A number of years ago, at just about this time of year, my family was enjoying a pleasant alfresco dinner in our Brooklyn backyard when a squad of marauding mosquitoes crashed the party. These were unusually shrewd and persistent critters, targeting our ankles and the backs of our necks, hovering just out of reach of our swats and slaps. They were ferocious biters, too. But when one landed on my forearm, I paused to admire this nefariously beautiful creation of evolution: She was engineered to be a sleek hypodermic needle with wings, and her sole purpose was to siphon off, like a gas bandit, a bit of blood, which she needs in order to lay her eggs.
And then I noticed the telltale pattern of white stripes on her legs. “Damn,” I said to myself. “That’s an Asian tiger!”
Only a mosquito nerd would get jazzed by insect markings during a backyard blood draw. But it wasn’t that long ago, in ecosystem time, that the Asian tiger first turned up in the United States. A couple of mosquito-control officers in Houston spotted the mosquitoes (the official name is Aedes albopictus) in a port area of Houston, where the insects had presumably hitched a ride inside some used tires imported from Asia. Experts now consider it the most invasive mosquito in the world.
This immigration story is important for three reasons. One, since that day in August 1985, the Asian tiger has spread to at least 37 states in the U.S., including New York, Pennsylvania, and virtually all of New Jersey, in part because its eggs can survive northern winters. Two, even battle-hardened insect-control officials noticed something special about this mosquito: It is a very aggressive biter. And three, it is the only resident mosquito in the New York area, as far as anyone knows, that can transmit many of the “exotic” tropical viruses knocking on our door, including Zika.
The Asian tiger is a close relative of Aedes aegypti, the mosquito that is driving the Zika epidemic in Brazil, Puerto Rico, and elsewhere in South and Central America. The good news is that Aedes aegypti can’t stand the winters in New York; in a press release last month the mayor’s office stated that the Aedes aegypti mosquito “has never been found in New York City.” (This technically isn’t quite true, but we’ll get to that later.) The bad news is that its hardier cousin — in addition to being its equal at incubating the Zika virus, according to laboratory tests — is deeply entrenched in the region. The possibility that it might cause some local cases of Zika this summer is, in the words of Centers for Disease Control and Prevention research entomologist Janet McAllister, “an area of concern for us.”
Mosquito populations are exquisitely sensitive to climate and temperature, so mosquito experts aren’t sure what this year will bring. The relatively mild winter usually augurs an early mosquito season, but the cool spring could delay the emergence of this year’s crop (New York City began applying larvicide in mid-May). Despite these annual uncertainties, the Asian tiger has settled in these parts as a major pest; it is an avid daytime biter, and in the past, Long Island residents who live in Asian-tiger territory have complained that their children can’t play outdoors because of their fierce attacks.
The Asian tiger can transmit a number of nasty viral diseases, including what Ilia Rochlin calls “the big three” — Zika, dengue, and chikungunya — as well as West Nile disease. “Albopictus is kind of a wicked mosquito,” says Rochlin, who is affiliated with Rutgers University’s Center for Vector Biology and was the lead author of a 2013 paper documenting how the Asian tiger has colonized the I-95 corridor from Philadelphia to New York and predicting that its northern expansion will eventually reach all the way to Maine, especially if temperatures rise with climate change. Already, more than 30 million people live in areas of the Northeast that are prone to dense infestations of Aedes albopictus.
Whether Zika is here to stay is another matter. As of May 25, New York City Health Department officials had reported 109 cases, including 17 women who were pregnant at the time of diagnosis and whose fetuses are thus at risk for developmental abnormalities, including small brain size (microcephaly). All those cases, however, were “imported.” People with the disease contracted it outside the U.S. and showed up here already infected. What public-health officials don’t know — and can’t predict — is whether the Asian tiger mosquito will spread Zika in New York City, in what they call a “local-transmission event.” “We think it’s possible,” says Dr. Jay Varma, the deputy health commissioner for disease control, “but not likely.”
The reason Varma believes local transmission is unlikely is that the exotic viruses similar to Zika have never gained a foothold in New York City. Despite a recent chikungunya epidemic overseas, he says, “we have never once had a locally transmitted case.” In a little-known episode in 2013, however, Suffolk County health officials reported that a case of dengue fever on Long Island was locally transmitted — meaning someone on Long Island had the disease and an “American-based” mosquito passed it on. The Asian tiger was almost certainly the culprit. “We know it can happen,” says McAllister. “We just don’t know how likely it is to happen.”
McAllister says a “perfect storm kind of situation” has to arise for transmission of a tropical virus to occur so far from the tropics. A mosquito has to bite somebody with a high level of virus in his blood; the virus has to incubate in the mosquito’s gut for about seven days after its “blood meal”; the same mosquito has to bite another person, so that the virus can be passed along. Lowering the odds even more: The Asian tiger isn’t a picky eater. It would be as happy biting a squirrel or your dog as you (the mosquito driving the epidemic in Brazil, by contrast, prefers to bite humans, lives indoors with them, and has the habit of taking small, frequent blood meals, which facilitates the spread of disease).
No one thinks a major, Brazil-like outbreak is likely here, but isolated cases may become part of the local ecosystem. Epidemiology is often about low-probability events superimposed on very big numbers: the number of mosquitoes out at any given time of year, the number of travelers from endemic areas, the amount of virus in their blood. Traffic between New York and the region where Zika is being transmitted peaks between May and August, according to the Port Authority. The density of New York’s population, the presence of Aedes albopictus, and the number of potentially infected travelers lead McAllister to believe that, sooner or later, local transmission of Zika is a distinct possibility.
Perhaps the larger point is not about Zika at all, but about the Asian tiger mosquito. Public-health officials know that it is effective at spreading many diseases. A 2007 Zika outbreak in Gabon was driven by Asian tigers, as was a chikungunya epidemic in Northern Italy. As Rochlin says, “It can transmit pretty much everything.” And there’s been a dramatic increase in these types of illnesses. “We’re seeing more frequent problems,” says the CDC’s McAllister. “We used to see a big mosquito-borne outbreak every ten years. Last year it was chikungunya. This year it’s Zika. And there’s probably something else that’s coming along as well. We just don’t know what it is.”
If this sounds a little alarmist, let’s return to that questionable claim in the mayor’s press release. It is true, in the modern sense, that Aedes aegypti “has never been found” in New York City. But that is a historically — one is tempted to say fatally — inaccurate assertion. The informal name for Aedes aegypti is the “yellow-fever mosquito,” and it sparked at least 19 significant epidemics in New York City in the 18th and early-19th centuries, sailing into town in water barrels aboard slave ships and commercial vessels. Those mosquitoes couldn’t survive a northern winter, but they hung around long enough to cause thousands of deaths in New York, Philadelphia, Baltimore, Boston, and other cities up and down the East Coast. This year’s Asian tigers are poised to emerge. Larvae have been detected on Long Island, and the blood-seeking females typically appear at the end of May. Mosquito bites are always annoying. This year’s carry the risk — unlikely but possible — of something worse.
*This article appears in the May 30, 2016 issue of New York Magazine.