Published On Feb 10, 2016
Beating Zika in the Wild
Fighting mosquitoes is no walk in the park. A disease ecologist describes the landscape of mosquito-borne diseases here in the United States.
The World Health Organization recently called the spread of the Zika virus a global emergency, and the Centers for Disease Control has put the United States on the highest level of alert. Research teams are developing vaccines and looking at how to treat people who have already been infected.
But a major front against Zika will be waged outdoors, in the world of the mosquitoes that carry the disease. Key players will be the disease ecologists, specialists that study how diseases move through animals, habitats and ecosystems.
Shannon LaDeau at the Cary Institute of Ecosystem Studies in Millbrook, N.Y., studies mosquitoes, especially those that carry deadly pathogens such as Zika. Of the 3,500 species of mosquitoes, she notes, only a few have carved out a niche among Homo sapiens—which means that they are “specialized at prospering in human-dominated places” and won’t surrender their turf without a fight.
What do we know about the mosquitoes that transmit Zika?
There are two main culprits to watch in the United States. The first species, Aedes aegypti, is found mostly in Florida. They’re responsible for spreading the virus in South and Central America. They bite humans almost 100% of the time. The other mosquito is Aedes albopictus, or the Asian tiger mosquito, which is found in much of the eastern United States. Humans make up only about 60% of their diets.
These two species affect how an outbreak might happen in these two regions. When a mosquito bites an infected human, and then a rat or a dog, it can interrupt the transmission cycle. So an outbreak carried by Ae. albopictus, which is more likely to bite another species after biting a human, would be slower.
And what else do we know about their habitats?
When both species are found in the same area, at least in the United States, Ae. aegypti seems to be more urban, with Ae. albopictus gravitating to more suburban areas. But they both live around humans.
These species breed only in container habitats here—discarded buckets and bottles that collect standing water, gutters that don’t drain, rain barrels, birdbaths and so on. And only around human settlements. ?
So we just need to get rid of water containers, right?
Yes. But it’s trickier than it sounds. People may see that their own little birdbath breeds mosquitoes, but they imagine that the true problem is really caused by their neighbors or local vacant lots.
In Brazil, the epicenter of the current outbreak, the problem is different. People store their own water, on their roofs or inside their homes. So it’s not just about dumping the debris of our North American backyards. You need to address these necessary water storage systems.
There was a lot of public health outreach around this two years ago, when Brazil hosted the World Cup and there was a big dengue fever outbreak. Dengue is also transmitted by Ae. aegypti, and people were told to remove standing water. But the places getting the most attention were where tourists were expected—and those aren’t the places where Zika hit.
In Brazil now they’re trying to control Zika by spraying insecticides. Does that work?
It’s really difficult to hit the mosquitoes. Chemicals that kill adults have limited efficacy unless they stick around in the environment, like DDT—but that persistence is why DDT is so damaging to other animals, too. It’s also difficult to spray chemicals that kill the larvae in a way that reaches all the container habitats.
Is there any way to improve ecological dynamics instead of using chemicals? Can we encourage natural predators like birds or bats?
Bats and swallows probably do eat a lot of adult mosquitoes. But there isn’t any documentation that proves they eat enough to really knock Aedes populations back. And in the containers where both Ae. aegypti and Ae. albopictus breed, the only really effective approach is to dump the containers.
There’s one type of mosquito, Toxorhynchites, that eats other mosquitoes’ larvae. Occasionally we find them in containers where they’ll eat Ae. albopictus. There have been some experiments on how to increase Toxorhynchites numbers, but they’re not as tolerant of urban conditions.
There has been talk about eradicating disease-carrying mosquitoes through the use of genetic tools or other high-tech means. If any of those happened, what would the impact be on their ecosystems?
I don’t know what’s on the horizon for these techniques. But it seems unlikely that eradicating any one mosquito species would have a detectable effect. If it’s the right species, it could have quite positive effects on human health. However, it would depend on how they’re targeted. The toughest species are generally the biggest pests. Eradicating one of these without hurting more sensitive species could be tricky.