On Tuesday, the CDC confirmed a case of sexually transmitted Zika in Dallas, Texas, in which a person spread the virus to a sexual partner after contracting it abroad. Prior to this, it was suspected that Zika could be sexually transmitted—traces of the virus were found in a patient’s sperm during the French Polynesia outbreak—but no one knew for sure.
While the outbreak is now growing quickly—the WHO director general, Margaret Chan, declared Zika to be “spreading explosively” through the Americas—the emergency is not so much Zika itself, but the neurological conditions associated with it. Since last May, Brazil has seen an uptick in cases of microcephaly (a birth defect of small head size which can signal brain damage) and Guillain-Barré syndrome (in which the immune system attacks the nervous system). The WHO “strongly suspect[s]” a causal connection between Zika and microcephaly, though the link hasn’t yet been proven; the connection with Guillain-Barré is likewise suspicious but not confirmed.
On its own, Zika isn’t much to worry about—it’s rarely fatal, and is typically milder than its more painful cousin dengue, though the two share symptoms like fever, rashes, and joint pain. Most people who are infected with Zika (four out of five of them) will never experience any symptoms at all. “For most of the non-pregnant population, there is no reason to think Zika presents a particular risk,” the CDC director, Tom Frieden, wrote for CNN.
While this is great news for the casual traveler who just wants to visit Rio and avoid a rash, it makes it difficult to estimate how many people have actually been infected in this outbreak. If you’ve been infected with Zika, chances are you won’t know it. It’s a lurker. And now that we’ve seen it can be transmitted sexually, that's doubly worrying.
Plus, it really complicates the picture for pregnant women. If Zika does cause microcephaly, a lack of Zika symptoms makes it hard to know who’s at risk for having a baby with the birth defect.
But the number of cases of microcephaly in Brazil may be a little overblown—the Brazilian health ministry has reported around 4,000 suspected cases since October 2015, but these are just suspected, not confirmed. Some of those babies may just have small heads, and no developmental problems. The Global Post reports that the number of confirmed cases is much lower:
So far, of the original thousands of suspected cases, Brazil has confirmed 270 instances of children born with microcephaly since October 2015. And, significantly, 462 cases have been discarded, according to the ministry’s numbers. So cases are being discarded at almost twice the rate that they’re being confirmed.
“When you’re studying something in epidemiology, there's always going to be a little bit of over-reporting compared to the confirmed number of cases of a condition,” says Chad Achenbach, an infectious-disease specialist at Northwestern Memorial Hospital and Northwestern University. But even if some of those suspected cases don’t pan out, the increase is still notable—from 2010 to 2014, microcephaly cases in Brazil were hovering around 150 per year.