What are spatial repellents and how do they work?
Spatial repellent are chemical products designed to be 'active' (requiring heat or electricity) or 'passive' (requiring no heat or electricity) and release volatile chemicals into the air within the treated space. Product examples that are currently available include mosquito coils, candles among others; however, many more types of spatial repellent products are waiting to be developed.
Spatial repellents elicit ‘spatial repellency’ which refers to a range of insect behaviors induced by airborne chemicals that result in a reduction in human-mosquito contact. These behaviors include:
- movement away from a chemical stimulus,
- attraction-inhibition and/or,
- feeding inhibition
How are spatial repellents different than other chemical products such as toxicants and contact irritants?
In the most simplistic forms, repellents drive mosquitoes away from a treated space, toxicants kill mosquitoes and contact irritants cause agitation. Repellents can operate from a further distance than irritants and toxicants which rely on contact between the mosquito and a treated surface to function.
Many compounds exhibit two or more modes of action, but they can be distinguished by the concentration or dose needed to achieve them. Spatial repellency occurs at low vapor phase concentration, contact irritancy requires higher doses and killing requires absorption of still higher levels.
What is the role of spatial repellents in vector-borne diseases control and elimination?
Malaria, dengue and other mosquito-borne diseases are public health problems in many parts of the world. Malaria control strategies that have demonstrated success include treatment of infected individuals with drugs, application of insecticide to reduce mosquito populations through indoor residual spray, and reduction of human contact with infected mosquitoes via long-lasting insecticidal nets. Dengue prevention currently relies entirely on vector control and is focused on routine larval source management and reactive space spray or other adult-focused campaigns.
As goals of malaria elimination and eradication are put forth and the scope of dengue continues to grow, new tools will be needed to compliment the limited number of available interventions and/or further optimization of current products will be required to meet public health demands. The need for spatial repellents for vector control can be highlighted as they demonstrate effect against outdoor/day/early-evening biting mosquitoes – areas of transmission where traditional interventions are not completely effective. Spatial repellents have shown effect against insecticide resistant populations and may also have added benefit in areas where traditional long-lasting insecticidal nets or indoor residual spray interventions may not offer full protection or have reached their efficacy limits – especially in areas with residual transmission or in areas where elimination is considered.
Control of disease in these areas will require new approaches and may be where spatial repellency would be most effective. Spatial repellents may be offered as stand-alone tools where no other interventions are currently in use; or, most likely, combined with existing interventions to augment efficacy of these other tools (i.e., combination strategies).