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

Abstract

Abstract. The expense and ineffectiveness of drift-based 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; failure has chiefly been attributed to inadequate participation of the communities involved. 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 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. Threshold levels were estimated to range between about 0.5 and 1.5 Ae. aegypti pupae per person for ambient air temperatures of 28ЊC and initial seroprevalences ranging between 0% to 67%. Surprisingly, 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 Ͼ60 in 25 sites in dengue-endemic or dengue-susceptible areas in the Caribbean, Central America, and Southeast Asia. If, for purposes of illustration, we assume an initial seroprevalence of 33%, 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% and 83%; however in Mexico and Trinidad, reductions of Ͼ90% would be required. A clearer picture of the actual magnitude of the reductions required to eliminate the threat of transmission is provided by the ratio of the observed standing 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 with ratios averaging approximately 25, the elimination of 24 of e...

Productividad de Aedes aegypti (L.) (Diptera: Culicidae) en viviendas y espacios públicos en una ciudad endémica para dengue en Colombia

“…En este sentido, y puesto que existe una gran correlación entre el conteo de pupas y los adultos emergidos que actúan como vectores (31), los valores de los índices por persona pueden indicar los umbrales de transmisión requeridos para alcanzar un incremento de 10 % o mayor en la seroprevalencia de los anticuerpos contra el dengue en el curso de un año (denominado año epidémico), teniendo en cuenta factores como la temperatura ambiente y la seroprevalencia de anticuerpos en la población humana (32). Según los datos de temperatura (27,9 °C en la época de lluvias y 30,4 °C en la época de sequía), y asumiendo una seroprevalencia de 67 % dada la condición endémica de la ciudad, los umbrales de transmisión serían de 1,38 y 0,35, respectivamente (10,32).…”

“…Según los datos de temperatura (27,9 °C en la época de lluvias y 30,4 °C en la época de sequía), y asumiendo una seroprevalencia de 67 % dada la condición endémica de la ciudad, los umbrales de transmisión serían de 1,38 y 0,35, respectivamente (10,32). De este modo, la transmisión del dengue en Girardot durante la época de lluvias permanecería baja, aunque no nula, con un índice por persona igual a 1, en tanto que en la época de sequía sería mayor, es decir, de 1,3.…”

High potential risk of dengue transmission during the hot-dry season in Nha Trang City, Vietnam

“…The observed number of pupae per person in both the survey periods was nearly the same; however, this number exceeded the disease transmission threshold only during the first survey conducted in July. When the seroprevalence of antibodies against the dengue virus and the rate of virus introduction are constant, the average monthly temperature influences the transmission threshold (Focks et al, 2000;Focks et al, 1993a). There was a large difference in the monthly average temperature between the 2 survey periods, resulting in a remarkable difference in the outbreak thresholds (0.38 in the first survey conducted in June and 2.32 in the second survey conducted in December).…”

“…aegypti pupae per person above which an outbreak of dengue presumably occurs) were estimated using simulation models developed to provide site-and weather-specific insights into the dynamics of dengue viruses and vectors (Focks et al, 1993a;Focks et al, 1993b). The transmission threshold was estimated as a function of the mean temperature, rate of virus introduction, and immunity of the target human population (Focks et al, 2000). The estimation 4 of the transmission threshold provides an overall target-an upper limit-of the number of pupae per person present in the environment that ensures that virus introductions would result in little or no transmission.…”

Evaluación de un método de barrido rápido sobre la superficie para estimar el número total de estados larvarios tardíos de Aedes aegypti (Diptera: Culicidae) en depósitos de agua de grandes capacidades: comparación con estimativos de pupas

“…The field study sites in Barranquilla have been described previously (4,6). In this study, all houses in three neighborhoods (Los Olivos I, Rebolo and Siape) (14 masl, average annual temperature 28 o C) were surveyed during the wet and dry seasons from December 2003 until August 2004 by 10 pairs of workers.…”