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 Specific research studies Entomology
Winning the war against malaria by beating its vector

We have won some significant victories against o­ne of human's greatest enemies, malaria, by targeting its vector. But are we doing enough, especially in Sub-Saharan Africa, to win the war?

In 2015, malaria caused an estimated 438,000 deaths in 95 countries. Almost half of the world's population is at risk of malaria, with most of them (88%) living in Sub-Saharan Africa (SSA), which is also where 90% of malaria deaths occur. Two countries alone, Nigeria and the Democratic Republic of the Congo, accounted for over 35% of these deaths. An estimated 97% of the population in Nigeria is at risk of malaria infection.
        Children, pregnant women and immunocompromised individuals have the highest malaria morbidity and mortality. The most vulnerable are children under 5; more than two thirds (70%) of all malaria deaths occur in this age group. In Nigeria, several studies found that malaria was the most common cause for death in children between o­ne and five years (South-West 1996-2000, South-South 2007-2008, South-South 2007-2011 and North-East 2012).

So, obviously it is paramount to beat malaria, and the o­nly way we have been able to do so in some countries in the past was by beating its vector, the Anopheles mosquito.

Previous victories

Malaria used to have a much larger geographical distribution than it has today. People in countries like Portugal, Spain, Greece and parts of the USA and Australia were all at high risk from malaria in 1946. The name malaria itself comes from Italy, where malaria infected up to two million, causing 15,000-20,000 deaths in the late 19th century. It was o­nly in 1981 that the World Health Organization (WHO) declared Australia malaria-free, following Yugoslavia in 1973 and the USA in 1970. Europe is the o­nly WHO region so far in which the  interruption of the indigenous transmission of malaria has been achieved in all its countries - in 2015!

What did all these country do to get rid of malaria? o­n o­ne hand, they improved diagnosis and treatment of malaria to eliminate the parasites in humans. o­n the other hand, through vector control interventions, they reduced the number of malaria carrying mosquitoes and also minimized human-mosquito contact.

Core vector control interventions

Vector control is considered a key intervention for global malaria eradication and has been in use for centuries. Currently, two large-scale vector control interventions are considered to be core by the WHO: indoor residual spraying and insecticide treated nets.

During indoor residual spraying (IRS), long-lasting residual insecticides are applied to surfaces in living areas to reduce mosquito density and longevity indoors. IRS led to "tremendous  accomplishments  in  malaria eradication  programmes in Europe, Asia and the Americas?" in the 1940s-1960s and also helped to reduce the incidence of malaria in Africa. Unfortunately, coverage of IRS has been declining between 2010 and 2014. In SSA, where IRS is more in use than elsewhere, coverage was o­nly 11% in 2010. This is problematic as coverage of IRS needs to be high for this method to be effective (usually over 85%).

Bednets reduce human-mosquito contact during the night. Several studies (Nigeria and Tanzania) have shown that a person who sleeps under a net is less likely to suffer from malaria. When used by pregnant women, bed nets improve pregnancy outcomes.The greatest protection is provided by insecticide treated nets (ITNs), such as long-lasting insecticide treated nets (LLINs).

Reasons to look further

Due to insecticide resistance and changing behaviours of the vector (Tanzania, Equatorial Guinea, Uganda and Benin), the effectiveness of IRS and LLINs are threatened. The development of new insecticides but also the use of supplementary vector control interventions and the development of new techniques should be a priority if we want to eradicate malaria.

One of the supplementary vector control interventions recommended by the WHO is larval source management (LSM). It aims at reducing the number of Anopheles mosquitoes by targeting potential breeding sites. Developed countries that were endemic for malaria concentrated o­n LSM for decades. It's such a successful intervention that, by 2011, all countries that successfully eliminated malaria had used LSM.

Due to the wide diversity in the classes and modes of action of different LSM methods, this is a solution to the above mentioned insecticide resistance. It can help to reduce the use of insecticides (thus slowing down the progress of resistance) and be employed where resistance has already emerged.

Despite its strong role in eliminating malaria elsewhere, LSM hasn't been rigorously employed in SSA as it's often regarded as unfeasible for the region; but this might just be a "myth". It has been argued that LSM can reduce malaria transmission in Africa and that its cost is comparable to other malaria interventions.

There are several other interventions that are being developed at the moment. An example is sterile insect technique (SIT). While it was difficult to produce enough sterile mosquitoes in the past, new technological developments are in the process of removing this barrier.

In the future, genetic vector control interventions should also become an option. Genetic modification is o­ne way of rendering mosquitoes sterile. It can also stop the transmission from mosquito to human while gene drive ensures that these genes will be passed to all offspring.

A technique that is being researched right now makes use of Wolbachia bacteria. This technique usually is mentioned in regard to dengue but current efforts are also directed towards malaria.

The current state of malaria vector control in Nigeria

The Nigeria Demographic and Health Survey 2013 showed that slightly over a third (36%) of people in participating households had had access to an ITN the night before the survey, and o­nly 12.9% had used their nets that night. IRS in 12 months prior to the survey was reported to have occurred in less than 2% of households; while LSM coverage hadn't even been included in the survey.

An increase of malaria vector control is promised by the Nigerian National Malaria Strategic Plan 2014-2020, which aims for the "transition from malaria control to malaria elimination in Nigeria". The plan includes a scale-up of IRS, universal coverage of LLINs and strategical use of LSM. Other potential interventions, such as SIT, GM and those using Wolbachia bacteria, are not discussed.

As you can see from the 2013 survey and the omission of current developments in the field of vector control in the 2014-2020 plan, Nigeria, along with much of SSA, still has a long way to go  ...


(Source: biomedcentral.com)  


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