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United States Department of Agriculture

Agricultural Research Service

Title: Transmission Thresholds for Dengue in Terms of Aedes Aegypti Pupae Per Person As a Function of Temperature and Herd Immunity

Authors
item Focks, Dana
item Brenner, Richard
item Hayes, Jack - UNIVERSITY OF TEXAS
item Daniels, Eric

Submitted to: Journal of American Society for Tropical Medicine and Hygiene
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 29, 1999
Publication Date: N/A

Interpretive Summary: Dengue and Dengue Hemorrhagic Fever are serious viral diseases of humans living in the tropics. It is transmitted from person to person through the bite of Aedes aegypti mosquitoes which breed in and around houses in water-holding containers such as laundry sinks, drums, flower vases, and discarded tires and trash. Because there is no vaccine, attempting to control dengue has always involved insecticide sprays or clean-up campaign aimed at reducing the number of mosquito-producing containers. Eliminating the threat of this disease has always been expensive and difficult because no one knew where or when mosquito populations within a city were high enough to be dangerous. USDA-ARS Scientists working at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, Florida have used computer simulation models that they developed earlier to make estimates of just how many Ae. aegypti pupae are necessary to allow transmission. The results of this paper will help dengue and mosquito control personnel to determine the risk of dengue using the simple survey. Also, the results may help scientists predict the consequences of expected climate change.

Technical Abstract: The expense and ineffectiveness of insecticide aerosols to control dengue epidemics has led to suppression strategies based on eliminating larval breeding sites. With the notable but short-lived exceptions of Cuba and Singapore, these source reduction efforts have met with little documented success. The present work attempts to estimate transmission thresholds for dengue based on an easily-derived statistic, the standing crop of Aedes aegypti pupae per person in the environment. We have developed these thresholds for use in the assessment of risk of transmission and to provide targets for the actual degree of suppression required to prevent or eliminate transmission in source reduction programs. The notion of thresholds is based on two concepts: the mass action principal- the course of an epidemic is dependent on the rate of contact between susceptible hosts and infectious vectors, and threshold theory- the introduction of a few infectious individuals into a community of susceptibles will not give rise to an outbreak unless the density of vectors exceeds a certain critical level. We use validated transmission models to estimate thresholds as a function of levels of pre-existing antibody levels in human populations, ambient air temperatures, and size and frequency of viral introduction. From a control perspective, the results are not particularly encouraging. The ratio of Ae. aegypti pupae to human density has been observed to range between 0.4 and >60 in dengue-endemic or dengue- susceptible areas. However, threshold levels range, for example, between ca. 0.5 and 1.5 for ambient air temperatures of 28oC and initial seroprevalences of 0 to 67%. Surprisingly, the size of the viral introduction was not seen to be especially significant.

Last Modified: 8/30/2014
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