Study Shows That Worse Dengue Epidemics Could Occur After Mosquitoes Develop Resistance To Insecticides Used

Academic Journal
Main Category: Tropical Diseases
Also Included In: Infectious Diseases / Bacteria / Viruses
Article Date: 02 May 2011 - 16:00 PDT


A new modelling study published Online First and in an upcoming Lancet shows that worse dengue epidemics could occur following insecticide-based vector control programmes, due to decreasing herd immunity and increased insecticide-resistance. The work also shows that six high-efficacy adult vector control applications per year to control dengue would have a cost-effectiveness ratio that will probably meet WHO's standards for cost-effectiveness. The Article is by Dr Paula M Luz, Evandro Chagas Clinical Research Institute, Oswaldo Cruz Foundation, Rio de Janeiro, Brazil, and colleagues.

About 50 million dengue infections occur every year worldwide, with an estimated 2•5 billion people at risk of dengue. The incidence of dengue is increasing, especially in metropolitan areas, mainly because of growing vector mosquito and human population densities. Incidence of dengue is especially high in developing countries, where control relies mainly on insecticides targeted at larval or adult mosquitoes.

In this study, the researchers did a cost-effectiveness analysis of 43 insecticide-based vector control strategies over a five year period, including strategies targeted at adult and larval mosquitoes, at varying efficacies (high-efficacy [90% mortality], medium-efficacy [60% mortality], and low-efficacy [30% mortality]) and yearly application frequencies (one to six applications). The modelling showed that all interventions caused the emergence of insecticide resistance, which, with the loss of herd immunity, can increase the magnitude of future dengue epidemics. It also showed that one or more applications of high-efficacy larval control reduced dengue burden for up to 2 years, whereas three or more applications of high-efficacy adult vector control reduced dengue burden for up to 4 years. The incremental cost-effectiveness ratios of the strategies for two high-efficacy adult vector control applications per year (over five years) was US$615 per disability-adjusted life year (DALY) saved and for six high-efficacy adult vector control applications per year was $1267 per DALY saved. Even this higher amount falls within WHO's cut-off point for cost-effectiveness*.

The authors say the work draws attention to the negative effect on herd immunity of reduced transmission and increased vector control. Since viral reintroduction can occur from other locations, dengue elimination is unlikely and more intense outbreaks could occur. They conclude: "Our model shows that when the effect of insecticide-based interventions wane because of the evolution of insecticide resistance, pronounced epidemics will occur, the severity of which depends on the specific intervention used. These findings draw attention to the importance of considering insecticide resistance and dengue transmission dynamics when implementing insecticide-based dengue vector control programmes in a specific setting."

In a linked Comment, Dr Eduardo Massad and Dr Francisco Antonio Bezerra Coutinho, University of São Paulo, Brazil, note that the findings by Luz and colleagues refer to use of high-efficacy adulticides, not the low-efficacy adulticides used in most dengue-control programmes. They conclude: "Practical hurdles with the use of adulticides have so far rendered this method to be a control strategy with limited effectiveness.To achieve the results obtained with the high-efficacy adult-control strategies proposed by Luz and colleagues, new insecticides and better methods of application are urgently needed."

*The authors defined Brazil-specific thresholds of $24 660 per DALY saved for a cost-effective intervention, and of $8 220 per DALY saved for a very cost-effective intervention, on the basis of the criteria of the WHO Commission on Macroeconomics and Health.

Link to article and comment

Source
The Lancet

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