Transmission thresholds for dengue in terms of Aedes aegypti pupae per person with discussion of their utility in source reduction efforts

The expense and ineffectiveness of drift-based insecticide aerosols to cont rol 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 documen ted success; failure has chiefly been attributed to inadequate participatio n of the communities involved. The present work attempts to estimate transm ission thresholds for dengue based on an easily-derived statistic, the stan ding crop of Aedes aegypti pupae per person in the environment. We have dev eloped these thresholds for use in the assessment of risk of transmission a nd to provide targets for the actual degree of suppression required to prev ent or eliminate transmission in source reduction programs. The notion of t hresholds is based on 2 concepts: the mass action principal-the course of a n epidemic is dependent on the rate of contact between susceptible hosts an d infectious vectors, and threshold theory-the introduction of a few infect ious individuals into a community of susceptible individuals will not give rise to an outbreak unless the density of vectors exceeds a certain critica l level. We use validated transmission models to estimate thresholds as a f unction of levels of pre-existing antibody levels in human populations, amb ient air temperatures, and size and frequency of viral introduction. Thresh old levels were estimated to range between about 0.5 and 1.5 Ae. aegypti pu pae per person for ambient air temperatures of 28 degrees C and initial ser oprevalences ranging between 0% to 67%. Surprisingly, the size of the viral introduction used in these studies, ranging between 1 and 12 infectious in dividuals per year, was not seen to significantly influence the magnitude o f the threshold. From a control perspective, these results are not particul arly encouraging. The ratio of Ae. aegypti pupae to human density has been observed in limited field studies to range between 0.3 and >60 in 25 sites in dengue-endemic or dengue-susceptible areas in the Caribbean, Central Ame rica, and Southeast Asia. If, for purposes of illustration, we assume an in itial seroprevalence of 33%, the degree of suppression required to essentia lly eliminate the possibility of summertime transmission in Puerto Rico, Ho nduras, and Bangkok, Thailand was estimated to range between 10% and 83%; h owever in Mexico and Trinidad, reductions of >90% would be required. A clea rer picture of the actual magnitude of the reductions required to eliminate the threat of transmission is provided by the ratio of the observed standi ng crop of Ae. aegypti pupae per person and the threshold. For example, in a site in Mayaguez, Puerto Rico, the ratio of observed and threshold was 1. 7, meaning roughly that about 7 of every 17 breeding containers would have to be eliminated. For Reynosa, Mexico, with a ratio of approximately 10, 9 of every 10 containers would have to be eliminated. For sites in Trinidad w ith ratios averaging approximately 25, the elimination of 24 of every 25 wo uld be required. With the exceptions of Cuba and Singapore, no published re ports of sustained source reduction efforts have achieved anything near the se levels of reductions in breeding containers. Practical advice on the use of thresholds is provided for operational control projects.