Climate-Based Descriptive Models of Dengue Fever: The 2002 Epidemic in Colima, Mexico

By Chowell, Gerardo; Sanchez, Fabio | Journal of Environmental Health, June 2006 | Go to article overview

Climate-Based Descriptive Models of Dengue Fever: The 2002 Epidemic in Colima, Mexico


Chowell, Gerardo, Sanchez, Fabio, Journal of Environmental Health


Introduction

Dengue is the most significant mosquito-transmitted flavivirus-caused disease in tropical areas around the world, including southeast Asia, India, the Western Pacific, and South America (Morens, Folkers, & Fauci, 2004). It affects approximately 100 million people every year. The exact number of cases is unknown because a large number of cases involve few or no symptoms (Kurane & Takasaki, 2001).

Dengue is transmitted by at least two species of mosquitoes, namely Aedes aegypti and Aedes albopictus. Aedes aegypti, the principal vector, can lay 100 to 200 eggs at once. Female mosquitoes are responsible for the transmission of the virus since males feed primarily on plants and flowers (Scott et al., 1993). The lifetime of a mosquito is approximately 15 to 20 days on average. Most mosquito breeding sites are generated by humans (e.g., old toys, water containers, and tires).

Infected mosquitoes transmit the virus by biting a susceptible host. Four dengue serotypes (Den-1, Den-2, Den-3, and Den-4) coexist in the world (mostly in the tropics) (Gubler & Kuno, 1997). Individuals acquire permanent immunity to each strain that infects them, but there is no evidence of cross-immunity. In humans, the dengue virus produces flulike symptoms for up to 14 days. In severe cases of dengue (dengue hemorrhagic fever), the case fatality ratio ranges from 5 percent for treated cases to 15 percent for untreated cases (Gubler & Kuno, 1997).

Dengue incidence is becoming endemic in regions where outbreaks used to be sporadic. Hence, control of dengue requires an understanding of the mechanisms and factors that facilitate the invasion, transmission, and persistence of the virus in populations.

Different aspects of the transmission dynamics of dengue are known to depend on climatological conditions; those aspects include the survival and development of the vector Aedes aegypti (Jetten & Focks, 1997; Li, Lim, Han, & Fang, 1985; Mourya, Yadav, & Mishra, 2004). The extrinsic incubation period (EIP) and the susceptibility of the mosquito have been observed to depend on temperature (Mourya et al., 2004). Furthermore, seasonal variations in temperature and rainfall have been observed to be correlated with levels of dengue infection, with a higher number of dengue cases associated with higher rainfall and temperature, probably because of increases in mosquito breeding sites during the rainy season (Koopman et al., 1991; Schultz, 1993). A set of general-circulation models of global climate change have made the association between small temperature rises and higher risk of dengue epidemics (Patz, Martens, Focks, & Jetten, 1998).

Factors that facilitate the invasion of the causal agent of dengue are complex and not well understood. In some regions, recurrent epidemics of dengue have been observed, while in other regions only sporadic outbreaks have occurred. The latter situation is the case in the state of Colima, Mexico (Figure 1), where the mosquito population of Aedes aegypti is endemic (Espinoza-Gomez, Hernandez-Suarez, & Coll-Cardenas, 2001). A significant outbreak occurred in Colima in 1997 (4,910 cases), and the most recent outbreak occurred in 2002 (the outbreak on which this paper is focused), with a total of 2,379 cases confirmed in the laboratory (no cases were reported in 2001) (Espinoza-Gomez, Hernandez-Suarez, Rendon-Ramirez, Carrillo-Alvarez, & Flores-Gonzalez, 2003). Several explanations are possible for the re-emergence of dengue in regions where it had been absent for a prolonged period of time. Possible explanations include the immigration of people infected with a new strain of the virus to which the population is susceptible to, loss of immunity of the population through births and migration, and the invasion of a new strain of the virus from local natural reservoirs as a result of environmental changes.

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For this paper, the authors analyzed the correlation between dengue incidence during the 2002 outbreak in Colima, Mexico, and climatological variables. …

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