recommendations are likely to put more pressure on pediatricians to follow up with patients and ask about possible maternal and congenital Zika exposure for every newborn, she said.

Since the CDC began tracking Zika pregnancies last year, there have been 1,751 completed pregnancies in the 50 states and the District of Columbia through July 11, according to the CDC. Of those, 91 babies were born with birth defects, and eight pregnancies resulted in miscarriage, still births or terminations.

But the number of new Zika cases has declined dramatically this year, with only 175 symptomatic cases reported in the continental United States as of Wednesday.

CDC officials said the new guidance is an attempt to weigh all considerations in the face of declining Zika cases and the limitations of the test.

“It’s not perfect,” said Henry Walke, CDC’s incident commander for Zika. “We’re trying to negotiate between false positives … vs. missing perhaps one true positive.” At this point, he added, “we feel the majority of positives using this test will be false positives.”

Laura Riley, vice chair of the obstetrics department at Massachusetts General Hospital, said specialists at the American College of Obstetricians and Gynecologists and the Society for Maternal Fetal Medicine worked with CDC on the latest update.

Any change in guidance is likely to be confusing at first, she acknowledged. But the test in question “is not quite as specific as we were hoping,” she said. “When the disease incidence decreases, the test performs even worse than it did when there was lots of disease. What the patient then gets is a potentially false positive, and that creates nothing but hysteria.”

Health-care providers will need to have much more detailed conversations with pregnant women with possible Zika exposure so they and the patient “can talk about whether or not to do the test,” she said. For example, with a pregnant woman who had traveled to Brazil, the epicenter of the epidemic, clinicians should ask how long she was there, if she got mosquito bites and when.

In places like Florida and Texas, which have had local transmission and see much travel with Zika-affected regions, local and state health departments may want to enhance their screening of asymptomatic pregnant women, CDC officials said.

Determining whether someone has been infected with Zika is enormously difficult. When someone is first infected, one kind of test can be used to look for genetic material of the virus. But because that genetic material usually is cleared from the blood and other body fluids and tissues within a few days, the testing window is very narrow.

The blood test that looks for Zika antibodies is also challenging. Using this test, it’s hard to tell the timing of the infection because antibodies can persist for more than 12 weeks. It’s also hard to differentiate between antibodies triggered by Zika and those produced by infections from related viruses, such as dengue and chikungunya. Many areas in South and Central America and the Caribbean that were hard hit by Zika have had outbreaks of dengue and chikungunya.

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