Increased Dengue Fever Incidence as a Result of El Niño Events and Higher Temperatures in Mexico

Increased Dengue fever (DF) transmission has been associated with higher temperatures and precipitation in tropical and subtropical regions. Due to climatic factors, in Mexico, DF is endemic year round. Colón-González et al. (2011) studied the effects of El Niño events, temperature, and precipitation on DF incidence in 12 Mexican provinces over a 23-year period. Multiple linear regression models were used to explore the relationship.  The incidence rate of infection was found to be significantly higher during El Niño events in the warm and wet season. Higher temperatures also had a significant effect, while precipitation was found to be not significant. This study complements previous findings on DF dynamics in the region and may be useful for the development of early warning systems.—Simone Berkovitz
Colón-González, F.,Lake, Bentham, G., 2011. Climate Variability and Dengue Fever in Warm and Humid Mexico. The American Journal of Tropical Medicine and Hygiene 5, 757–763.

            Dengue fever is a mosquito-borne infectious disease caused by the dengue virus. It is present in over 100 tropical and subtropical countries and approximately 50–100 mill­ion cases are reported each year. As a vector-borne disease, the influence of climate on transmission has been the subject of numerous studies. Due to Mexico’s climate, DF is endemic across the country and transmission occurs year round. Previous research in Mexico has shown that increases in temperature, sea surface temperature, precipitation, and the presence of El Niño events have been associated with an increase in DF. However, past studies have only looked at DF incidence in shorter time series and smaller geographical areas. Therefore, Colón-González et al. found it desirable to analyze 23 years ( 1985-2007) of reported DF cases across 12 Mexican provinces in order to examine the associations between temperature, precipitation, and El Niño events on DF incidence.
            The study utilized monthly DF notifications from 12 warm and humid provinces of Mexico, which encompassed 60% of all total cases in Mexico. In order to measure the strength of El Niño, monthly sea surface temperatures  (SST) were obtained. Monthly minimum and maximum temperatures and precipitation were obtained for each province. From the data it was found that year-round climate has two distinct seasons: cool and dry from November to May and warm and wet from June to October. The DF data were converted to a Cumulative Incidence Rate (CIR) based on the region population, which was than linearized by taking the natural logarithm (Ln-CIR). The authors used linear regression models, using the Ln-CIR as the dependent variable, which were separated by El Niño, non-El-Niño, warm and wet season, and cool and dry season.
            The authors found that in the presence of an El Niño event, the risk of DF infection is 2.77 times higher and a significant association between DF incidence and the strength of El Niño was found. The authors also found a significant relationship between increased temperature and DF incidence, which was still significant when the El Niño temperatures were removed. However, the effect of precipitation was found to be not statistically significant.
            The authors state that although the effects of El Niño were found to be significant, the reason behind the influence is not completely clear. Lower temperatures are associated with decreased transmission due to increased development time and larval mortality. Rising temperatures increase transmission by shortening the development time. In addition, feeding is increased because mosquitoes digest blood faster at higher temperatures. Human behavior may also play a role, as time spent indoors during the wet and warm season often leads to greater vector-host contact. Precipitation was not found to be significant because there is enough rainfall year round to create breeding sites. Human storage of water creates additional breeding sites independent of rainfall.
Global climate change, along with an increase of severe weather events, will increase the prevalence of Dengue fever, especially in warm and humid regions.  Climate change is likely to increase the warm and wet seasons, which will elevate disease transmission. These results can be used to predict future incidences in the region, which can provide opportunities to improve the control measures of the disease and strengthen population adaptation. 

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