Texas lifestyle limits transmission of dengue virus

Emerging Infectious Diseases, Jan, 2003 by Paul Reiter, Sarah Lathrop, Michel Bunning, Brad Biggerstaff, Daniel Singer, Tejpratap Tiwari, Laura Baber, Manuel Amador, Jaime Thirion, Jack Hayes, Calixto Seca, Jorge Mendez, Bernardo Ramirez, Jerome Robinson, Julie Rawlings, Vance Vorndam, Stephen Waterman, Duane Gubler, Gary Clark, Edward Hayes

Univariate analysis indicated a significant association between IgM seropositivity and five variables: absence of air-conditioning, fewer room air-conditioning units, the presence of an evaporative cooler, no travel outside the Laredo/Nuevo Laredo area, and shorter distances to neighboring houses (Table 3). IgG seropositivity was significantly associated with absence of central air-conditioning, fewer room air-conditioning units, smaller plot size, and a shorter distance to neighboring houses (Table 4).

On multivariate analysis, backward selection of variables yielded two that remained significantly associated with IgM seropositivity: absence of air-conditioning (odds ratio [OR] 2.6; CI 1.2 to 5.6) and no history of travel beyond Laredo/Nuevo Laredo in the previous 3 months (OR 2.0; CI 1.0 to 4.0). IgG seropositivity was associated with absence of air-conditioning (OR 2.4; CI 1.5 to 4.0), a history of crossing the border during the previous 3 months (OR 1.8; CI 1.1 to 2.8), and a greater number of occupants per household (OR 1.1; CI 1.0 to 1.2). By using the calculated prevalence ratio of 2.6 as an estimate of the relative risk of dengue in houses without air-conditioning, the proportion of dengue infections attributable to lack of air-conditioning in Nuevo Laredo was 55%, i.e., 55% of cases of dengue in Nuevo Laredo would not have occurred if all households in Nuevo Laredo had air-conditioning.

Discussion

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[degrees]C (mean maximum 40.0[degrees]C; mean minimum 24.4[degrees]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.

Ae. aegypti 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.