Working Group II: Impacts, Adaptation and Vulnerability

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9.7.2. Dengue

Dengue is a disease that is caused by four closely related viruses that are maintained in a human-Aedes aegypti-human cycle in most urban centers of the tropics (Gubler, 1997). The geographic distribution of the dengue viruses and mosquito vectors (Aedes aegypti and A albopictus) has expanded to the point that dengue has become a major tropical urban health problem (Gubler, 1997, 1998b). Dengue is primarily an urban disease; more than half of the world's population lives in areas of risk (Gubler, 1997, 1998b). In tropical areas of the world, dengue transmission occurs year-round but has a seasonal peak in most countries during months with high rainfall and humidity. Major factors causing epidemics include population growth, rapid urbanization, lack of effective mosquito control, and movement of new dengue virus strains and serotypes between countries (Gubler, 1997, 1998b).

The global resurgence of dengue in recent years has resulted in increased imported dengue and cases of local transmission in the United States and Australia. As with malaria, the number of cases is small and sporadic (Gubler, 1989, 1997, 1998b). By contrast, Mexican states bordering the United States have had repeated large epidemics of dengue (Gubler, 1989, 1998b; Reiter, 1997; Rawlins et al., 1998). The difference in vulnerability may be caused by differences in living standards and human behavior, which in the United States decrease the probability that vector mosquitoes will feed on humans. It is unlikely that climate change will affect these factors and cause increased epidemic dengue activity in temperate zone developed countries. Modeling the Impact of Climate Change on Dengue

To date, all published studies regarding evaluations of the possible impact of global climate change on dengue transmission have involved modification of the standard equation for vectorial capacity (VC) (Jetten and Focks, 1997; Martens et al., 1997; Patz et al., 1998a). Temperature affects the rate of mosquito larval development, adult survival, vector size, and gonotrophic cycle, as well as the EIP of the virus in the vector (Focks et al., 1993a,b, 1995).

Modeling studies (Jetten and Focks, 1997; Martens et al., 1997; Patz et al., 1998a) suggest that a warming projection of 2°C by 2100 will result in a net increase in the potential latitudinal and altitudinal range of dengue and an increase in duration of the transmission season in temperate locations. However, they also ignore the complex epidemiological and ecological factors that influence transmission of dynamics of dengue. Changes in potential transmission in areas that currently are endemic for dengue are projected to be limited. As with malaria, models indicate that the areas of largest change of potential transmission intensity as a result of temperature rise are places where mosquitoes already occur but where development of the virus is limited by temperature during part of the year. However, these models do not incorporate demographic, societal, and public health factors that have been responsible for eliminating dengue from temperate areas. Transmission intensity in tropical endemic countries is limited primarily by herd immunity, not temperature; therefore, projected temperature increases are not likely to affect transmission significantly. Moreover, in subtropical developed areas, where transmission is limited primarily by demographic and societal factors, it is unlikely that the anticipated temperature rise would affect endemicity (Gubler, 1998b).

Table 9-3: Temperature thresholds of pathogens and vectors. Tmin is minimum temperature required for disease transmission. Tmax for the pathogen is upper threshold beyond which temperatures are lethal. Tmax for vectors are not provided. Temperatures are in degrees Celsius. Note that temperatures assume optimum humidity; vector survival decreases rapidly as dryness increases. There is considerable variation in these thresholds within and between species (Purnell, 1966; Pfluger, 1980; Curto de Casas and Carcavallo, 1984; Molineaux, 1988; Rueda et al., 1990).
Disease Pathogen Tmin Tmax Vector Tmin for Vector
Malaria Plasmodium falciparum 16-19 33-39 Anopheles 8-10 (biological activity)
Malaria Plasmodium vivax 14.5-15 33-39 Anopheles 8-10 (biological activity)
Chagas'disease Trypanosoma cruzi 18 38 Triatomine bugs 2-6 (survival)
20 (biological activity)
Schistosomiasis Cercaria 14.2 >37 Snails
(Bulinus and others)
5 (biological activity)
25±2 (optimum range)
Dengue fever Dengue virus 11.9 not known Aedes 6-10
Lyme disease Borrelia burdorferi Not yet determined Ixodes ticks 5-8

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