RESUMO
The expense and ineffectiveness of drift-based insecticide aerosols to control dengue epidemics has led to suppression strategies based on eliminating larval breeding sites. 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 2 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 susceptible individuals will not give rise to an outbreak unless the density of the 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 the size and frequency of viral introduction. Threshold levels were estimated to range between about 0.5 and 1.5 Ae. aegypti pupae per person for ambient air temperatures of 28 degrees C and initial seroprevalences ranging between 0 percent to 67 percent. Suprisingly, the size of the viral introduction used in these studies, ranging between 1 and 12 infectious individuals per year was not seen to significantly influence the magnitude of the threshold. From a control perspective, these results are not particularly 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-epidemic of dengue-susceptible areas in the Caribbean, Central America, and South East Asia. If, for purposes of illustration, we assume an initial seroprevalence of 33 percent, the degree of suppression required to essentially eliminate the possibility of summertime transmission in Puerto Rico, Honduras, and Bangkok, Thailand was estimated to range between 10 percent and 83 percent; however in Mexico and Trinidad, reductions of >90 percent would be required.(AU)