Potential Impact of Integrating the Sterile Insect Technique into the Fight against Disease-Transmitting Mosquitoes

More than three thousand million people live with the risk of malaria. Due to the widespread resistance of mosquitoes to insecticides and of parasites to chemotherapies, previous gains made in disease reduction are being reversed. In addition to this perennial threat, there is now a rapid invasion of Aedes mosquitoes across the globe and the associated spread of the arboviruses (arthropod-borne viruses) they carry. One half of the worlds population is now at risk of dengue, and chikungunya (having emerged from Africa) is an increasing public-health problem in Asia and the Americas. The economic and social costs of these diseases is so great that, in some areas, they have slowed the development of nations. Current vector-control methods are inadequate (especially against container-breeding species) because they are losing their effectiveness, the global burden of mosquito-borne diseases is increasing, and no specific drugs or effective global vaccines are available to treat or prevent the diseases. Therefore, there is a need for additional suppression methods to be applied as part of Integrated Vector Management (IVM). Since the early 2000s, there has been a renewed interest in applying the sterile insect technique (SIT) against mosquito vectors of disease. The explosive outbreaks of the Zika virus (and associated birth defects) across the tropics increased the urgency. The recent availability of technology to rear and release the sterilized males of many mosquito species on a large scale has increased the expectation that the SIT could help reduce the suffering caused by mosquito-borne diseases. Much progress has been made in developing the SIT technology for mosquitoes, based on historic SIT efforts and the experiences gained in the successful large-scale application of the technique against agricultural pest species. The SIT is a suitable technology for suppressing mosquitoes because: (1) they can be mass-reared in a laboratory, (2) natural sexual dimorphism in many species aids sex separation, and (3) females become refractory after mating. There has been a perception that mosquitoes are more vulnerable than many pest species to damage during handling, sterilization, and release. However, technological and methodological improvements can cope with this lower robustness, and indeed take advantage of their smaller size and weight. Nevertheless, the need for perfect sex separation for male-only release to preclude any biting and disease transmission by released females, remains a technical bottleneck to scaling the SIT beyond small-scale pilot trials. As a remedy for this, genetic sexing strains are being developed. However, until they are available, combining the SIT with cytoplasmic incompatibility conferred by Wolbachia infection (incompatible insect technique (IIT)) has been proposed as an advantageous strategy. The advantage of including the IIT is that Wolbachia infection may prevent potential disease transmission by any released females, whereas sterilization guarantees that such females cannot reproduce, avoiding the loss of the cytoplasmic incompatibility due to Wolbachia establishment in the target population. Another advantage of simultaneous IIT use is that it enables the radiation dose to be minimized. Other challenges remain, particularly in release technology and quality control. Nevertheless, in recent years, pilot trials have been conducted or have been initiated, e.g. China, Germany, Greece, Italy, Mauritius, Mexico, Singapore, and Thailand, achieving encouraging results in suppressing adult populations of Aedes species. Area-wide releases, focused on urban and suburban settings, appear particularly promising in terms of sustainable and cost-effective IVM of Aedes vectors (eventually provided commercially by the private sector) because they can protect many people concentrated in relatively small areas. In the case of Anopheles vectors, the SIT may become a useful complementary tool, especially against outdoor-biting species which are not well-controlled by mosquito nets.