Tucson may be part of the Sonoran Desert, but to the mosquito Aedes aegypti, it's a tropical paradise. Unlike other mosquito species, this one survives in small--very smallamounts of water, in a cup, in a watering can, in dishes under potted plants or in trash left around the yard that accumulates water during the monsoon season.
But more threatening than a nasty bite, this mosquito transmits dengue fever, a virus endemic to more than 100 countries around the world (see sidebar). Although Tucson has never had an outbreak of dengue, it does support year-round populations of A. aegypti that are able to carry the virus from person to person.
Henry Hagedorn, a professor of entomology at the University of Arizona, along with research associate Frank Ramberg and a team of undergraduate students, is studying the ecology of this mosquito to find out if it's going to be an important vector of dengue in Tucson. To be a threat to public health in Arizona, both the mosquito and the virus must be present. So far, the researchers have determined that Tucson has high numbers of A. aegypti, but no dengue fever. Yet its presence here as a potential carrier of dengue makes it a potential medical threat in Tucson, according to Hagedorn.
"It lives around humans, breeds in our back yards, and in our homes," he says. "Controlling the mosquito is a matter of controlling the backyard breeding sites."
Hagedorn and his team have run a series of tests in Tucson to analyze parts of the insect's life cycle and its feeding habits. So far they have focused on the mosquito's egg-laying behavior, the frequency of its blood meals, and how often it bites animals other than humans. Also under study are the various ways people make their homes and back yards attractive habitats for the insect. Findings so far:
- A. aegypti mosquitoes are in Tucson all year round, according to results from egg-laying traps set out in a local neighborhood, but the largest numbers are here in July, August and September, the monsoon months.
- The mosquito is biting multiple hosts and thus could transmit the virus if it were carrying it: about 40 percent of the mosquitoes attempting to bite a researcher in a Tucson neighborhood had taken a previous blood meal and had developed a batch of eggs. This is the same average as the A. aegypti mosquito shows in Thailand and Puerto Rico, where dengue is endemic, according to Hagedorn.
- Only 50 percent of the mosquitoes tested have bitten humans, according to preliminary tests using antibodies to detect blood types of different animals in captured A. aegypti mosquitoes. If this is confirmed, the threat of dengue transmission would be reduced.
- Humans create the conditions the mosquito larvae can breed in, and they provide shelter for the adults from the heat of the day. Researchers found that open windows and doors, standing water and lots of shrubs near the home provide opportunities for mosquitoes to remain in the area.
- Each step of the research has raised more questions regarding the mosquito's blood feeding habits over its lifetime, how long it lives, and how far it flies between meals, all important factors in determining the vector ability of A. aegypti. Further tests are underway.
"We need to find out how the population in Tucson is related to those in Phoenix, and other cities in Arizona, Texas and Mexico," Hagedorn says. Since the mosquito prefers living in areas with moisture, the adult insects can't survive flying across long stretches of empty desert. So how are they moving? And how fast are they moving? Hagedorn believes it's possible that the mosquitoes in Phoenix may have arrived there not by flying from Tucson, but by hitching rides in cars or trucks.
To keep the mosquitoes from taking up residence, Hagedorn advises homeowners to focus on environments in and around the home that attract A. aegypti, including shrubbery that adults hide in, and places in and around the home that contain water. Window and door screens can prevent mosquitoes from entering the home; pet access ports may allow more than pets in.
"If you have an evaporative cooler that is not well maintained, mosquitoes can breed in it," Hagedorn says. Windows are often left open while using the cooler. "The best way to avoid dengue fever is to reduce the access the mosquito may have to you."