Mosquito magnet? Theory is up to scratch
Katie Warchut
The Arizona Republic
Aug. 21, 2003
Mosquitoes and the West Nile Virus
The West Nile virus, a disease transmitted to humans by mosquitoes, was
unknown in the United States until 1999. Mosquitoes bearing the virus
have appeared in Arizona, and officials are spending more to monitor and
fight the virus.
Only female mosquitoes bite; they need blood to produce eggs. A single
female can lay hundreds of eggs in her lifetime.
Mosquitoes need water to complete their cycle; increased summer watering
provides prime habitat.
The mosquito Cules tarsalis, shown here, is one of the more than 40
soecies in Arizona and a prime carrier of the West Nile virus.
How the Virus is transmitted
1. Infected birds carry West Nile virus and serve as a resevior.
The virus is passed on to the mosquito when it bites an infected bird;
the cycle continues when infected mosquitoes bite healthy birds.
2. Infected mosquito passes the virus on to humans.
3. Symptoms of the disease are simular to the flu and may last
three to six days. Only 20 percent of people infected become ill.
The elderly and people with weakened immune systems are most at risk.
No one in Arizona has been diagnosed withthe virus. Last year,
284 people died in the United States and more than 4,100 individuals became
ill.
The West Nile virus' arrival worries Valley residents like Irene Hunting,
who feels like she spends her camping trips scratching at red welts while
her husband seems immune to the blood-hungry mosquitoes.
"I think I'm particularly tasty to them," said Hunting, who lives in central
Phoenix.
It's not her imagination.
Mosquitoes can detect factors on people that tell the insects where to
find blood. The number and strength of these factors vary from person to
person.
Some people, such as Hunting, are mosquito magnets. "They're not detecting
blood, but they're getting clues," said Will Humble, head of the Arizona
Department of Health Services' epidemiology and disease control division.
Mosquitoes identify humans and animals by the carbon dioxide they exhale.
And the bigger a person is, the more carbon dioxide he or she will put out,
Humble said. A group of people would be even more susceptible because it
will emit a larger concentration of carbon dioxide.
Heat and sweat also attract mosquitoes, both of which vary from person
to person.
Once a mosquito gets close enough, both bacteria and chemical scents attract
it to the skin. Skin produces more than 340 chemical scents, according to
a U.S. Department of Agriculture study.
Bacteria also appear on the skin in different combinations.
"Taking baths and showers can change the bacteria, but it doesn't necessarily
help," Humble said.
Perfumes, deodorants and hair sprays also could play a role.
"Anything that changes the basic clues that it's looking for would make
a difference," he said.
Scientists have tested susceptibility to bites by age, gender or ethnic
background but have yielded no conclusive evidence, said Ulrich Bernier, a
research chemist in the mosquito and fly research unit of the USDA.
The good news, Bernier said, is that some mosquitoes are picky about their
victims. The species that transmits the West Nile virus prefers birds.
"We're their last choice," he said.