Aedes aegypti density and the risk of dengue-virus transmission

Using genetically modified mosquitoes to control vector-borne diseases will require specific, quantitative targets for the extent to which populations of competent mosquito vectors need to be reduced in order to produce predictable public-health outcomes. Unfortunately, dengue researchers do not have an entomological measure for predicting the risk of human dengue infection and disease that is as effective as they would like. The situation is further complicated by the fact that contemporary dengue control is based on the assumption, which has not been thoroughly tested, that a reduction in adult Aedes aegypti population densities will decrease risk of virus transmission. Ae. aegypti eradication is not considered feasible and there are no commercially available dengue vaccines or clinical cures. Herein we discuss four interrelated questions that need to be addressed for the proper evaluation and implementation of genetically modified mosquitoes for dengue control. In specific terms, what is an acceptable level of dengue risk? What are the mosquito densities necessary to achieve that goal? What is the best way to measure entomological risk? Because most dengue risk factors are likely to exhibit spatial dependence, at what geographic scale are the components of dengue transmission important? We conclude with two recommendations for improving dengue surveillance and control. First, there is an urgent need for field-based prospective longitudinal cohort studies on the relationships among measures of Ae. aegypti density, dengue incidence, and severity of disease. Second, new rapid, inexpensive, and operationally amenable methodologies are needed to evaluate and monitor the impact of vector-control strategies on disease reduction. Unless competent mosquito vectors are eliminated entirely, predicting and evaluating success following release of genetically modified Ae. aegypti will require a more thorough understanding of the relationship between vector density and the risk of human disease