Date Published: February 17, 2009
Publisher: Public Library of Science
Author(s): Michael A. Johansson, Francesca Dominici, Gregory E. Glass, Eduardo Massad
Abstract: The four dengue viruses, the agents of dengue fever and dengue hemorrhagic fever in humans, are transmitted predominantly by the mosquito Aedes aegypti. The abundance and the transmission potential of Ae. aegypti are influenced by temperature and precipitation. While there is strong biological evidence for these effects, empirical studies of the relationship between climate and dengue incidence in human populations are potentially confounded by seasonal covariation and spatial heterogeneity. Using 20 years of data and a statistical approach to control for seasonality, we show a positive and statistically significant association between monthly changes in temperature and precipitation and monthly changes in dengue transmission in Puerto Rico. We also found that the strength of this association varies spatially, that this variation is associated with differences in local climate, and that this relationship is consistent with laboratory studies of the impacts of these factors on vector survival and viral replication. These results suggest the importance of temperature and precipitation in the transmission of dengue viruses and suggest a reason for their spatial heterogeneity. Thus, while dengue transmission may have a general system, its manifestation on a local scale may differ from global expectations.
Partial Text: The dengue viruses are the most widely distributed and damaging arthropod-borne viruses (arboviruses) affecting humans. The viruses and their predominant mosquito vector, Aedes aegypti, are endemic to most of the tropical and subtropical regions of the world, where they cause seasonal epidemics of varying size. The seasonal nature of transmission may reflect the influence of climate on the transmission cycle. Increases in temperature and precipitation can lead to increased Ae. aegypti abundance by increasing their development rate, decreasing the length of reproductive cycles, stimulating egg-hatching, and providing sites for egg deposition ,,,. Higher temperature further abets transmission by shortening the incubation period of the virus in the mosquito .
In the distributed lag model including temperature at lags of 0, 1, and 2 months and precipitation at lags of 1 and 2 months, monthly variation in temperature was positively associated with monthly variation in dengue incidence in most municipalities (Figure 1). The global association (average short-term association across all municipalities) was positive and statistically significant at all three lags. Short-term associations were significant for monthly maximum and minimum temperature but weaker than those observed for average monthly temperature. Monthly variation in cumulative precipitation was significantly associated with monthly variation in dengue incidence in some, but not all, municipalities at lags of 1 and 2 months (Figure 1). Globally, this association was significantly positive only after accounting for local effect modification by long-term climate. These findings were robust to the addition of further temperature and precipitation lags.
The associations between temperature, precipitation, and dengue transmission reported here are strong and consistent through time. Moreover, these associations depend on local characteristics and have a biological interpretation. Together these associations suggest an important relationship. It is critical, however, to consider the extent of the role which temperature and precipitation may play in increasing dengue incidence. The spline smooth in the current analysis reduces the extensive inter-annual variation in incidence observed in endemic areas like Puerto Rico such that the analysis effectively isolates association on finer, monthly, temporal scales. Thus, while we have reported a significant association between climate and dengue incidence, it is on a month-to-month time scale and does not show that warmer years (for example) consistently exhibit higher overall incidence. Though temperature and precipitation may also influence the magnitude of yearly transmission, this analysis does not demonstrate that. Studies on the relationship between multi-year climate variation and dengue incidence are inconsistent and at most account for only part of inter-annual variation in dengue transmission ,,,,,, (Puerto Rico manuscript in preparation; M.A.J., D.A.T. Cummings, & G.E.G.). Cogent alternative hypotheses suggest the importance of intrinsic factors related to the interactions of the four serotypes of dengue virus with human populations ,,,.