An infectious disease expert breaks down Zika’s threat to pregnant women in the US

Many pregnant women around the world are alarmed by the resurgence of the Zika virus.
Many pregnant women around the world are alarmed by the resurgence of the Zika virus.
Image: AP Photo/Felipe Dana
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In mid-January, a mother in Hawaii received the devastating news that her newborn was suffering from microcephaly—a condition in which babies are born with smaller heads and abnormal brain development. Testing confirmed that the mother had been infected with the Zika virus during her pregnancy, most likely during her time living in Brazil in the spring of 2015.

Pregnancy is already an unsettling time for many women. Confronted with laundry lists of things to do and things to avoid, it’s difficult to sort out what is truly a threat to one’s self or one’s fetus. Now the resurgence of the Zika virus, a tropical disease transmitted by mosquitos, has women in the US and around the world struggling to understand ways to avoid exposure and protect themselves and their babies.

A brief history of Zika

Zika isn’t a new phenomenon. We’ve known about it since the 1950s, but at the time infections were considered relatively mild and geographically constrained to parts of equatorial Africa and Asia. This made it a low-priority pathogen—one of many “neglected tropical diseases.”

But in 2007, Zika caused an outbreak in Micronesia’s Yap islands. In 2014, it spread to French Polynesia and other Pacific islands, and by 2015 the virus had made it to Brazil. Zika transmission has now been identified in 24 countries in Latin America and the Caribbean. Zika infection has also been documented in travelers returning home to Denmark and the U.S. after visiting the affected countries, though no spread has yet been documented in those countries.

In most people, Zika causes a fever, rash, pain in the joints and muscles, and a headache. It’s not always easy to diagnose, as these symptoms are very similar to other mosquito-borne viruses, such as dengue and chikungunya. And it wasn’t until the current outbreak in Brazil that physicians recognized that Zika could be harmful to developing fetuses. Cases of microcephaly soared in the country, with 3,893 suspected cases just in the month of October. Since doctors hadn’t observed this effect with Zika cases in Africa and Asia, right now we don’t know how common microcephaly is in pregnant women exposed to the disease.

Is it safe for pregnant women to travel to Latin America?

Because this is such a fast-developing epidemic, it’s difficult for pregnant women to know what to do. Despite the uncertainty, the US Centers for Disease Control and Prevention have recently issued interim guidelines for pregnant women who may be traveling to affected countries. Their first suggestion: just don’t go. An updated list of affected locations can be found here.

However, this list includes any country that has confirmed Zika transmission—even if the outbreak is localized to a single region. So if a trip can’t be postponed or rescheduled, pregnant woman should remember to check out transmission at the local level rather than relying on country-wide reports.

For US women who are traveling to affected countries, avoiding mosquito bites is key. Pregnant women, or women planning to get pregnant in the near future, should take care to wear protective clothing, stay inside in areas with screens whenever possible, and use insect repellents. The CDC notes that repellents containing DEET, picardin, and IR3535 are safe to use during pregnancy.

Pregnant women who develop symptoms consistent with Zika infection should be tested for the virus and monitored during their pregnancy. And pregnant women who don’t experience symptoms but have traveled to areas with outbreaks should let their physician know about their travel history when they return home. That way, doctors can schedule fetal ultrasounds to check for microcephaly.

How likely is Zika to spread to the US?

While the US hasn’t yet experienced any transmission of the Zika virus, the mosquito that carries it is established throughout the South. There are concerns that the mosquito’s range could expand farther north due to global warming, putting up to 200 million Americans at risk.

The good news is that the US tends to have better mosquito control programs and more screened and air-conditioned buildings than a lot of the countries currently affected. That reduces the country’s exposure to mosquito bites, and therefore the chance of the infection spreading. And unlike West Nile, America’s last major imported mosquito-borne virus, Zika doesn’t have an animal reservoir. The West Nile virus could be spread by birds that flew from one place to another. But in order for Zika to spread, it needs to be transmitted via a mosquito that bites one person infected with the virus, then bites another human.

Impoverished areas in the US that lack protective A/C and window screening could remain vulnerable, however. So while the US has some factors working in its favor, we need to be alert to the possibility of contagion.

Lots of questions, few answers

There are still more questions than answers about Zika in pregnancy. There is some evidence that Zika could be transmitted via sex. That means it’s possible that even women who are not infected, but who have partners who have contracted Zika while traveling, may be vulnerable to infection.

Despite the current media narrative, it’s actually unclear how common microcephaly (or other birth defects) are after Zika infection. Right now the assumption is that Zika is causing these defects, but scientists can’t say that with 100% certainty until more tests are done.

And we also don’t know if the defects only happen during a particular trimester or developmental window. There is no conclusive evidence regarding whether mild or apparently asymptomatic infection could still result in birth defects. This last item is a particularly critical missing piece of information. Current guidelines are based primarily on symptomatic infections, but only about 20% of those infected actually show symptoms. And finally, we don’t know how the virus may be causing these defects.

Preventative measures

Unfortunately, we also lack a vaccine or treatment for the virus, though the outbreak has certainly spearheaded calls for one—including from US president Barack Obama. But even assuming scientists successfully develop an effective treatment, it will still take two to three years at a minimum to identify, test, and market such a vaccine.

Elsewhere, in Latin American countries including El Salvador, Brazil, and Jamaica, officials have begun suggesting women delay pregnancy for up to two years because of the outbreak. But that’s unlikely to be a viable alternative, particularly in countries where unplanned pregnancies are common and women lack access to many kinds of birth control.

With millions already affected around the world, what is clear that we need answers—and we need them yesterday. In the meantime, many US women who are pregnant or may soon become so have quickly added Zika to the list of things to worry about alongside sushi, deli meats, and soft cheeses.

For now, the safest plan is probably to avoid traveling to affected countries when possible. If that’s not doable, or if Zika starts spreading across the continental US, expectant mothers should stay in touch with their physicians, keep them appraised of their travel history and any possible mosquito exposures or infections, and keep bug spray on hand. The Zika outbreak is frightening, and it’s understandable that mothers are concerned. But with so many unknowns, it’s important to balance caution with perspective.