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"Water that has been standing in the river channel is breeding mosquitoes, including the Culex species, which are potential vectors for encephalitis viruses. This problem is of even greater concern lately since the Western equine encephalitis virus has been isolated from mosquitoes collected in the same general area of Tempe in July and August," Levy stated in his September 7 letter.
Levy warned that "this virus can cause illness in both humans and horses."
Health officials recommended that Tempe channelize the river west of Town Lake and remove vegetation growing in the river bed. The vegetation -- mostly salt cedar -- is providing shelter for mosquitoes to breed in shallow ponds. The ponding is caused by steady flows from the city's street drainage system, which has an outfall immediately west of the downstream dam.
On September 9, Tempe asked the Corps of Engineers for permission to channelize the river bed and clear the salt cedar. The wetlands are not only breeding mosquitoes, but a variety of water fowl and migratory birds have been seen in the area.
The Corps of Engineers has not yet formally responded to the city's request, says Cindy Lester, who is with the agency's permitting section. Lester says the corps must first ask the U.S. Fish & Wildlife Service to make sure there are no endangered species that would be affected by the work Tempe wants to do.
Mosquitoes are vectors for the transmission of encephalitis from small creatures, usually birds and rodents, to humans. Birds that live near bodies of standing water are susceptible to encephalitis infection, according to the Mayo Clinic's Web site, HealthOasis. If a mosquito feeds on an infected bird, the mosquito becomes a lifelong carrier of the virus. The mosquito will transmit the infection to the next bird it feeds on, which will in turn spread it to more mosquitoes.
Although mosquito-borne encephalitis in humans is rare, it can be very dangerous. The risk of occurrence is highest during the warm months when birds and mosquitoes reproduce. There is no specific treatment for viral encephalitis.
Mosquito traps placed in the Salt River bed began turning up large numbers of encephalitis-carrying mosquitoes -- primarily Culexmosquitoes -- in July. Traps set one-eighth to a quarter-mile west of the downstream dam were capturing about 100 mosquitoes per night, says Rick Amalfi, laboratory director of Aquatic Consulting & Testing, which contracts with Tempe to monitor water quality in Town Lake and adjacent areas.
A half-mile downstream from the dam, the numbers soared -- 300 to as many as 900 mosquitoes caught in traps each night. Amalfi says the state-mandated alert level is 20 mosquitoes caught per night.
Amalfi, who conducted a survey of mosquito populations prior to the filling of Town Lake, says this year's mosquito population in the Salt River bed was about triple the year before. Amalfi says the heavy monsoon rains contributed to the growth of habitat suitable for mosquitoes. In addition, the steady flow from the Tempe storm drain is feeding water into the wetlands.
In contrast, the number of mosquitoes captured in traps directly adjacent to Town Lake was very low, typically about three a night, Amalfi says. The lake's depth -- ranging from five feet to 16 feet -- and concrete edge eliminate most mosquito-breeding habitat.
The infected mosquitoes breeding in the Salt River bed, however, can travel up to two miles in a night, Amalfi says.
The city and the county launched an aggressive mosquito-control operation last summer that lasted until early November.
More than 60,000 acres were treated with insecticides countywide.
County Environmental Services manager Al Brown says this is the first year that mosquitoes infected with Western equine encephalitis were found in the Salt River bed. He says their discovery could be the result of better testing, or possibly the presence of more birds attracted to the 2.2-mile-long Town Lake.
Residents near Town Lake should expect that pesticide applications will become routine.
"It is important to keep studying the mosquito population and applying pesticides as necessary," Brown says. "I don't see the problem going away. Is it going to become worse? I don't know."
Residents living near the lake were not formally notified by either the county or the city because the most toxic chemical used -- malathion -- doesn't legally require advance notice, officials say.
"Though not required, we discussed the issue of public notification during the planning phase of this work," says Tempe water management division manager Don Hawkes. "Our conclusion was that no notification was necessary."
Hawkes says the insecticide fogging operation -- malathion by the county and Anvil by the city -- did not come within 300 feet of any residences.
"The fogging was also done late in the evening or in the early morning with calm air conditions and when the majority of citizens were asleep," he wrote in an e-mail obtained under the state Public Records Act. "All this information contributed to our decision that public notice was unnecessary."
Maricopa County fogged the area on at least two occasions with malathion, says John Townsend, county vector control manager. The county also applied a larvicide -- Altacide -- directly to standing water in the river bed on two occasions. Altacide is a growth-hormone regulator that disrupts the evolution of mosquito larvae.