The Current State of Vector Control
Popular vector control methods such as long lasting insecticidal treated nets (LLINs) and indoor residual spraying (IRS) have halved the prevalence of malaria in sub-Saharan Africa over the last 15 years. While these types of vector control have been highly effective in preventing millions of cases of malaria, they are only suitable against endophagic (feeds indoors), endophilic (rests indoors) and anthropophilic (feeds predominantly on humans) mosquitoes. Endophagic mosquito behaviour has even started to adapt to human sleeping habits. Female mosquitoes are starting to host-seek earlier in the evening, when many people are working or relaxing outside, and before the majority of people have gone to bed. This change, coupled with the emergence of insecticide resistance leads researchers to continually rethink the vector control strategies we utilise. We often do this by improving on the methods we currently use, as well as designing novel chemistries and control methods that can target both in and outdoor host-seeking mosquitoes.
A whole series of new vector control methods have been proposed, ranging from sterile/genetically modified males, to toxic sugar baits and targeting the larval stage. While these techniques have plenty of potential, they come up against various obstacles that prevent distribution throughout affected areas; mainly the cost, but also public distrust in these techniques.
Although not used widely in the tropics, spatial insecticides (SIs) are a potentially inexpensive and safe method of reducing the burden of malaria by deterring outdoor host-seeking mosquitoes from entering homes or outdoor communal areas. SIs work by using volatile chemicals that diffuse through the air to repel or kill mosquitoes.
Many existing SI products use pyrethroids, such as transfluthrin and metofluthrin, which both become volatile at ambient temperatures in Africa, Asia and South America. Others use less volatile chemicals such as allethrin. However, even at low doses most SIs can cause deterrence, feeding inhibition and reduced egg laying.
Currently, most volatile insecticides are produced as coils or candles, which both require an active heat source to produce the vapour effect essential to deter host-seeking females. Although these methods have been shown to reduce the number of malaria cases, they are impractical, short-lasting, produce smoke plumes and can be expensive. There is also evidence to suggest that they often do not kill mosquitoes, which may increase the risk to other unprotected people/houses in the community. The counter argument is that unprotected people/houses may have some level of protection due to the diffusion of the volatile insecticides in the local area.
Evidently, there are both pros and cons to the use of mosquito coils, candles or any other active product. However, a more passive method, with a similar or greater impact than active methods, would be more desirable to users. This would require a more cost-effective method to release the volatiles that would need little to no energy, and less upkeep by those in the community.
The Promising Future of Spatial Insecticides
Research into SIs and their potential is ongoing. Indeed, my PhD is focusing on SIs and looking at new ways that we can assess the effects of new volatile chemistries/products on mosquito behaviour and longevity, alongside a range of potential sub-lethal effects. Other studies are concentrating on designing simple low-technology SI products such as emanators.
A recent study found that hessian strips treated with transfluthrin prevented three-quarters of outdoor bites by Anopheles arabiensis, Culex spp. and Mansonia spp., and over 90% protection on warmer nights when the evaporation rate was higher, for at least 1 year. They also discovered that non-users in the community saw reduced biting at a distance of 5m from the strips for Anopheles, and they found no evidence that non-users were at an increased risk of bites for up to 80m. This study suggests that low cost hessian strips may be able to provide long-term protection against some of the most dangerous mosquito species. One of the most important things to take away from this study is that these strips work outdoors, where LLINs and IRS are impractical.
Volatile insecticides are also useful in the fight against other vector-borne diseases such as dengue, transmitted by Aedes aegypti mosquitoes. A study found that female Aedes exposed to just 5% metofluthrin formulations were inhibited from biting due to rapid knockdown and mortality (80-90% in just 1 hour). They also found that mosquitoes became disorientated, stopped landing on hosts and stopped seeking resting sites after just a few minutes of exposure.
More Work to be Done
Used in conjunction with LLINs and IRS, SIs could be highly successful in reducing biting rates, and thus disease risk in endemic regions. Research is showing the potential for SIs to be exceptionally useful in the fight against various vector-borne diseases. Presently, resistance to transfluthrin and metofluthrin is limited and this is something that we can capitalise on, whilst being cautious that we limit the risk of resistance. Ultimately, our understanding of mosquito behaviour in the presence of SIs is limited and necessitates expansion.