If you were bitten by a mosquito last summer, or the summer before that, chances are it was an Asian tiger.
This nasty, prolific, people-loving mosquito is one of the most invasive in the world. Less than 20 years after arriving in the Philadelphia region, Asian Tigers have proven themselves to be "vicious biters," said Jon Gelhaus, curator of entomology at the Academy of Natural Sciences.
Weather forecasts suggest that conditions could be ripe for a lot of mosquitoes this summer. On the bright side, this isn't the variety that has been spreading the Zika virus.
But it's not entirely out of the question.
Scientists who study mosquitos say the Asian tiger can carry Zika and similar, more familiar viruses such as dengue, but for a number of reasons it's lousy at transmitting them.
"We haven't seen it with dengue and we haven't seen it with chikungunya," said Neil O. Fishman, an infectious-diseases physician at the University of Pennsylvania.
But viruses and the mosquitoes that carry them move in a sort of evolutionary dance, changing over time in ways that benefit each.
And that is what's causing some scientists to view the Asian tiger mosquito — a.k.a. Aedes albopictus — with more concern.
One factor that makes Asian tigers more of an annoyance than a threat is the fact that people are not their first choice of food. Surprising shifts can occur in both animal and virus, though.
For example, researchers at Johns Hopkins found a few years ago that the dengue virus "turns on" mosquito genes to make the insects hungrier, so they will bite more and spread more disease. Scientists in Brazil are currently studying the possibility that birth defects linked to Zika infection during pregnancy are more likely if a woman had previously been infected with dengue.
Although men can transmit Zika to women through sex, most infections worldwide have been caused by Aedes mosquitoes.
In the Americas, two species in that genus can carry Zika:
- Aedes aegypti is behind the rapid spread of the virus through most of Central and South America, infecting millions of people and likely causing some women who got it while pregnant to have babies with abnormally small heads, a condition known as microcephaly.
Aegypti used to show up every summer in Philadelphia during colonial times. It was responsible for the 1793 yellow fever epidemic that killed 10 percent of the city's population. But it has not thrived locally since the accoutrements of modern living — screens, sewers, air-conditioning — cut off much of its human food supply.
- Aedes albopictus, with its distinctive Asian tiger stripe, is considered an invasive species — thanks to the global economy. Mosquitoes laid their eggs in scrap tires in Asia that were shipped all over the world. Rain filled the tires with water at their new destinations, triggering the eggs to hatch and the adults to colonize nearly every continent.
In the United States, albopictus was first detected in Houston in 1985, and spread rapidly north and east as the tires were transported by truck. The hitchhikers arrived in the Philadelphia region in the mid-1990s. By 2013, they were in every county in New Jersey. Pennsylvania has reported them in 38 counties, but they tend to cluster in densely populated areas. These are urban mosquitoes.
Nickname: Yellow fever mosquitoScientific name: Aedes aegypti
Appearance: Brown body with white stripes on black legs. Banjo-shaped white marking on the thorax.
Native to: Africa; spread to the tropics in many parts of the world with early European traders. Range extends into southern U.S.
In the news: Considered the primary vector of Zika virus in South and Central America. Also carries dengue and chikungunya viruses.
Notable: Responsible for Philadelphia’s deadly 1793 yellow fever epidemic. Rarely shows up here now due to better mosquito control, window screens and air-conditioning, and is not considered a threat locally.
Nickname: Asian tiger mosquitoScientific name: Aedes albopictus
Appearance: Black and white body with white stripes on black legs. Telltale single white stripe runs from middle of head to dorsal thorax.
Native to: Asia; a variety likely from Japan spread to U.S. as eggs laid in a shipment of used tires in mid-1980s; has spread north and east mainly along interstate highways.
In the news: It’s the top nuisance mosquito in the Philadelphia area. But is considered an inefficient transmitter of Zika, dengue and chikungunya partly because it often bites mammals other than humans.
Notable: Rarely seen in rural areas. Like aegypti, has adapted to live with humans.
Albopictus is not known to spread Zika in the wild. But laboratory experiments show that the mosquito meets the three critical criteria to cause disease: It can become infected by feeding on an infected animal or human. Its gut is a hospitable place for the virus to replicate. And the virus can migrate between gut and salivary gland, making its bite infectious.
Still, to cause disease the mosquito has to be interested in biting a host carrying the virus — and then biting again to transmit it.
"This is an odds situation," Scott C. Crans, a field researcher at Rutgers University's Center for Vector Biology, said by email. So far, 312 people in the U.S. are known to have picked up Zika, all linked to travel outside the country.
The Asian tiger "is very likely to feed on a pet dog, cat, squirrel or rat" instead of a human, Crans said, and could die naturally without ever encountering one.
"What I am getting at here is you need a lot of virus introductions in the human population to get it into the mosquitoes, and a lot of mosquitoes in the population that feed mainly on people," said Crans, who coordinates training for mosquito programs around the state. "So it is possible we will see some spotty local transmission in the U.S., but not likely anything as explosive as we are seeing in tropical areas," especially in the Northeast.
The biggest concern: The virus could mutate, making transmission easier for the Asian tiger. That has not happened with dengue and chikungunya, which have been around here longer — for the dengue virus, decades longer.
And Aedes albopictus has proved itself eminently adaptable, and a highly successful nuisance that is showing more fondness for human blood.
Unlike the Culex mosquitoes that engorge themselves on a single blood meal at dawn or dusk, spreading West Nile virus, the Asian tiger snacks rapidly throughout the day. It is fast and relatively painless at the moment its proboscis plunges into your flesh, making it harder to swat. Mosquito-control agencies say it now accounts for the majority of complaints, overtaking even the notorious hordes of salt marsh mosquitoes at the Jersey Shore.
Dina M. Fonseca, a Rutgers professor of entomology, has been studying Asian tiger behavior in different places. DNA analysis of blood from trapped mosquitoes found that albopictus was nearly 50 percent more likely to feed on humans in suburban Monmouth County than in inner-city Trenton. Her team surmised that the pit bulls kept in the city's low-income neighborhood offered a preferable meal.
Then they started collaborating with a group in Baltimore, "and oh, boy, we saw rats as a major source," Fonseca said. "We never saw rats in Trenton."
Those variations make eradication harder. In Trenton, the mosquito laid eggs in piles of garbage. Its Monmouth County relatives favored corrugated gutter spouts. Flower pot saucers, wheelbarrows, even bottle caps are popular all over.
"Container mosquitoes" aren't killed by insecticide sprayed from trucks or planes.
"The entire population has to participate in the control," said George Dimopoulos, a professor at Hopkins' Bloomberg School of Public Health.
Mainly, that means getting rid of standing water — and persuading your neighbors to do the same — in every conceivable place, every week.
Depending on the weather, the first eggs could hatch at the end of this month. Adult Asian tigers start emerging in late May or June, with the bulk in July and August.
The highest numbers so far were counted in 2012, when spring came early and warm. The latest 90-day forecast from the National Weather Service calls for warmer than normal temperatures through June.
"If it continues like this, I am holding my breath," Fonseca said, "it could be a banner year for Asian tiger mosquitoes."