This summer's aerial assault on mosquitoes spreading West Nile virus in Sacramento is just the latest front in a battle against the disease-bearing insects that began a century ago.
Ever since scientists in the late 19th century figured out that bloodsucking mosquitoes can transmit deadly pathogens to people, the little winged bugs with the piercing snouts have been the target of public health officials.
Not every type of mosquito is capable of transmitting nasty germs. Only a tiny fraction of the estimated 3,500 species of mosquitoes worldwide are considered important vectors, or transmitters, of disease.
But some of those sicknesses can be quite awful, including malaria, dengue fever, yellow fever and elephantiasis - a condition caused by a parasitic worm that causes gross enlargement of the limbs, head and/or genitals.
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For that reason, it's little wonder that mosquitoes have come to be vilified. Besides, even if catching a disease from a mosquito bite isn't a big concern - and until 1999, when West Nile virus showed up in New York, it hadn't been a big concern in the U.S. for years - it's no fun being bitten by a bug.
"Nobody likes mosquitoes except us guys who get paid to study them," said Bill Reisen, a research entomologist at the UC Davis Center for Vector-Borne Diseases, adding that he doesn't really like them, either.
"I don't like getting bit any more than anyone else," he said.
But he and other mosquito researchers do find the creatures, with their peculiar lives and lifestyles, fascinating.
Mosquitoes live on every continent of the world except Antarctica and come in a variety of sizes and colors - some drab, some a brilliant metallic blue, green or purple, said Tom Zavortink, a researcher at the UC Davis Bohart Museum of Entomology who specializes in classifying mosquitoes.
Mosquitoes start their lives in water, which is why eliminating standing water is so important in limiting their numbers. Only as adults do they fly, and only the females drink blood, something they do to get the protein they need to lay eggs.
But blood-drinking is not a universal trait. Some of the biggest mosquitoes known, of the genus Toxorhynchites, don't need blood.
"They are big, gigantic mosquitoes, and their larvae are like little alligators that eat up all the other mosquito (larvae)," Reisen said.
Apparently, they get enough protein as larvae to last their entire lives.
It's the mosquitoes that drink blood that can transmit disease, by picking up viruses, nematodes or protozoa from one person (or other animal) and passing them on to another.
At the same time, not all bloodsucking mosquitoes pass germs around.
Tom Scott, a UC Davis entomologist who has studied mosquitoes for 30 years, described a fairly complex set of circumstances that has to occur before a mosquito becomes a good vector. For one thing, the mosquito has to be able to survive the infection. If the pathogen is fatal to the mosquito, it won't have a chance to pass it on.
Secondly, the pathogen has to be able to multiply inside the mosquito. (This situation is particularly interesting in the malaria parasite and the mosquito species that carry it. Scott said the parasites have sex only in the gut of the mosquito.)
Thirdly, the mosquito has to live long enough after acquiring an infection to transmit it to another organism.
Fourth, the mosquito population must be dense enough for the pathogen to be passed around efficiently. And, in the case of human diseases, the mosquito has to be the kind of bug that feeds on people at least sometimes.
Zavortink estimates that less than 1 percent of the 3,500 species of mosquitoes in the world are important vectors.
California is home to 53 species of mosquitoes, Reisen said, of which only two are important in transmission of West Nile virus. Those are known as Culex tarsalis and Culex pipiens.
West Nile virus is a relatively new disease for the United States, first detected in this country in 1999 on the East Coast. The illness was first recognized internationally in 1937, when the West Nile pathogen was identified in the African country of Uganda.
The disease is wide-ranging in its effects. Most people who contract an infection don't even know it. An estimated 20 percent become ill, usually with flulike symptoms such as headache, fever and body aches.
Less than 1 percent of people with the virus become seriously ill with neurological problems such as brain swelling (encephalitis) or paralysis.
The disease can be fatal. At last count, it had killed six people in California this year.
The first person to implicate mosquitoes in disease was the British parasitologist Sir Patrick Manson. In the late 19th century, he discovered through work in China that the insects were key in transmitting the roundworm that causes elephantiasis.
Scott said Manson encouraged scientist Ronald Ross to study the role of mosquitoes in malaria, research for which Ross won the Nobel Prize. The link to mosquitoes led to a variety of global campaigns to eradicate the species associated with various diseases.
Following the invention of the pesticide DDT in 1939, the World Health Organization, U.S. Agency for International Development and other agencies set out to eradicate malaria in developing countries by spraying inside houses, Reisen said.
Regionally, the effort sometimes worked. Reisen said he was in Nepal in the 1970s when the DDT campaign wiped out the malaria mosquito Anopheles minimus locally. But globally, the attempt to eradicate malaria fizzled. The discovery of long-lasting harmful effects of DDT on the environment gave the pesticide a black eye and led to its ban in many countries.
Today, Reisen said, the WHO's effort to roll back the disease focuses on the use of mosquito netting impregnated with pyrethroids, a synthetic and more powerful form of the insecticide pyrethrin that is being sprayed in Sacramento and Yolo counties against West Nile virus.
"The campaigns are now to eradicate the disease, not necessarily the mosquito," Reisen said.
Human cases in 2005
Regional counties: Sacramento, 58; Stanislaus, 30; Butte, seven; Sutter, three; San Joaquin, four; Yolo, four; Placer, five.
Where to get information
* To report a dead bird, call (877) 968-2473
* Sacramento and Yolo county residents can request mosquitofish, report untreated pools of standing water, get aerial spraying information and sign up for e-mail notification of local insecticide treatments by calling the Sacramento-Yolo Mosquito and Vector Control District at (800) 429-1022 or (916) 685-1022, or at www.fightthebite.net.
* Placer County residents can get West Nile information and obtain mosquitofish by calling the Placer Mosquito Abatement District at (916) 435-2140. For other information, call the Placer West Nile virus line at (530) 889-4001 or go to www.placermosquito.org or www.placer.ca.gov/wnv.
* Butte County residents can go to www.buttecountypublichealth.org or call (800) 339-2941.
* Anyone with concerns about the health effects of spraying can call the California Poison Control number at (800) 876-4766.
To reduce the risk of catching West Nile virus, the Sacramento-Yolo Mosquito and Vector Control District recommends:
* Use an effective mosquito repellent containing ingredients such as DEET, Picaridin or oil of lemon eucalyptus.
* Repair tears in door and window screens.
* Drain standing water.
* Wear long pants and long sleeves outdoors when practical.
* Avoid being outside at dawn and dusk, when mosquitoes are most active.
What's in the pesticide
Here are the ingredients to EverGreen Crop Protection EC 60-6, the pesticide that is being aerially sprayed over Sacramento County:
* Active ingredients: Pyrethrins (a killing agent extracted from chrysanthemum flowers), 6 percent; piperonyl butoxide (known as PBO, this "synergist" slows an insect's ability to break down pyrethrins and is classified as a "possible" cancer-causing agent by the U.S. Environmental Protection Agency), 60 percent
* Inert (nonactive) ingredients: Glycol ethers (a solvent and inactive byproduct of PBO), less than 1 percent; petroleum distillates (refined kerosene, a solvent used to dissolve the active ingredients), about 5 percent.
* Remainder: A proprietary mixture designed to dissolve and bind all ingredients together. It includes ionic and anoinic surfactants (soap chemicals found in dishwashing soaps and laundry detergents; these enable the solution to be diluted in water or oil) and sorbitan monooleate (an oil emulsifier and stabilizer that allows the product to be mixed with oil for application.)
About the writer:
- The Bee's Edie Lau can be reached at (916) 321-1098 or firstname.lastname@example.org.