Reduction in dengue cases observed during mass control of Aedes (Stegomyia) in street catch basins in an endemic urban area in Colombia

Highlights

• The major breeding site of Aedes aegypti in Buga was the street catch basins.

• Aedes aegypti from Buga showed resistant to the insecticides used by public health.

• During the intervention Buga showed decrease in the number of dengue cases.

• Better entomological data collection and analysis improve vector control strategies.

Abstract

Dengue incidence continues to increase globally and, in the absence of an efficacious vaccine, prevention strategies are limited to vector control. It has been suggested that targeting the most productive breeding sites instead of all water-holding containers could be a cost-effective vector control strategy. We sought to identify and continuously control the most productive Aedes (Stegomyia) breeding site in an endemic urban area in Colombia and followed the subsequent incidence of dengue. In the urban area of Guadalajara de Buga, southwestern Colombia, potential breeding sites inside and outside houses were first characterized, and local personnel trained to assess their productivity based on the pupae/person index. Simultaneously, training and monitoring were implemented to improve the dengue case surveillance system. Entomological data and insecticide resistance studies were used to define the targeted intervention. Then, a quasi-experimental design was used to assess the efficacy of the intervention in terms of the positivity index of the targeted and non- targeted breeding sites, and the impact on dengue cases. Street catch basins (storm drains) were the potential breeding site most frequently found containing Aedes immature stages in the baseline (58.3% of 108). Due to the high resistance to temephos (0% mortality after 24 h), the intervention consisted of monthly application of pyriproxyfen in all the street catch basins (n = 4800). A significant decrease in catch basins positivity for Aedes larvae was observed after each monthly treatment (p < 0.001). Over the intervention period, a reduction in the dengue incidence in Buga was observed (rate ratio 0.19, 95% CI 0.12–0.30, p < 0.0001) after adjusting for autocorrelation and controlling with a neighboring town, Palmira, This study highlights the importance of street catch basins as Aedes breeding sites and suggests that their targeted control could help to decrease dengue transmission in such areas.

Introduction

Dengue viruses (DENV), transmitted to humans by Aedes aegypti, are responsible for dengue fever (DF) and severe dengue (SD). The number of infected people each year is unknown but has been estimated at 50–100 million new DF infections occur each year, causing ∼500,000 cases of SD and >20,000 deaths (Gubler, 2001, Gubler, 2002, Gubler, 2004), and possibly many more (Bath et al., 2013). The incidence of DF and SD is increasing due to the expansion of mosquito populations favored by uncontrolled urban development, inadequate vector control programs (Scott and Morrison, 2010), and global human mobility (WHO, 2006). No vaccine has yet been licensed (Wan et al., 2013),and hence, prevention programs rely on vector control and avoidance of mosquitoes bites. Moreover, vector control is expected to continue after a dengue vaccine is deployed to maximize its impact.

In Colombia, dengue vector control during epidemics focuses on the ultra-low volume (ULV) application of insecticides in houses around reported severe dengue cases, and on the streets of the most affected neighborhoods, together with community campaigns to promote the removal of unused containers in and around houses (Ministerio de la Protección Social, 2012). Routinely, periodic (3 to 4 times per year) entomological surveillance is carried out by sanitation technicians on randomly selected premises. The sanitation technicians record the presence of potential breeding sites and calculate Stegomyia indices. Positive containers that cannot be removed are treated with larvicides, mainly Temephos (Ministerio de la Protección Social, 2012). Although a national integrated vector control strategy is in place, dengue transmission has not decreased in the country. During the biggest outbreak in 2010, dengue incidences reached 666 cases per 100,000 inhabitants, 624 for DF and 41 for SD (SIVIGILA, 2013). The following years the morbidity rates for non-severe cases were 122.8 and 241.9 in 100,000 inhabitants for 2011 and 2012, respectively (SIVIGILA, 2013).

One limitation of the current dengue vector control strategies is that the continuity of the routine entomological surveillance and interventions can be disrupted by short term contracting of personnel. A second limitation is that vector control interventions are based on national guidelines rather than on local environmental and epidemiological characteristics. In order to make informed decisions and improve vector control at local level, it is necessary to have better data collection and analysis (Morrison et al., 2008). For example, most entomological surveillance programs do not distinguish the type of breeding sites and mosquito productivity, variables that are important in designing vector control strategies (Focks, 2003, WHO, 2006). Monitoring the production of late-instar larvae and pupae is putatively more informative than traditional Stegomyia indices but they are not yet routinely used (Alexander et al., 2006, Focks, 2003, Focks and Alexander, 2007, Focks et al., 2000). There is some evidence that identification and targeting the most productive breeding sites can be as effective in lowering entomological indices as targeting all water-holding containers, with lower implementation costs (Tun-Lin et al., 2009). Here, we describe a vector control strategy, its operationalization, the prioritization and control of breeding sites, and the subsequent entomological and epidemiological results, in an endemic town in southwestern Colombia.