Monday, April 23, 2018
Health

As the Aedes aegypti mosquito spreads globally, so does the risk of epidemics

Of all the mosquito species that populate the planet, few have proved themselves more resilient or more deadly to humans than the Aedes aegypti. The epidemics fueled by this tiny mosquito stretch across hundreds of years and include millions of victims.

Yellow fever, dengue fever, chikungunya. And now Zika, which has spread to more than 50 countries and can cause an array of severe birth defects.

"One of the most efficient killers in the world," Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, told the Washington Post this year in discussing A. aegypti.

These days, travel, global commerce and a warming planet only seem to be helping the mosquito to again flourish after widespread eradication efforts in the first half of the 20th century. That could mean more outbreaks of more diseases in more places.

Writing Thursday in the journal Science, Yale University's Jeffrey R. Powell, a professor of ecology and evolutionary biology, detailed how this species now breeds year-round in locations where it once didn't exist.

"These expansions are putting at risk large human populations that never experienced aegypti-borne viruses and therefore have no immune defenses against them," he wrote. "This greatly increases the likelihood of severe epidemics."

Powell noted that researchers have documented two subspecies of A. aegypti — the human-loving "Aaf" and the "Aaf" form traditionally found in forests — interbreeding in certain parts of the world from Argentina to Africa.

"The consequences of increasing hybridization between the two subspecies remain unclear," he continued, although warning that it could lead to increased genetic variation and still more spread of disease.

Part of what has made A. aegypti such a formidable foe and such an efficient disease transmitter is its ability to adapt. It has evolved to thrive in the densely populated places, particularly urban environments littered with old tires, trash and other open containers. It can breed in spots as tiny as a bottle cap. Its larvae don't necessarily need water to survive, and eggs can lie dormant for a year or more — only to hatch once submerged in water. The sticky eggs glue themselves to containers as common and varied as the insides of old tires and the edges of birdbaths.

"It's one of those pests, like cockroaches, that has evolved over the last 15,000 years to exploit changes in human behavior and habitation," Ronald Rosenberg, acting director for the Division of Vector-Borne Diseases at the Centers for Disease Control and Prevention, said at a conference this year.

In an interview last week, Powell called for far stronger mosquito-control campaigns in the United States and other countries.

"Rather than treating each disease after there's an outbreak, why not spend more money trying to control the mosquito?" he said.

Aside from yellow fever, no vaccine exists for other aegypti-borne diseases such as dengue, chikungunya and Zika — although scientists have been racing to develop a Zika vaccine. Powell said scientists have linked the species to "hundreds" of other viruses, mostly circulating among primates in Africa, that could one day cause the next global outbreak.

"We know they are there," he said. "We know it's going to happen again."

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