For the first time in the Western Hemisphere, researchers have detected the Zika virus in Aedes albopictus, the mosquito species known as the "Asian tiger," a finding that increases the number of U.S. states potentially at risk for transmission of the disease.
During the summer months when U.S. mosquito populations are at their peak, albopictus is more ubiquitous than the Aedes aegypti mosquito that has been the primary vector of the spread of Zika elsewhere in the Americas. Unlike the aegypti mosquito, which is mostly present in southern United States and along the Gulf Coast, the albopictus has a range as far north as New England and the lower Great Lakes.
The discovery was reported recently by the Pan American Health Organization after researchers in Mexico confirmed the presence of Zika in Asian tiger mosquitoes captured in the state of San Luis Potosi and sent them to government labs for testing.
U.S. health officials say they had anticipated the finding and have already encouraged states within the range of the Asian tiger mosquito to prepare for Zika. Scientists had previously identified the Asian tiger as the primary vector for Zika during a 2007 outbreak in the West African country of Gabon.
U.S. health officials say the latest discovery should serve as a wake-up call to state and local governments that have assumed their populations were too far north to be at risk.
"There are officials who have been saying we don't have Aedes aegypti, so we don't need to be worried or have a plan," said Janet McAllister, an entomologist at the U.S. Centers for Disease Control and Prevention. "What CDC is saying is: You need a plan in place because albopictus could transmit Zika in your area, and you need to take it seriously."
While the methods for eliminating the aegypti and albopictus mosquitoes are similar, they are not identical, experts say, because the two species have significantly different behavioral and breeding patterns.
Unlike the aegypti mosquito, which thrives in urban areas by laying eggs in discarded food containers and old tires, the Asian tiger mosquito lives outdoors, laying its eggs in tree stumps and holes, McAllister said. It doesn't try to follow humans indoors and prefers leafy forests to dense urban environments. It's especially fond of suburban back yards and sprawling city parks.
Marcos Espinal, director of communicable diseases at the Pan American Health Organization, said researchers are still trying to determine how effectively the Asian tiger can spread disease in comparison to aegypti.
"Scientists will not be surprised if it's a competent vector, but we need to find out more," Espinal said in an interview.
CDC entomologist McAllister said there are several reasons to doubt that the Asian tiger mosquito will be able to drive the kind of Zika outbreak ravaging Brazil, where more than 1,000 infants have been born with undersize heads and severe brain damage probably caused by a Zika-related infection during the early stages of fetal development.
For one, the albopictus mosquito is "a more aggressive biter," McAllister said, as it feeds on humans, raccoons, squirrels or any other warmblooded animal it finds.
"Once it starts taking a blood meal, it will stay on that person until it's completely full," she said, instead of jumping from person to person — precisely the promiscuous behavior that makes the aegypti species so effective at spreading infections.
The aegypti variety is much more adapted to humans, she said, as it flies low around human ankles to avoid detection, then eats quick, short meals that reduce its chances of getting swatted.
"It won't take a complete blood meal if people are actively moving around, so that's what makes aegypti a super-spreader," McAllister said. "Whereas the albopictus tends to stay put."
Specimens of the Asian tiger species found in the United States have been traced back to northern Japan, reflecting the mosquito's ability to survive colder weather. With a distinctive black-and-white coloring, the species was first detected in North America at the Port of Houston in 1985, and later showed up in the Port of Los Angeles. In both instances, scientists suspect the larvae arrived in shipments of used tires.
Albopictus eggs have also arrived from Asia in the ornamental plants sold as "lucky bamboo" in U.S. department stores.
In Europe, albopictus populations have increased so fast that the European Center for Disease Prevention and Control, a European Union agency, has labeled it the world's most invasive mosquito species.
Eliminating albopictus larvae can be more difficult than controlling aegypti populations because the species can reproduce in a variety of outdoor environments, said Thomas Inglesby, the director of the University of Pittsburgh Medical Center's Center for Health Security.
The discovery of Zika in the Asian tiger mosquito means that states and counties in the insect's range need to rethink their control plans, he said.
"We have a big country where the responsibility for this rests with a lot of little jurisdictions, so we don't have a lot of good data about how well states and localities are prepared," Inglesby said. "Mosquito-control programs around the country are a patchwork.
"I think some states and local governments have been focusing on aegypti and less on albopictus, but this finding makes clear that both will require control measures," he added.
The White House has asked Congress for nearly $1.9 billion to help state and local health officials prepare for a Zika outbreak in the United States this summer, including funds for spraying, larvacide and other mosquito-control methods. The request has been stalled for two months, and Senate Republicans have countered with a $1.1 billion proposal.
In the meantime, the Obama administration has been paying for Zika preparations using funds previously approved for fighting Ebola.
According to the most recent CDC figures, 426 Zika cases have been reported in the 50 U.S. states by travelers who acquired the virus abroad or had sex with an infected partner. An additional 596 cases have been tallied in U.S. territories, mostly in Puerto Rico, where mosquitoes are spreading the disease.
There have been no reports of Zika transmission by mosquitoes in the 50 U.S. states, but the CDC has warned that is likely to change as insect populations increase with warmer weather.