Tropical Storm Allison caused extensive flooding in Houston, which has caused the mosquito population to explode in the region.
But summertime is mosquito season for the rest of the country as well. Recently, mosquitoes have been promoted from annoyance to health hazard. New York and New Jersey have spraying programs to kill them because of the West Nile virus.
Andrew Spielman, ScD, an expert on mosquitoes and senior specialist in tropical illness at Harvard University, joined The Early Show to discuss the problem of mosquitoes, what can be done to get rid of them, and how to prevent them from biting.
Interview with Andrew Spielman
What's happening in Houston and why is it so bad?
In general, the water level in the puddles is higher than usual and there is more of an accumulation of water. That means that more eggs are hatching. Mosquito larvae spend the winter in the egg stage. The eggs can remain in that stage for years or even decades. In Houston, they've had a massive hatch: The water is deeper, there are more eggs hatching under the water, and more adults are emerging.
Let's demystify the mosquito. Which one bites?
Only the female bites. There are species that never feed on people.
Are all people fair game for mosquitoes or do they prefer some people to others?
There's an old paper out of Germany that says women are more attractive to mosquitoes than men. Particularly women when they are ovulating. And some people are more attractive than others are. We don't know why. There's a lot of research. And the biting varies with each species of mosquito.
What makes a mosquito bite so uncomfortable?
It's not the bite that produces a reaction and causes pain. As mosquitoes bite, they salivate into the wound, and it's the body's reaction to the saliva that's painful. On first exposure to a particular kind of mosquito you don't have a reaction. After repeated exposure [more bites], the body's immune reaction starts to increase. And then you have the pain associated with the bite. It's interesting to note that after repeated bites over several weeks the immune reaction decreases and you don't feel the bites. There are many species of mosquito and in any area there can be 20 or more species. Immunity builds up for each mosquito individually. You can be immune to one type and not the others.
Aside from being annoying do they pose a serious health problem? Do all mosquitoes carry disease?
Absolutely not. Some mosquitoes carry a vector [a disease] and others don't, and it's a complicated matter. There are many factors that determine if a mosquito is vector or not. If you look at the West Nile virus that has been of concern in New York, each person has a certain chance to get it, but each person's risk is slight. In New York, there were seven deaths out of a population of 10 million. It's a terrible burden on society as a whole when infections start accumulating. It also becomes a major psychological problem.
When you hear that high C [whining sound of a mosquito] in the middle of the night, you wonder if that bite is loaded. Actually your chance of getting a disease is about one in a million. And your chance of contracting a disease varies with age. Middle-and-older-aged people are more at risk. Although some children have been affected, that is extremely unusual.
So, what are the best ways to kill mosquitoes and keep the population down?
There are many species and each one is different. Essentially that translates into different kinds of interventions. Source reduction is one strategy: You do something about the larvae. The common house mosquito is found in street drains, abandoned swimming pools, ditches that aren't draining well, gutter down spouts that don't drain properly, and rain barrels. Any vessel that holds a gallon of water should be removed. If you can find the breedng sources, you can usually do something about it. Reduce the abundance of larvae and that reduces the population.
There are devices that are effective--citronella candles and mosquito coils. The coils include pyrethrum. They burn slowly, emitting a smoke that keeps mosquitoes away and actually kills them.
What's the best way to prevent them from biting?
Repellants that contain DEET are usually effective. An adult is best off with a DEET concentration that runs between 20 and 30%. A child needs lesser concentration. For a child, overexposure to DEET has resulted in injury. It's extremely infrequent, but use a lesser concentration for a child.
What attracts a mosquito?
Vision is an important part of a mosquito honing in on a person. Their eyes adjust mainly to movement. And contrast is important. If the background is dark and you have light clothing on, the mosquito will sense your presence. For instance, in a forest, it's better for you to wear clothing that allows you to blend in with the background.
Carbon dioxide is an important stimulus. The more active a person is, the more carbon dioxide we exhale. Lactic acid is also a product of activity, and that is also exhaled. Mosquitoes hone in on those substances. If you want to attract mosquitoes, stand on a little hill in a gentle breeze and wave your arms and that will attract them to you. A gentle breeze is less than 6 miles an hour. Mosquitoes can handle wind up to 6 miles an hour.
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