The first local transmission of the Zika virus from mosquitoes in the continental United States could appear in the coming months as the weather warms up and the bloodsuckers start stirring — although how far and wide the virus spreads here will depend on one black-and-white striped insect.
The yellow fever mosquito, Aedes aegypti, has been propelling the Zika virus in the ongoing outbreak in Latin America and the Caribbean. But a close relative called Aedes albopictus thrives in a much broader expanse of US territory. And if albopictus, also known as the Asian tiger mosquito, proves adept at spreading Zika, it greatly expands the map where cases — and possibly the birth defects and paralysis that global health officials suspect Zika can cause — could start to pop up.
While aegypti mostly huddles along the hot and humid Gulf Coast — stretching from Texas to Florida — albopictus is seen from Kansas through Ohio and even into southern New York.
Aegypti mosquitoes excel at transmitting Zika, but less is known about how efficiently the virus can be spread through albopictus. Researchers found evidence that albopictus was responsible for carrying the disease in Gabon in 2007, and scientists in Singapore have been able to infect albopictus with Zika in a lab.
Fortunately, in the fight against Aedes albopictus, we humans have one thing going for us: They’re just not that into us. Or at least not as much as aegypti mosquitoes are.
Aegypti mosquitoes cause such immense problems for people because they make our homes their homes. They like to camp out underneath beds and in closets. And their preferred breeding sites are man-made: flower pots, water basins, tires, and trash that collect just the little bit of water mosquitoes’ eggs need to hatch.
And their meal of choice? Human blood.
“From the day it emerges as an adult mosquito, it’s near humans,” said Chris Barker, an epidemiologist at the University of California, Davis, of aegypti. “The mosquito is very likely to bite one human and then another human.”
Albopictus, on the other hand, will bite us when we’re near. But the species doesn’t have the thirst for our blood that aegypti does.
“They tend to feed on a big variety of vertebrates,” said Carol Blair, a virologist at Colorado State University. “They’re not as choosey as Aedes aegypti. They’ll bite dogs, cats, cows, all kinds of mammals.”
Behaviorally, albopictus mosquitoes also keep their distance, often choosing to lay their eggs in more natural settings like tree holes. They’re more outdoorsy than their aegypti cousins, sometimes residing in forests or, when they’re among people, at least having the courtesy to stay in the yard.
“Aedes albopictus is a more catholic feeder,” Louis Lambrechts, who studies mosquito-virus interactions at the Institut Pasteur in Paris, wrote in an email.
Together, those characteristics of albopictus mean there’s less of a chance that it contracts a virus by biting one person and then transmitting it by biting another after it has become infectious.
“It dilutes the possibility of transmission,” said Dawn Wesson, who studies dengue, West Nile virus, and other vector-borne diseases at Tulane University. “But it doesn’t take it away entirely.”
What remains to be seen, experts say, is how effectively Zika can infect and spread through albopictus mosquitoes specifically. Just because a mosquito takes a blood meal with the virus present doesn’t mean the mosquito will become infected. And just because a virus infects the mosquito does not mean the mosquito itself then becomes infectious to people.
Once it’s ingested with the blood meal, the virus first has to invade cells in the mosquito’s midgut. Then the virus needs to sneak past the immune system to multiply throughout the insect — and mosquitoes “can fight off infections in some cases,” said Stephanie Richards, a medical entomologist at East Carolina University. “Not exactly like in mammals, but they have some mechanisms.”
The virus must finally reach the salivary gland, from which it can be passed to the person providing the mosquito’s next meal.
It takes an average of about seven to 10 days for a mosquito to become infectious, depending on environmental factors. Aedes mosquitoes tend to only live about two weeks once they hatch in the wild.
“Aedes albopictus is a more catholic feeder.”
Louis Lambrechts, Institut Pasteur
The cases studied in Gabon and lab work in Singapore show albopictus mosquitoes can transmit Zika, but experts say they need to do more research to determine just how capable albopictus mosquitoes are at transmitting the virus, especially in the Americas. Scientists are also studying whether both aegypti and albopictus mosquitoes infected with Zika can pass the virus to their offspring, which happens rarely with other viruses like dengue and chikungunya.
Elements like humidity and temperature can alter how well-paired a mosquito species and viral strain are, and geographic differences exist. A population of albopictus mosquitoes buzzing around Baton Rouge, for example, may have more resistance to Zika than a group flittering in Philadelphia.
Viruses also change. A mutation in a certain strain of chikungunya about a decade ago increased its ability to spread through albopictus mosquitoes in places around the world.
Overall, experts said they wouldn’t be surprised if Aedes albopictus caused some local spread of Zika in the United States — although any US outbreaks are expected to be small and short-lived, thanks to well-built homes, air conditioning, and screened windows.