By Dr Leo Braack and Dr Htin Kyaw Thu
The history of malaria vector control is a fascinating subject, the stuff of riveting books and many lessons that are to be learnt, previous experiences and lessons that we forget and then “re-discover” after much time and opportunity has been lost. It is interesting to note that ever since Ronald Ross discovered in 1897 that malaria parasites are transmitted by mosquitoes, the primary strategy for malaria control has in fact been vector control, more than a century of relying primarily on mosquito control as our main weapon to reduce malaria. At first the world responded to the discovery of Ronald Ross by draining ditches, swamps and other breeding areas as best possible, or adding oil and “Paris Green” to water to kill larvae, and also protecting household occupants by screening windows with mosquito mesh. It made a massive difference, among other things enabling the building of the Panama Canal, which had been aborted in a previous attempt because multiple thousands of labourers were dying of malaria and yellow fever. Then came World War II, and with it the discovery of DDT as an extraordinarily effective insecticide, plus the discovery of Resochin (Chloroquine). This combination of DDT and chloroquine so fired up the imagination of public health officials that the WHO embarked on the Global Malaria Eradication Programme (GMEP) in 1955, pouring massive resources into this ambitious agenda, which did actually achieve much good in clearing malaria from substantial parts of North America and Europe. But then resistance set in within mosquitoes and parasites against these “Silver Bullet” tools, and it led to the GMEP being abandoned in 1969, in favour of a more realistic malaria control objective rather than eradication.
Post-1969, after a few decades of trying to keep a lid on malaria as best possible, new developments such as the arrival of new insecticides, drugs, and Insecticide Treated Bednets, gave rise to renewed talk of a global onslaught against malaria once more. It led to the WHO Roll Back Malaria initiative in 1998, and a full-on acceptance of global malaria eradication after the Bill & Melinda Gates Forum meeting of 2007. Billions of insecticide-treated bednets made their way into every corner of the malaria-endemic world, making impressive gains in reducing malaria, until a deadlock was reached around 2015, and since then the malaria fight has pretty much stagnated. The magic of Long-Lasting Insecticidal bedNets and Indoor Residual Spraying had more or less reached their limit, and insecticide resistance together with outdoor biting, with other contributory factors, was causing a frustrating morass of residual malaria that conventional approaches and chemicals could not breach, with notable exceptions of a few countries resolutely soldiering their way through to malaria elimination.
Where are we in Asia Pacific?
Asia Pacific has made huge advances in reducing malaria, truly remarkable achievements that deserve many accolades. But this region too sits with residual malaria over large areas, specifically the Greater Mekong Subregion, which is the smouldering focus of drug resistance. Several countries are within a hairsbreadth of achieving malaria elimination, following the examples of Sri Lanka and China. But in these settings it is no longer a simple matter of throwing bednets at the problem or spraying the insides of houses, it needs a more nuanced and specifically targeted approach. Malaria has been driven into corners and pockets, much of it in remote rural areas where infection occurs from mosquitoes biting people who go into forests or rubber plantations and spend nights there, unprotected by bednets, or by migrant people crossing remote forested areas not within easy reach of health workers. In these situations it is no longer a matter of mainly vector control, it requires a strong inter-disciplinary approach that involves excellent surveillance capacity to quickly detect positive malaria cases and identify new or persisting foci of infection, which then must be investigated to understand which groups of people are being infected and where these infections are taking place and treated. Often it is Forest-Goers who may be involved in illegal forest activities, reluctant to inform case investigators where they have been and where they may have picked up infections, and even self-treat but only partially and so become infective sources that constantly infect mosquitoes with parasites wherever they go, setting up foci of infection in the forest and in local villages. These foci have to be detected, the people need to be identified and effectively treated, and the people they were living or working with must be investigated to see if they are also carriers of infective parasites. But, importantly, communities in these rural areas need to be effectively engaged, so they can understand and actively participate in the needs of the malaria endgame. It ensures compliance with basic malaria reduction measures. Ensuring that people back in the villages are supplied with bednets and that the bednets are being used and are still effective. Mosquitoes have to be collected and tested to see if insecticide resistance may have developed. You may ask, why check bednets if much of the remaining residual transmission is due to outdoor biting, or early biting when people are not protected by bednets? Because the bednets still provide the overwhelming majority of protection. Remove these bednets and malaria will resurge with a vengeance, potentially creating a massive and rapid return of the rampant infection.
So our battle against residual malaria has become more of an intensified, better integrated, and focused strategy, requiring effective surveillance, vector control, parasite control, case management and community participation, all contributing to finding and smothering foci of infection. In theory at least. It is easier said than done in reality, because of the remote locations where much of this takes place. But entomologists play a critical role in all of this. This is becoming a problem, as in recent years it has become clear that entomological capacity has diminished, with an older generation of experienced entomologists retiring, and not enough younger entomologists coming up to fill those vacant positions.
Why do we need entomologists?
Given that malaria control has relied largely on vector control for more than a century, it is essential that this expertise is retained. Entomologists are the people who identify which species of mosquitoes predominate in a particular area, which species are the most important to control and focus on, and understand where these mosquitoes feed (indoors or outdoors), where they rest (indoors, outdoors), where they breed. All these factors and understanding are necessary to understand which control tools are the most appropriate to use and how effective they are likely to be or may require supplementary actions. Very importantly, is also the entomologists who need to monitor how effective the bednets or insecticidal spray remain, given that mosquitoes develop resistance to particular classes of insecticide and the chemicals are no longer effective to kill the mosquitoes. The entomologists must then make the recommendations about what alternative products should be used, and how they should be used, and where they should be used. Entomologists too are generally the people who devise and administer the vector control programs, meaning where to focus control efforts given limited budgets, how intensive, what coverage, the human resources required, transport and all the details to make the vector control program effective. Remember, vector control is the number one strategy for malaria control in most countries, and you better get your vector control that right otherwise you will have outbreaks of the disease, with people dying. And then we are only talking about malaria. Medical entomologists are also responsible for vector control against dengue, zika, chikungunya, leishmaniasis, filariasis, and others, which involve different mosquitoes or flies, or even ticks for some diseases such as Crimean Congo haemorrhagic fever.
But it is not just having entomologists appointed on staff. Many medical entomologists graduate from university with impressive degrees and lots of excellent and necessary theory in their heads, but they have not acquired the experience and skills of particular vector control techniques that are necessary to implement and maintain vector control programs. And so there is a need for capacity building training courses to ensure these new medical entomologists have the practical skills necessary to plan and run insectaries, do the insecticide resistance tests, and many other tasks and skills required for effective vector surveillance and control.
It has become clear that in Asia Pacific there is a general shortage of entomologists and entomological skills. We need these people not just to implement vector-borne disease control programs, we also need them as critically important researchers, to undertake the necessary scientific studies that underpin control programs. These are the people that help to develop new insecticides, test these new products, develop new control tools and methods, advance our understanding of vector biology and disease epidemiology.
Why is there a shortage of medical entomologists?
This is an interesting question. It seems that much of the answer relates to job incentives and work fulfilment. Career advancement opportunities are often poor in vector-borne disease departments where senior positions may be very few and so people lower down the hierarchy are stuck in junior positions for very long with little prospect for better-paid and more challenging managerial positions. Lower down the rung, training is sometimes inadequate for optimal performance of jobs which leads to a lack of job fulfilment and job satisfaction.
Medical entomologists have made major contributions in reducing the burden of many vector-borne diseases over the last century. They include plague (transmitted by fleas), typhus (transmitted by lice), onchocerciasis (also known as River Blindness, transmitted by small blood-sucking flies), filariasis (also known as elephantiasis, transmitted by certain mosquitoes), sleeping sickness (transmitted by tsetse flies), and major contributions in the fight against malaria. That fight against vector-borne disease is not anywhere near over. Look at dengue, as only one of several examples. Dengue is on a steep trend of increase globally and will be our next major mosquito-borne public health challenge, and not an easy challenge to overcome. Closely associated are Zika, and chikungunya, and several others, such as yellow fever, West Nile virus, and several more.
Let us not neglect our medical entomologists, but instead encourage them with incentives and creating opportunities for them to have effective and satisfying careers.