In the Eye of the Tiger

There is no shortage of mosquitoes in North America, and adding one more variety might seem like just a minor uptick in summertime’s itchy-scratchy. But the Asian tiger mosquito, Aedes albopictus, comes with some particularly irritating characteristics. It’s an aggressive hit-and-run biter that frequently lives in close contact with humans. It’s a daytime feeder that dines on humans, dogs, livestock, birds and a host of wild animals. “The feeding behavior of Aedes albopictus is very catholic,” says Duane Gubler, a vector-borne disease expert at Duke–National University of Singapore. “It feeds on everything.”

People in the southeastern United States are already well acquainted with the Asian tiger, named for its black-and-white stripes. But these mosquitoes are creating quite a buzz as they drift northward into more temperate climates along the East Coast, where their eggs can survive even cold winters.

The buzz would be just so much hand-wringing if it didn’t include an alarming public health component: The Asian tiger turns out to be a competent vector for a raft of diseases, some lethal. It can carry dengue fever, yellow fever, chikungunya virus, West Nile fever and two forms of encephalitis named for St. Louis, Mo., and La Crosse, Wis. Among these diseases, only yellow fever is preventable by vaccine. Ominously, dengue has already gotten a toehold in southern parts of the United States. 

Kicking the competition

The Asian tiger mosquito has joined a rogues’ gallery of invasive species — including zebra mussels, red imported fire ants and Africanized bees — now established in North America. In Florida and other subtropical parts of the Deep South, it has shown clear signs of displacing a predecessor-in-crime, the Aedes aegypti mosquito, which is best known for spreading yellow fever before that scourge was quelled by a vaccine. While it’s unclear whether the Asian tiger has muscled out resident mosquitoes farther north, one glance at a map shows its territory expanding.

Like many invader species, the Asian tiger arrived by hitchhiking. It departed its native habitat in Southeast Asia as larvae tucked in tiny pools of rainwater in stacks of used tires. In the last couple of decades, the international trade in used tires has carried those larvae around the world.

But the notion that the Asian tiger might be a public health concern in temperate zones didn’t arise until it fueled a 2007 outbreak that started in a small Italian town, Castiglione di Cervia, about a two-hour drive south of Venice on the Adriatic Sea. There a man who had been bitten by mosquitoes on a trip to India fell ill with chikungunya virus. Shortly after the man became ill, so did others in the area, reporting high fever, rashes and debilitating joint pain. It had been a mild winter, and the Asian tiger mosquitoes were out in force. Investigators believe the sick man was bitten by the mosquitoes, which then bit other people, transmitting the disease he’d picked up in India.

Travelers have been returning home from tropical sojourns with exotic mosquito-borne diseases since the days of stagecoaches and sailing ships. But in those days, temperate-zone mosquitoes were unable to transmit those ailments from returning travelers to their friends, family and neighbors. Once an infected person returned home, the chain of contagion was cut.

Today, air travel allows more and more people to crisscross the globe in a matter of hours. But even more important, another globe-hopper has made it possible for infected people to spread some of their unwanted pathogenic souvenirs: the Asian tiger mosquito. In fact, a growing number of scientists have concluded that Aedes albopictus may be an underestimated threat to public health in the temperate zones of the United States and Europe.

Asian tiger takeover

An evolutionarily diabolical trick has allowed the Asian tiger to displace resident A. aegypti mosquitoes in many places where A. aegypti had dominated for hundreds of years. The scheme relies on the fact that although the two closely related species can’t produce offspring, they can mate. When a male Asian tiger mates with a female A. aegypti, chemicals in his semen render her sterile. Preventing A. aegypti from reproducing is a huge advantage to the Asian tiger and allows it to rapidly overpower resident populations. “This is one of the key components of how the Asian tiger mosquito is managing to be so competitive,” says Irka Bargielowski, an entomologist at the University of Florida in Vero Beach.

Her recent research, published February 19 in the Proceedings of the National Academy of Sciences, shows that A. aegypti may be learning to avoid mating with the Asian tiger, which would make it less likely that A. aegypti will be displaced.

But there are other dangers. Just as other species of mosquitoes are adapting to the presence of the Asian tiger, so too are the pathogens it transmits. Take the chikungunya virus. Public health officials studying the appearance of chikungunya on the Indian Ocean island of Réunion in 2005–2006 noted that there were very few A. aegypti mosquitoes there. Since the island’s Asian tigers were not known to transmit chikungunya, officials were stumped. However, when researchers sequenced the Réunion Island strain of the virus, they found that it had mutated. The Asian tiger could now pass the virus on very efficiently.

That mutated strain has continued to spread. Epidemics of chikungunya have occurred in India, Malaysia, Singapore and Sri Lanka. Since many of these areas are home to both A. aegypti and the Asian tiger mosquito, it’s difficult to say exactly which species is responsible for the bulk of transmission. Still, one of the Indian Ocean chikungunya strains is the same as the one identified in Italy in 2007, and its adaptation to the Asian tiger makes it the most threatening to Europe and North America.

Quantifying the risk

Laura Harrington, an entomologist at Cornell University, wanted to estimate the threat of a chikungunya outbreak in the United States. To transmit chikungunya, the mosquito must first bite an infected person and then pass along the infection by biting another person. If the mosquito bites a cat or dog, the virus is stopped dead in its tracks. Harrington and colleagues planned for a worst-case scenario and assumed that the mosquitoes would feed almost exclusively on humans.

To quantify the threat, Harrington and her colleagues calculated the likelihood of a chikungunya outbreak if the virus were brought to the United States by a single traveler. The analysis focused on three East Coast cities: New York, Atlanta and Miami. Published last November in PLOS Neglected Tropical Diseases, the study incorporated information about local temperature and other factors to determine the number of mosquitoes, including Asian tigers, that would be present at the time the virus was imported.

In Miami’s tropical climate, an infected traveler could spark a chikungunya outbreak year-round. For New York and Atlanta, however, Harrington and colleagues discovered a distinct seasonal pattern. In New York, the risk of an outbreak approached 38 percent if an infected person arrived in August. In Atlanta, the high-risk period was longer — June through September. Although these risks are likely over-estimated since the Asian tiger doesn’t usually feed on humans as frequently as assumed in the calculations, the numbers still indicate a significant probability of chikungunya spreading in North America, thanks in part to the Asian tiger.

“No one has really been talking about this, even among public health officials and scientists. And now I can say, ‘Hey, if this can happen in Italy, this could happen here in the U.S.,’ ” Harrington says.

Although the potential range of the Asian tiger extends into far more temperate areas than A. aegypti’'s, plenty of places on Earth are still far too cold for the mosquito. As the Earth’s temperature slowly warms, however, areas even farther north will become hospitable for the Asian tiger mosquito. A group of researchers led by Cyril Caminade at the University of Liverpool modeled the potential future spread of the Asian tiger throughout Europe in a study published in October in the Journal of the Royal Society Interface.

Besides temperature, Caminade and colleagues found that rainfall was a significant predictor of the future habitat of the Asian tiger. As expected, they found that warming climate will probably push the range of the mosquito northward. Interestingly, southern Europe, where the mosquito is currently thriving, may become less hospitable with climate change since the area will become drier as well.

“Since the paper came out, people have been telling me how they have found the mosquito in exactly the areas that the model predicted,” Caminade says. “They are not scientists, so we cannot be sure, but the mosquito was detected in places where France and Germany share a border.”

Although scientists generally agree that global warming will expand the range of the Asian tiger mosquito, some, like Gubler, have doubts about its public health significance. Although the Asian tiger can transmit dengue in a laboratory setting, it happens less frequently in real life. When compared to A. aegypti, the Asian tiger is much less of a threat in Gubler’s opinion because it is much less likely to spread viruses when it bites.

“We need to focus on the people and animals that are moving these viruses around the world. The main drivers of the re-emergence of these diseases aren’t Aedes albopictus, they’re humans,” Gubler says.

Still, the Asian tiger has been responsible for several significant outbreaks of chikungunya — and a dengue outbreak in Hawaii in 2001. It also brings the potential for outbreaks to areas that have never before been at risk. Scientists agree that part of the solution to this problem is to increase mosquito-control efforts by reducing places for females to lay eggs and by the use of properly applied pesticides. Then, as Gubler explains, it won’t matter what species of mosquito is currently causing problems or how many humans are traveling.

Carrie Arnold is a science writer living in Virginia.

Invasion of a vector

As the Asian tiger mosquito establishes itself in temperate climates, it brings with it the ability to spread tropical diseases to those with no immunity.

• 1894 Asian tiger mosquito, Aedes albopictus (originally Culex albopictus), first described scientifically
• 1979 Asian tiger emerges in Europe (Albania)
• 1985 First continental U.S. breeding population established
• 1990–91 Asian tiger populations found in Italy
• 1999 Asian tiger emerges in France
• 2004 Asian tiger invades Croatia
• 2005–2006 Chikungunya outbreak on Réunion Island in Indian Ocean
• 2007 Chikungunya outbreak in and around Castiglione, Italy