An introduction to ecological challenges concerning the use of genetically-modified mosquitoes for disease control

Vector-borne diseases such as malaria and dengue constitute a major obstacle to socio-economic development in much of the tropics and remain high on the list of priorities for the improvement of public health. Sadly, financial as well as logistical constraints prevent a rapid amelioration of this situation (Reducing risks, promoting healthy life 2002). Unlike other infectious diseases, vector-borne diseases stand out because of their complex way of transmission, requiring passage from man to man or animal to man through an arthropod vector. This method of transmission implies the simple principle that removal of the vector will lead to the elimination of the disease. Indeed, the control of malaria, dengue, and other vector-borne diseases has relied – to a large extent – on vector control and, in certain areas (malaria in Western Europe, the Soviet Union and India, and dengue in the Americas), has been extremely successful. The introduction of synthetic insecticides (DDT, dieldrin a.o.) in the twentieth century created great optimism that vector-borne diseases could be controlled or even eradicated (Najera 1989). It is now realized that this optimism was unjustified. The main reasons were the development of insecticide resistance in the arthropod vectors, the sociological resistance to recurrent house spraying and a lack of political will to consolidate eradication efforts sufficiently well-funded for an adequate coverage of all disease-endemic regions (Greenwood and Mutabingwa 2002). Today the control of vector-borne diseases depends on a variety of methods including indoor spraying with insecticides, insecticide-treated bed nets, drainage and other means to eliminate mosquito-larval habitats, bioinsecticides, insect-growth regulators for larval control, and biological control (Curtis 1991). All such methods are combined with other health-control interventions such as drug treatment and vaccination. Although these methods help to reduce the disease burden by interruption of transmission, they do not remove or eradicate the pathogen and they leave the vectors to thrive in their natural habitats. It has proven increasingly difficult to eradicate an arthropod vector, and where successful control has been reported, this was often the result of temporary interruption of transmission to clear the human reservoir of the pathogen as has been the case for malaria. Hence we know of ‘anophelism without malaria’ in many countries around the Mediterranean and the continental USA (Bruce-Chwatt and De Zulueta 1980). In other areas, such temporary successes have led to a rebound effect, a.o. in India and Sri Lanka. A different approach to vector-borne disease control is the proposition of using genetic methods to either reduce the density of the vector population or replace competent vectors with genetically modified counterparts that have been made