Malaria affects over 200 million people every year, mostly in sub-Saharan Africa. The 2018 WHO World Malaria Report showed that in 2017, almost half of the world’s population were at risk of malaria and nearly half a million people died from the disease, the majority of deaths being in children under the age of five. The malaria parasite is spread through the bites of infected female mosquitoes, mostly of the species Anopheles gambiae, as they take their blood meals. Malaria is preventable and curable. However, whilst campaigns based on indoor spraying and the distribution of bed nets have made great progress, these approaches are reaching the limits of their efficacy, and even in areas where nets are distributed free, the use of nets is not universal, nor are they always in good repair.
The most effective way to monitor the disease is to monitor populations of An. gambiae mosquitoes. However, current tools for sampling mosquitoes are time-consuming and labour intensive, making them expensive and difficult to standardise. Data between countries and regions cannot be reliably compared and interpreted. Furthermore, the current WHO-approved method for outdoor mosquito surveillance, the Human Landing Catch, is performed by trained field technicians collecting mosquitoes that land on their exposed leg. This exposes collectors to malaria and to other mosquito-borne diseases such as dengue fever, giving rise to serious ethical issues.
What is NRI's solution?
NRI has carried out work on modelling the populations of different vector species and how they are impacted by different approaches to their control to improve understanding of the variations that take place in vector distributions. This is important in improving the ability to predict when and where mosquito populations will be high. This knowledge allows the improved allocation of resources for vector control activities.
Building on this understanding of vector populations, the institute has undertaken research on the spatial movement patterns of mosquitoes throughout their lifetime. Quantifying the spatial population dynamics of mosquitoes from larval through to adult stages can provide crucial insights into the effectiveness of local intervention activities by identifying where individual mosquitoes have come from and how that influences their exposure to control methods and the likelihood of them spreading disease. As part of the institute’s work with collaborators in Sweden and Burkina Faso, the team demonstrated that whole-body bacterial flora of An. gambiae mosquitoes gave each mosquito a specific bacterial profile, providing precise information that showed how mosquitoes formed clear local populations with a close affinity to individual villages. This is very important for understanding the spread of malaria within and between villages.
To solve the problem of unethical and difficult-to-standardise surveillance methods for malaria mosquitoes, the institute has developed a mosquito trap that exploits the blood-seeking behaviour of mosquitoes by mimicking the sensory stimuli that a mosquito follows when searching for a person to bite. These include the look, smell and temperature of warm-blooded hosts. The team has incorporated these stimuli into a trap that lures mosquitoes towards it and then captures them when they land. So far, the trap has been trialled in Benin, Cameroon, Kenya, Burkina Faso and Indonesia, with successful results leading to planned trials in Ethiopia, Malawi, and Tanzania. The team was asked by the Asia-Pacific Malaria Elimination Network to train National Malaria Control Programme entomologists from Vietnam, Malaysia, Cambodia, Sri Lanka and Thailand in the use of the trap and support an operational research programme testing the trap across these countries.
Innovations and outcomes
The team used cutting-edge 3D tracking to directly observe mosquito flight towards humans in naturalistic twilight conditions. After quantifying their behavioural responses to humans and artificial stimuli, the team designed a simple trap that incorporates the cues to which the mosquitoes responded in the laboratory. Named the ‘Host Decoy Trap’ (HDT), it is risk-free for operators, standardised and reproducible and can be used outdoors, where mosquito surveillance has previously been difficult. Preliminary data shows the trap catches up to ten times more malaria mosquitoes than Human Landing Catches, and up to five times the number of mosquitoes from other genera, including those capable of transmitting lymphatic filariasis and dengue fever. These promising results indicate the HDT is a viable alternative to current sampling methods, and suggest a potential future role as an outdoor mosquito control tool.
The HDT is patented in the EU (16744842.2) and US (15/746,557). The trap design is under non-exclusive license with a German-based mosquito trap manufacturer, Biogents AG, internationally recognised for their participation in applied research and use of their existing products in national mosquito control programmes. While research and development are ongoing, a protype trap has been commercialised and sold. In addition, a publicly available protocol for making a ‘DIY’ version of the trap using locally available materials has been viewed over a hundred times, offering researchers or public health officials the ability to make traps for research use or local surveillance programmes.
In addition to work on An. gambiae, the institute is applying its expertise to Aedes aegypti, the species of mosquito responsible for transmitting Zika, dengue, and other deadly diseases such as yellow fever and chikungunya. Our team is investigating Ae. aegypti egg laying sites. Once an egg hatches it can take as little as 8–10 days from an egg to an adult, and this rapid reproduction makes understanding the choice of oviposition sites a key stage in vector control. Together with collaborators in Brazil, the team has been working on the chemical composition of the water that Ae. aegypti lay eggs in and on the water containers that they choose. This understanding will inform future development of targeted traps or repellents to reduce the incidence of people getting bitten by this mosquito and minimise the risk of being infected by dangerous diseases.