Humans vs. the Mosquito: An Age-Old Battle

There’s an old proverb: “If you think you are too small to make a difference, try spending the night with a mosquito.” It’s an inspiring sentiment—but of course the insects do far worse than ruin a single night’s sleep; they are responsible for carrying and spreading some of the most lethal diseases in human history.

“The Aedes aegypti mosquito is notorious,” says Stephen Morse, professor of Epidemiology. “It carries a family of viruses that have posed a threat throughout history: yellow fever, dengue, chikungunya, and—most recently—Zika.” Fortunately, humans do have effective tools to fight the Aedes aegypti and all the diseases it carries.

A brief history of yellow fever

Of all the viruses carried by the Aedes aegypti, yellow fever has historically been considered the most dangerous due to its high fatality rate. Originating from West Africa, yellow fever made its way across the Atlantic Ocean with the slave trade, spreading across the Caribbean and the tropical regions of North and South America.

Throughout early American history, there were several deadly outbreaks of yellow fever, the worst of which hit Philadelphia in 1793, killing an estimated 5,000 people—about 10 percent of the city’s population. Yellow fever also posed such a deadly threat that the very first attempts to build the Panama Canal failed. Around the turn of the 20^th century, scientists proved two important facts to help fight yellow fever: mosquitoes could spread human diseases—and yellow fever was carried by the Aedes aegypti mosquito.

These discoveries led to a breakthrough: in 1936, Max Theiler—who was awarded the Nobel Prize for his work—created the yellow fever vaccine. “The development of the vaccine was really a remarkable tour de force,” says Morse. “That’s what tamed yellow fever for most of the wealthy world. It’s one of the oldest vaccines, but today it remains one of the safest, most effective vaccines we have.”

Controlling Yellow fever: A victim of its own success

Immunization, paired with mosquito control efforts, was so effective in the Western Hemisphere that yellow fever was largely relegated to the history books—a problem of the past. But today, the world is seeing a resurgence of yellow fever, and the other diseases carried by the Aedes aegypti pose a greater threat than ever.

Right now, Angola is experiencing its worst yellow fever outbreak in thirty years—in the last year, more than 200 people have died of the disease. Already, the virus has spread to neighboring countries like Kenya and the Democratic Republic of Congo, and about a dozen international workers have carried it back with them to China.

How could a yellow fever outbreak happen when an effective vaccine has been available for many decades? “In many parts of the world, the vaccine is only made widely available when you have an outbreak,” explains Morse. “It’s been used not for prevention but emergency response. We have a vaccine that saves lives, but unless you’re wealthy or a traveler from the West, we don’t really use it broadly until there’s a severe outbreak—an outbreak which almost inevitably has already caused deaths.”

In its emergency stockpile, GAVI, a public-private partnership committed to increasing access to immunization in poor countries, has just five million doses of the yellow fever vaccine—not nearly enough to cover the population of Angola and its neighbors. With just six vaccine manufacturers in the world, the World Health Organization has warned that Angola’s yellow fever outbreak “constitutes a potential threat for the entire world.”

Simultaneous battles: yellow fever and Zika

Compounding this problem is another virus carried by the Aedes aegypti: Zika. Until very recently, Zika was considered mild, especially when compared with the fatality rate of yellow fever. But just last month, scientists confirmed the connection between the virus and very severe birth defects. The top Aedes aegypti experts and vaccine companies are working now to rapidly develop a vaccine for Zika.

For many in public health, the best way to tackle Zika and yellow fever lies not in vaccines, but in stepping up mosquito control efforts. The Aedes aegypti is highly adaptable to human beings: it bites during the day, it can breed in the tiniest amounts of water, and will often stay in or near homes. After World War II, many countries in South and North America dedicated extensive, effective—and very expensive efforts—to control the mosquito, including the spraying of pesticides like DDT.

“Mosquito control was a victim of its own success,” says Morse. “The mosquito problem wasn’t as glaring, so the efforts weren’t sustained—and the mosquitoes came back. That’s the irony in today’s Zika outbreak: if it had happened fifty years ago, we would probably be better off. The virus likely wouldn’t have been able to establish itself in South America, because the mosquito had been largely controlled.”