Dengue and Climate

Dengue viruses are transmitted by Aedes mosquitoes, which are highly sensitive to environmental conditions. Temperature, precipitation, and humidity are critical to mosquito survival, reproduction, and development and can influence mosquito presence and abundance. Additionally, higher temperatures reduce the time required for the virus to replicate and disseminate in the mosquito. This process, referred to as the “extrinsic incubation period”, must occur before the virus can reach the mosquitoes salivary glands and be transmitted to humans. If the mosquito becomes infectious faster because temperatures are warmer, it has a greater chance of infecting a human before it dies.

Although environmental factors are important they are not the only factors critical to dengue transmission. Virus must be present, there must be sufficient numbers of humans still susceptible, or non-immune, to the virus, and there must be contact between those susceptible humans and the mosquito vectors. As recently as the 1940s, large dengue outbreaks were documented in the United States reaching places as far north as Boston. Today, the situation has changed significantly. Reasonable climate, competent mosquito vectors, and susceptible human hosts are all still present in the continental United States, and dengue viruses are frequently reintroduced by infected travelers. Transmission in the U.S. is rare, however, because there is insufficient contact between infected humans, vector mosquito species, and susceptible humans to sustain transmission. Studies on the US-Mexico border, for example, suggest that the restriction of transmission there is due to the limitation of contact between human hosts and mosquito vectors that comes with low housing density and the use of air conditioning and screens.

In countries where transmission does routinely occur, short-term changes in weather, particularly temperature, precipitation, and humidity, are often correlated with dengue incidence. These associations, however, do not describe the occurrence every few years of major epidemics in these areas, suggesting that long-term climate variability does not regulate long-term patterns in transmission. A more important regulator of epidemics might be the interplay of the four different dengue serotypes. The level of prior exposure of a human population to each of the dengue serotypes may be a more critical determinant of whether a large epidemic occurs than climatic cycles.

Globally, the reported incidence of dengue has been increasing. Although climate may play a role in changing dengue incidence and distribution, it is one of many factors; given its poor correlation with historical changes in incidence, its role may be minor. Other important factors potentially contributing to global changes in dengue incidence and distribution include population growth, urbanization, lack of sanitation, increased long-distance travel, ineffective mosquito control, and increased reporting capacity.

Some CDC references for further reading: