Whether a tropical disease linked to a Brazilian epidemic of severe birth defects spreads widely in the U.S. now hinges on a small, dark mosquito with white spots on its legs, a plague for centuries.
Aedes aegypti is the main carrier of the Zika virus that has infected more than a million people in Brazil. The bug lives year round in Gulf Coast Coast states from Florida to Texas, covering an area populated by 22.7 million people.
But while some Americans are panicking about Zika, which has been diagnosed in a dozen people in the U.S. so far (all of whom got it while traveling elsewhere), it’s entirely possible that the virus won’t have much of an impact here.
Viewed as a mild tropical illness until its arrival in Brazil last year, Zika is now linked to nearly 4,000 cases of microcephaly, an abnormally small brain.
“Zika’s disturbing march may not stop there,” National Institutes of Health chief Francis Collins warned on Tuesday, in an agency summary that noted the release of U.S. Centers for Disease Control and Prevention guidelines for testing infants with possible Zika infections.
Some 200 million people in the U.S., mostly on the East Coast and Midwest, live in places where mosquitoes carrying Zika might extend their range in summer months, Collins added.
But that doesn’t necessarily mean that Zika will spread there. After all, the same mosquitoes also carry yellow fever, but that disease is rarely seen in the U.S., with 10 cases in the last two decades
The fact is that Aedes aegypti mosquito “is a bit of a homebody,” infectious disease expert Daniel Lucey of Georgetown University told BuzzFeed News.
The mosquito likes to bite people in homes, but most Americans have screens and air-conditioning. What’s more, the bug doesn’t travel far from where it's born, which limits its ability to spread disease. Swamps are drained near most U.S. homes, and public health services try to control mosquito populations. (In countries without screened homes or strong public health services in Latin America, he views the situation as “very serious,” for the same reason.)
The big question for Americans is whether Zika will repeat the pattern of West Nile Virus, which spread from New York to California between 1999 and 2003. West Nile was different because it travelled inside dozens of species of mosquitoes as well as in birds. It killed at least 119 people last year, according to the CDC.
Alternatively, Zika might remain bottled up like the tropical Chikungunya virus, its viral cousin, which is also carried by Aedes aegypti. Mosquito bites have infected people in Florida since 2014, but the disease hasn’t flown farther on mosquito wings.
“During the [Chikungunya virus] outbreak only very few locally transmitted cases occurred in the United States,” mosquito-borne disease expert Moritz Kraemer of the University of Oxford told BuzzFeed News by email. He called Chikungunya a “more appropriate” comparison to Zika virus.
Every year, more than 6 million travelers fly from Brazilian airports near Zika-infected regions to the U.S., raising the risk of travelers transferring the disease to mosquitoes that go on to bite uninfected people. But last year, travelers in 45 states returned home from tropical regions infected with Chikingunya without subsequently spreading the virus.
“The U.S. public health infrastructure is strong enough to detect outbreaks of disease and deter its spread,” travel medicine expert Phyllis Kozarsky of Emory University told BuzzFeed News. “Everyone is being told to be very alert for signs of Zika.”
Even if Zika virus can be carried farther north by the hardier Asian Tiger mosquito, Aedes albopictus, a question still under study, Kraemer suggests the spread of Zika in the U.S. will still mirror Chikungunya. That would mean small, contained outbreaks of the disease rather than a nationwide spread.
Still, a lot remains to be seen. Zika is a mild virus and most of its victims, perhaps 80%, exhibit no symptoms, which means they won’t go to a doctor and could escape detection while the disease spreads. Zika requires time- and resource-intensive genetic testing for a conclusive diagnosis.
“At this time, there is no strain on CDC testing capacity,” agency spokesman Thomas Skinner told BuzzFeed News by email.
Ironically, one of the factors limiting the spread of Zika might be the Asian Tiger mosquito, which only arrived in Baltimore in 1987 in water inside recycled tires. The vicious tiger mosquito has since outcompeted Aedes aegypti in much of North America, lowering its population and, perhaps, keeping Zika bottled up.