The recent Zika virus outbreak has alarmed public health officials by expanding its range — but it’s not alone. It’s just the latest in a number of mosquito-borne illness to spread beyond their endemic areas in recent years. Experts say that the combination of a number of factors have ignited their rapid spread: climate change, urbanization, and easy access to travel. That means outbreaks of this kind are here to stay, and could potentially get even worse.
For most of its existence, Zika was more or less isolated to Asia and Africa. Since its discovery in the 1940s through 2007, there are records of only 14 people with infections. Now, though, the virus is expanding at lightning speed throughout South and Central America; somewhere between 500,000 and 1.5 million have been infected in Brazil alone since early 2015.
These viruses, once foreign to the Americas, pose a substantial concern
The outbreak is reminiscent of dengue, which was once considered a mainly Asian disease; it has taken hold in the tropical Americas, becoming a big problem in the years since the 1990s. Incidences of dengue have increased 30-fold in the past 50 years, and the World Health Organization estimates there are 50 million cases worldwide each year. A state of emergency was just declared in Hawaii over a recent dengue outbreak. But it’s not just Zika or dengue — other mosquito-borne illnesses, including Chikungunya, West Nile, and Japanese encephalitis virus are gaining ground, too. And it's thanks to rising temperatures and a growing population that's more mobile than ever before. "Everyone is acting like Zika is new, but it’s just the latest in a series of similar events," says Bill Reisen, a zoologist at UC San Diego.
Now these viruses, once foreign to the Americas, pose a substantial concern for developing nations in the region. (Developing nations have fewer resources to implement mosquito control initiatives, which might hinder the diseases’ spread in humans). That’s not the only problem, though. Doctors in the Americas are unfamiliar with these diseases. "The doctors we train in all the universities in the US are not exposed to these diseases, so there’s no research about it," says Eliseo Eugenin, of the Public Health Research Institute Center at Rutgers University. But Eugenin and Reisen agree that these outbreaks are only going to continue, and soon scientists are going to have start paying attention.
Distribution of the Aedes aegypti mosquito; stronger concentrations in yellow and red areas. (Kraemer et al.)
It makes sense that these viruses are growing in scope: the mosquitoes that carry them are expanding their reach. Many mosquito-borne illnesses like Zika are spread by two main species: the Aedes aegypti and the Aedes albopictus. Originally from Africa and Southeast Asia, the two invasive mosquito types can now be found in tropical and subtropical regions throughout the globe. They first arrived with the slave trade, and today’s international shipping have helped to ferry them overseas.
The mosquitoes that carry these diseases are expanding their reach
But that was just the start of the mosquitoes’ expanding range. With average global temperatures rising each year, the mosquitoes can be found even farther beyond those tropical areas now. The Aedes aegypti doesn't normally live in areas where winters dip below 50 degrees Fahrenheit (10 degrees Celsius); the mosquitoes can't survive in that kind of cold. But more northern areas are experiencing warm winters, allowing the Aedes aegypti to expand its territory. "Before, the mosquitos was mainly concentrated in the tropics, but now the mosquito has been seen all the way up to Canada," says Eugenin. "They’re everywhere."
The mosquitoes also have more people to bite. The world's population has tripled in just the past 60 years. Now more than half of the world's population lives in urban areas — up from just one-third in the 1950s. Poor urban housing often isn’t equipped with proper piping, so many residents end up storing water in and around their homes. Stagnant water is the perfect place for the Aedes aegypti mosquito to reproduce. These trends have created a perfect breeding ground for mosquitoes in urban areas. "You have this expanding human population moving into cities, and in lesser developed countries, the urban infrastructure has not been able to keep up," says Reisen.
"The mosquitoes are here; the diseases are here."
After gaining ground in developing nations, these viruses are increasingly making their way to countries like the United States, now that people have easy access to international travel. Typically mosquitoes don’t travel very far from where they live, but people can transport the diseases they carry over long distances. It's very simple for someone to travel to a country infected with a mosquito-borne disease, get bitten by a carrier insect, and return back to the US where local mosquitoes can transmit the virus to someone else. About 80 percent of people who get Zika have no symptoms, but they have enough of the virus in their system that they can transmit Zika to mosquitoes that bite them. "So you have people that are essentially healthy carrying viruses around with them as they come and go," says Reisen. "It’s not a wonder that we’ve had a cascade."
Fortunately, viruses like dengue and Zika, which transmit from mosquito to person to mosquito, won't find much of a toehold in the United States. Most Americans have easy access to air conditioning and wind screens, to help shield them from mosquito bites. Here, the real threat is from viruses spread from mosquitoes to other animals besides humans. Those are much harder to eradicate, because transmission continually occurs in the wild. An example is West Nile Virus, which came to the United States in 1999 and caused a deadly outbreak in 2012.
But other countries in the Americas aren’t so lucky. Developing nations like Brazil don’t have the resources to implement major mosquito control initiatives, and citizens don’t always have easy access to air conditioning. Plus, doctors in these countries don’t have as much money as US doctors to conduct research on viruses like Zika. It may be harder for doctors in these areas to recognize the signs and symptoms of these diseases. Because of this, Eugenin and other health experts say more research on mosquito-borne illnesses is crucial. Climate change isn't going away, and the population is only going to get bigger. "The mosquitoes are here; the diseases are here."