Volume 4, Number 4—December 1998
Aedes aegypti in Tucson, Arizona
To the Editor: The highly domestic mosquito species Aedes aegypti, a tropical, nonnative vector of dengue and yellow fever, has been identified in several desert communities, including the city of Tucson and the border towns of Douglas, Naco, and Nogales (1). Ae. aegypti has now been found in the southern Arizona communities of Benson and Sahuarita Heights, which indicates that its distribution is probably expanding. Moreover, mosquito surveillance indicating that Ae. aegypti populations in Tucson are established throughout the city supports local concerns that the mosquito poses a public health risk in Arizona's second largest metropolitan area.
Since it was first detected in 1994 on Tucson's west side, the mosquito has been found in the central downtown and university districts, northern foothills, and the east and south sides (both in residential areas, including schools and parks, and in business districts). Between 1995 and 1997, almost 200 adult Ae. aegypti (female and male) were recovered in CDC CO2 traps from 26 sites in Tucson by vector biologists from the Arizona Department of Health Services, Pima County, and the University of Arizona Veterinary Science Department.
These trapping events occurred between May and October of each year and were associated with either routine arbovirus surveillance or nonsystematic Ae. aegypti "spot-checks." Many of the sites were sampled more than once and 30% contained Ae. aegypti on multiple occasions. Catches from individual sites contained 1 to 40 adult mosquitoes (mode = 1).
A larval survey of the city has not yet been conducted because of limited staff and problems associated with oviposition trapping, an important part of Ae. aegypti larval surveillance (2). Initial attempts at ovitrapping by state, county (1), and later university personnel were not successful because the hay infusion water in ovitraps evaporated rapidly in Tucson's arid climate. However, the few larvae recovered from household containers suggest that Tucson's urban environment is providing a breeding habitat. Since the local climate requires full-time vigilance of ovitraps, this method of surveillance appears too labor intensive for the present.
In 1997, the University of Arizona Entomology Department initiated a mosquito survey of the city. This multiyear project is funded by the Entomology Department, the city of Tucson, and the Pima County Health Department. Neighborhood associations in Tucson were surveyed for their perceptions of the magnitude of the mosquito problem in their areas. On the basis of survey results, the city was divided into regions: north, east, south, west, and central. Four trapping stations were established in each region for a total of 20 sites spanning the metropolitan Tucson and outlying areas. The five regions were surveyed for mosquitoes through use of CO2 traps approximately every 10 days starting July 1, 1997; traps were set in the late afternoon and collected in the late morning. Daytime CO2 trappings were not effective.
The Entomology Department's 1997 surveillance data suggest that the central part of the city is the most heavily infested. Of 95 adult Ae. aegypti trapped, 49.5% were from the central region of Tucson, 18.9% from the west side, 17.9% from the east side, 10.5% from the north side, and 3.2% from the south side. The mosquito populations appeared to fluctuate with the weather, increasing in size after rainfall. Long-term trapping and future larval surveys should shed more light on this association.
Pima County and the University of Arizona Veterinary Science Department trapping activities in 1997 also produced evidence of Ae. aegypti in two communities near Tucson. Six adult mosquitoes were recovered in the town of Benson, 30 miles southeast of Tucson, and seven were trapped in Sahuarita Heights, 15 miles south of Tucson. The presence of the mosquitoes in these communities, as well as in Douglas, Naco, and Nogales, demonstrates that Arizona's smaller desert communities are also susceptible to Ae. aegypti infestations. The humidity emitted by home evaporative coolers may be crucial for the survival of tropical mosquitoes, such as Ae. aegypti, in Arizona's arid climate (N. Monteny, pers. comm.).
Genetic analysis of the Ae. aegypti collected from southeastern Arizona, Texas, and Mexico is under way at the University of Arizona Ecology and Evolutionary Biology Department to determine the structure, history, and origin(s) of the reemergent mosquito populations. Preliminary findings from mitochondrial DNA sequences suggest that Ae. aegypti in Arizona represent a single (panmictic) population, which indicates frequent local migration. More extensive sampling is necessary to confirm these results and determine a point of origin.
A community outreach program has been developed to inform the public about Ae. aegypti breeding and control in Tucson. Public involvement will be a key factor in the control of these urban breeders. Major emphasis will also be placed on programs for children and teachers as both groups can be instrumental in maintaining long-term interest in this problem. As these programs are developed, they can be expanded and amended to meet the needs of other infested communities in southern Arizona. A mosquito control abatement district is under consideration in a central part of Tucson. The primary purpose of this district would be to provide approximately 10,000 homeowners with information on controlling Ae. aegypti breeding on their property.
Just how long the Ae. aegypti infestation will last is difficult to assess. Records of the city's earlier infestation indicate the mosquito was present for at least a 15-year period (1931 to 1946) (1,3,4). Since their identification in early 1998 summer mosquito samples from Tucson, adult Ae. aegypti have been part of the city's local environment for at least 5 consecutive years (1994 to 1998). Their continued presence and the abundant breeding habitat provided by the expansion of Tucson's urban landscape suggest that Ae. aegypti could survive for an extended period.
- Engelthaler DE, Fink TM, Levy CE, Leslie MJ. The reemergence of Aedes aegypti in Arizona. Emerg Infect Dis. 1997;3:241–2.
- Reiter P, Amador MA, Colon N. Enhancement of the CDC ovitrap with hay infusions for daily monitoring of Aedes aegypti populations. J Am Mosq Control Assoc. 1991;7:52–5.
- Murphy D. Collection records of some Arizona mosquitoes. Entomol News. 1953;14:233–8.
- Bequaert J. Aedes aegypti, the yellow fever mosquito, in Arizona. Bulletin of the Brooklyn Entomological Society. 1946;41:157.