The University of Arizona

Background | CLIMAS

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Background

Since the first human cases of the mosquito-borne West Nile virus (WNV) were detected in the Southwest in 2003, more than 800 people in the region have tested positive for the disease, and at least 26 have died from it, according to state health records. Most people who become infected with the virus will have no symptoms. Others will develop mild symptoms such as headache, body aches, and fever. A small number of those infected, especially the elderly, may develop more severe symptoms, including high fever, severe headache, neck stiffness, and encephalitis or inflammation of the brain. Little is known about the distribution in Arizona of the mosquitoes (Culex tarsalis and Culex quinquefasciatus) that carry WNV. Even less is known about the climatic and environmental characteristics of their habitats.

In tropical locations, Aedes aegypti is the primary vector of dengue viruses and yellow fever. Dengue fever, which causes flu-like symptoms and occasionally death, is not circulating in the Southwest, but Ae. aegypti has become more abundant and has expanded its range in Arizona from Tucson west to Yuma and northwest to Tempe. The insects typically bite humans during the daytime, often around the ankles. Across the world’s tropics, it is estimated that tens of millions of people are annually infected and there are hundreds of thousands of cases of severe dengue hemorrhagic fever.

The life cycle of a mosquito is sensitive to variations and changes in weather and climate conditions. Ae. aegypti lays her eggs along the edge of containers such as discarded soda bottles, potted plants, or old tires that hold water. The eggs will only hatch into larvae when additional water is added to the container and the eggs are re-submerged. Extra water may be supplied by rain, a sprinkler, or other processes related to human activities. As a rule of thumb, a container must hold water for at least three days to support mosquito development.

Once hatched, the maturation of the larvae to a pupa and then an adult mosquito is dependent upon a variety of environmental factors, including temperature. Mosquitoes are cold blooded, meaning that their body temperature is similar to the surrounding environment. In the Southwest, high summer temperatures will shorten a mosquito’s life; prolonged below-freezing temperatures will kill the insect. In tandem with weather, a complex set of environmental and social characteristics will influence the abundance of a mosquito population in any given year and location.

The increasing presence in Arizona and New Mexico of mosquitoes that transmit human diseases has renewed government and public interest in controlling the insects.