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.

Research Methods

We used a spatial model, the Genetic Algorithm for Rule-set Production (GARP) to provide insight into the climatic and environmental characteristics of the habitats of mosquitoes that transmit WNV. The GARP model combines ecological niche theory and empirical field work, through a combination of artificial intelligence programming and statistical optimization, to inductively define a theoretical area where a species can sustain a population. For Ae. aegypti, this niche may be characterized by a variety of biophysical and societal variables. Different spatial and temporal patterns of these variables form the best predictive models for each mosquito species. We explored novel methods of spatially analyzing an ecological niche to further understand WNV transmission at different spatial and temporal extents. We derived a rule set describing mosquito presence, which was iteratively developed using a process of data sampling, rule selection, evaluation of the goodness of fit, preservation or rejection of a rule, and additional rule generation until there was minimal improvement in predictive accuracy.

Our dengue fever research used data on the presence or absence of Ae. aegypti at 68 sites in Tucson and Nogales, Arizona and Nogales, Sonora (Mexico) during the months of July, August, and September. We also used microclimatic variables and housing characteristics data.

We used multivariate logistic regression models (a variant of standard linear regression used when the dependent variable is a dichotomy, such as success/failure) to determine the relative importance of the independent variables to mosquito presence the week before mosquito collection and the four days of mosquito collection. We generated regression models for the months of July, August, and September. In addition to individually analyzing presence/absence of the mosquitoes in each month of each year, a Generalized Estimating Equation (GEE) was used to analyze presence/absence for each month across all years of data collection. A GEE is a covariance pattern model that directly models the correlation of repeated measures on each site.

Research Outcomes

Results of the WNV study show that physical properties, such as the composition of the soil profile and the depth to bedrock are important variables to define the fundamental niche of both Culex species. Cu. tarsalis preferentially breeds in perennially wet environments such as irrigated fields, wetlands, and lakes, and is most influenced by annual rainfall and maximum temperature; maximum temperatures are not detrimental or conducive to Cu. quinquefasciatus. Cu. quinquefasciatus exhibits a bimodal population cycle that is influenced by both late winter/spring and summer rainfall. The predicted fundamental niche of Cu. tarsalis is a viable representation of the maximum spatial extent of the mosquito. There is a clear effect of slope and both frontal and monsoonal rainfall on the spatial heterogeneity of viable mosquito habitat. The predicted Cu. quinquefasciatus fundamental niche appears to be conservative.

Preliminary results from our dengue fever investigations show a strong association between summer season vegetation and Ae. aegypti presence. In a subtropical location, microclimatic variables influence Ae. aegypti presence/absence in a manner that is consistent with mosquito physiology. Before the onset of the monsoon, increased maximum temperatures are detrimental to Ae. aegypti presence. Controlling for vegetation, relatively warmer and drier locations are favorable for Ae. aegypti during the monsoon. After the monsoon ends, greater relative humidity is conducive to Ae. aegypti presence. Differences in human ecology between the three study sites appear to exert an important control on Ae. aegypti presence/absence.

Related Publications

Uejio, C. 2007. Mosquito. In The Encyclopedia of Environment and Society, Paul Robbins (ed.). Sage Publications