Given the proximity of the two cities, the difference in transmission rates cannot be attributed to climate. Moreover, the mean daily temperature for August, the peak month of transmission, was 32.2°C (mean maximum 40.0°C; mean minimum 24.4°C), several degrees higher than the mean for the hottest months on Caribbean islands where dengue is common. Indeed, summer temperatures throughout the range of Ae. aegypti in the southern United States are hotter than in many tropical regions where the disease is endemic.
Despite mosquito control campaigns on both sides of the border, Ae. aegypti
infestation rates in Laredo were remarkably high. The Breteau Index was on a par with that observed during major dengue epidemics in Puerto Rico (CDC, unpub. data). The House Index (the percentage of houses with at least one infested container) was 37%, seven times higher than the level (5%) equated with a “high risk” of dengue transmission by the World Health Organization (7
). Thus, vector populations cannot account for the low rate of transmission on the Texas side of the border.
is closely associated with human habitation and readily enters buildings to feed and to rest during periods of inactivity (8
). In this context, casual observation supported the association of lack of air-conditioning with dengue transmission. In Laredo, most shops, restaurants and other public places are air conditioned and have closed windows and self-closing doors, as do houses in residential areas, even in low income neighborhoods. By contrast, in Nuevo Laredo, many shops, bars, and restaurants are open to the street, and the windows and doors of houses are left open, particularly in the daytime. Thus, there is less opportunity for mosquito/human contact in Laredo than in the Mexican city.
More than 85% of all buildings in Texas are fully air conditioned (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA; available from: URL: http://www.ashrae.org/
). Indeed, air-conditioning is ubiquitous in many parts of the United States. To maximize heating/cooling efficiency, windows are usually fully glazed and are often kept permanently closed. Thus, most people spend much of their daily life sequestered in sealed buildings. Even if infected mosquitoes gain entry to such buildings, the artificially dry atmosphere lowers their survival rate, and the cool temperature extends the extrinsic incubation period, reducing the likelihood of transmission. Presumably, when denied access to humans, mosquitoes must seek other hosts. In Puerto Rico and Thailand, some Ae. aegypti
feed on dogs, even when humans are readily accessible (9
). In Laredo, we observed that large dogs were housed in outdoor kennels at many homes. Whether these animals are an important blood source for the species would be an interesting topic for future research.
The dollar cost of electricity is similar in Laredo and Nuevo Laredo, but income, as indicated by per capita gross domestic product, is much higher in Texas than in Taumalipas (). The proportional cost of maintaining air-conditioning for an entire dengue season is therefore much higher for the average family in Mexico and is unaffordable for the majority. Thus, the ultimate determinant of dengue prevalence in this setting is socioeconomic rather than environmental.
Estimated cost (US dollars) of air-conditioning a house in Texas vs. Taumalipas, Mexico