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households were selected by a modified version of the cluster survey of the world health organization expanded program on immunization ( 4 ) .
first , we mapped the population of each census block in laredo and in a major portion of nuevo laredo ( sector 1 ) . in each city
, 30 clusters were chosen from these census blocks by using a selection probability proportional to population .
four city blocks were randomly chosen from each of these clusters , and individual houses in one or more of those blocks were selected at random ( where block maps were available ) or systematically from a randomly chosen starting point .
binational teams , each composed of an epidemiologist , a nurse , and an entomologist , conducted the surveys .
a blood sample was obtained by fingerstick from a randomly selected resident ( ages 1865 ) .
a short questionnaire solicited general household information ( number of inhabitants , type of construction , proximity to neighboring houses , number of bedrooms , presence and type of air - cooling system , and the presence and quality of window screens ) . demographic data and travel histories of the blood donors
serum samples were tested for anti - dengue immunoglobulin m ( igm ) by igm antibody - capture enzyme - linked immunosorbent assay ( mac - elisa ) , and for anti - dengue igg by igg - elisa and mixed dengue antigens ( 5,6 ) .
cary , nc ) and sas - callable sudaan ( research triangle institute , research triangle park , nc ) software .
risk factors for igm and igg seropositivity were assessed by multivariable weighted logistic regression , accounting for stratification by country , clustering within each city , and different numbers of surveys per cluster .
surveys were completed in 622 households ( 309 in laredo , 313 in nuevo laredo ) , and 516 persons ( 228 in laredo , 288 in nuevo laredo ) provided blood samples .
igm seropositivity ( table 1 ) was lower in laredo ( 1.3% ; 95% confidence interval [ ci ] 0 to 3% ) than in nuevo laredo ( 16% ; ci 12% to 20% ) .
igg seropositivity ( table 1 ) was also lower in laredo ( 23% ; ci 17% to 28% vs. 48% ; ci 41% to 55% ) .
conversely , mosquito - infested containers were more abundant on the texas side of the border : the breteau index ( the number of infested containers per 100 houses ) was 91 in laredo versus 37 in nuevo laredo .
eighty - two percent of homes in laredo had central or room air - conditioning versus 24% in nuevo laredo . in laredo ,
evaporative coolers ( a low - technology air - conditioning device that cools and humidifies air by drawing it from outdoors through a continually wetted screen ) were less prevalent , a greater proportion of houses had intact screens , the average distance between houses was greater , and fewer persons lived in each house ( table 2 ) .
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 .
given the proximity of the two cities , the difference in transmission rates can not be attributed to climate . moreover , the mean daily temperature for august , the peak month of transmission , was 32.2c ( mean maximum 40.0c ; mean minimum 24.4c ) , 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 .
the breteau index was on a par with that observed during major dengue epidemics in puerto rico ( cdc , unpub .
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 can not account for the low rate of transmission on the texas side of the border .
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 .
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 ,
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 .
aegypti feed on dogs , even when humans are readily accessible ( 9,10 ) . in laredo
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 ( table 5 ) .
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 .
it has frequently been stated that dengue , malaria , and other mosquito - borne diseases will become common in the united states as a result of global warming ( 1114 ) .
such predictions often refer to vectorial capacity , a simple model that incorporates the population density , biting frequency , and daily survival probability of the vector , and the extrinsic incubation period of the pathogen ( 15,16 ) .
although the vectorial capacity model has proved useful for interpreting entomo - epidemiologic data , particularly for transmission of malaria ( 17 ) , it does not incorporate factors like air - conditioning , use of vaporative coolers , and the behavior of mosquitoes and humans .
if the current warming trend in world climates continues , air - conditioning may become even more prevalent in the united states , in which case , the probability of dengue transmission is likely to decrease .
if the economy of mexico continues to grow , the use of air - conditioners may gain momentum south of the border .
urban dengue is common in most countries of the americas , but has been rare in the united states for more than half a century . in 1999 
 we investigated an outbreak of the disease that affected nuevo laredo , tamaulipas , mexico , and laredo , texas , united states , contiguous cities that straddle the international border . 
 the incidence of recent cases , indicated by immunoglobulin m antibody serosurvey , was higher in nuevo laredo , although the vector , aedes aegypti , was more abundant in laredo . 
 environmental factors that affect contact with mosquitoes , such as air - conditioning and human behavior , appear to account for this paradox . 
 we conclude that the low prevalence of dengue in the united states is primarily due to economic , rather than climatic , factors .