The conniving, malaria-transmitting mosquito has been deciding the fate of certain historical events for as long as we can remember.
The signature irritating whine of the tiny, bloodsucking insect has been tied to humanity’s struggle to prosper since the dawn of Homo sapiens thousands of years ago.
Humankind has made so much progress in bending nature to our will that we sometimes forget our own place in it. We have seemingly been fighting a losing battle throughout our existence.
With global attention now firmly on the Covid-19 pandemic, the fight against malaria, one would assume, has taken a back seat. However, scientists have still been at work and there seems to be light at the end of the tunnel.
Malaria is transmitted to humans and other animals by female Anopheles mosquitoes when they “bite” and feed on the blood, so protecting them could, in turn, protect people.
The disease is caused by a one-celled parasite called a Plasmodium, which is picked up by female Anopheles mosquitoes from infected people when they bite to obtain blood needed to nurture their eggs.
Mankind has been locked in a deadly struggle with this parasite that is estimated to have killed between 150 million and 300 million lives, accounting for two to five per cent of all deaths.
An estimated 40 per cent of the world’s population still resides in malaria endemic areas, making its threat to life, prosperity and good health as potent as ever.
The World Health Organization reports there were 228 million cases of malaria in 2018 and most of those infections were in sub-Saharan Africa. Of the more than 400,000 people who died during that period, most (67 per cent) were children under the age of five. In 2018, 19 countries in sub-Saharan Africa and India carried almost 85 per cent of the global malaria burden. Six countries accounted for more than half of all malaria cases worldwide: Nigeria (25 per cent), the Democratic Republic of the Congo (12 per cent), Uganda (five per cent), and Côte d’Ivoire, Mozambique and Niger (four per cent each).
The battle to control the parasite has been on since the times of the ancient Egyptian, Greek and Chinese empires, with artemisin being prescribed as a cure. Plant-based therapy has always been our mainstay in managing the disease.
A physician once wrote in 340 AD about the medicinal properties of artemisia annua, the sweet wormwood plant. “To reduce fever, take a handful of sweet wormwood, soak it in a sheng of water, squeeze out the juice and drink it all,” wrote Ge Hang.
And it has been a long journey from then until 2019 when the pilot phase of the RTS,S vaccine was launched in Kenya, Ghana and Malawi. The first of its kind.
Now, scientists have discovered that the Microsporidia MB microbe fully prevents mosquitoes from Plasmodium falciparum infection.
Last week’s report on the discovery of a microbe that stops mosquitoes from transmitting malaria has renewed hope that the disease, which annually kills more than 400,000 people globally, could one day be eradicated.
In what has been hailed as a significant milestone, a team of scientists in Kenya and the UK discovered a bug (microbe) in female Anopheles mosquitoes which can interrupt transmission of Plasmodium falciparum, the parasite which causes malaria.
Research published in the Nature Communications journal revealed that none of the examined mosquitoes around Lake Victoria in Kenya, which were found to be carrying a microbe called Microsporidia MB, had the malaria parasite. Microorganisms, or microbes, are microscopic organisms, which exist in a single-celled form or in a colony of cells.
Further lab experiments confirmed the microbe gave the mosquitoes protection, leading the team to assert that the finding has “enormous potential” to provide a safe, biological way of fighting malaria.
“This is a strong indication that current control measures are insufficient and additional novel strategies to control Anopheles mosquito populations or their capacity to transmit Plasmodium parasites are needed if we are to make further inroads in reducing malaria incidence,” the study notes.
Microsporidias are fungi, or at least closely related to them, and most are parasites. This bug lives in a mosquito’s gut and genitals, where it produces spores. It is found in five per cent of mosquitoes in a high-risk region around Kenya’s Lake Victoria, where the researchers focused their work, showing that this could be a strategy to control Anopheles mosquito populations or their ability to transmit Plasmodium parasites.
Scientists have been looking for naturally occurring microbes in mosquito populations in the hope of using them to eradicate malaria and other mosquito-borne diseases, such as chikungunya and dengue fever.
“The data we have so far suggests it is 100 per cent blockage, it’s a very severe blockage of malaria,” said Dr Jeremy Herren from the International Centre of Insect Physiology and Ecology. “It will come as quite a surprise. I think people will find that a real big breakthrough.”
Not all mosquitoes enjoy human blood. However, those that prefer humans to cattle are also much more likely to transmit malaria to humans. The 13-member team worked with a malaria mosquito called Anopheles arabiensis, which is known to readily bite cattle in the absence of humans. This mosquito also bites people outdoors.
Its ability to transmit malaria is, therefore, generally lower than that of other mosquitoes such as Anopheles gambiae or Anopheles funestus, which generally prefer humans to other animals and bite mainly indoors.
“In the Western parts of the country where there is a high burden of malaria, the Anopheles gambiae is more common compared to the arabiensis species that were studied. For this reason, the team will need to prove that what was found to be ideal in the lab can be effective in the field,” explained Eric Ochomo, a senior research officer and head of entomology section at the Kenya Medical Research Institute-Centre for Global Health Research.
These concerns will be foremost if scientists are to successfully immunise the mosquitoes that transmit the most malaria in Kenya
For several years, the team of scientists examined wild mosquitoes to identify gut microbes with the potential to block pathogen transmission.
They then artificially introduced the microbes into uninfected mosquitoes, making them immune to malaria infections. They also showed that these infections did not affect the natural survival of the mosquitoes.
“It is rare that we see so much rigour and so much beauty in mosquito research,” said Dr Fredros Okumu, a parasitologist and entomologist, who is the director of science at the Ifakara Health Institute in Tanzania.
WHAT FUTURE HOLDS
So, what does this finding mean to the fight against malaria? Does it mean that the pesky night prowler will quit hunting for its delicacy - human blood - even if it is not carrying malaria?
Dr Herren believes this might just be the first step. “I think it’s the first step towards developing a much-needed new tool in the fight against malaria,” he told HealthyNation.
But, he added that the method would only stop mosquitoes from transmitting malaria, not get rid of them. While huge progress has been made through the use of insecticide-treated bed nets and spraying homes with insecticide, this has stalled in recent years.
“Mosquitoes are very good at evolving resistance to all the ways we develop to try and kill them. This method won’t suffer from that limitation,” he said.
WHO has warned that the number of deaths caused by malaria in sub-Saharan Africa could double to 769,000 as efforts to tackle the disease face disruptions by the coronavirus pandemic.
If the technique works, it will address concerns about the insect’s resistance to insecticides commonly used for house spraying or on bed nets.
Malaria prevention in Kenya and elsewhere heavily relies on bed nets treated with pyrethroids, against which most mosquitoes are already resistant.
As the use of insecticides in public health and agriculture increases, so does the level of the resistance, significantly compromising efforts to fight malaria.
The new development could provide an excellent opportunity to go around this problem by creating what Dr Okumu referred to as “a complementary mosquito immunisation programme to be used alongside bed nets and house spraying”.
But, Dr Herren has a cautionary word for the prospects of the process.
“Although the finding is very promising, it will be difficult to predict clinical effects at this stage. We hope to develop a ’spreading’ strategy for the microbe and if successful with that, it will become a much-needed new tool to complement insecticide-treated nets and indoor residual spraying for vector control and, hopefully, the combination will be sufficient to eradicate malaria,” said Dr Herren.
Still, experts have welcomed the research. “The study by Dr Jeremy Herren and team was excellent and highly commendable. I am confident it can be done and the researchers involved have the required level of motivation to transform this field,” said Dr Okumu.
From a clinical perspective, however, Chris Odero, a clinician, said even if available on a wider scale, this discovery may not be the silver bullet.
“Previous studies have found a positive role of fungi in killing mosquitoes and these have yet to prove successful in real-world settings. It will still need to be used with other malaria preventive strategies,” said Mr Odero, who is also a policy, access and introduction technical advisor at PATH.
Dr Jesse Gitaka, a malariologist, said we were not out of the woods yet as far as the war against malaria was concerned. “We need concerted efforts and investment in methods like insecticide-treated bed nets, drugs, tests, public education and indoor residual spraying. Funds should also be committed for better research tools and resources as done by this group,” he said.
The researcher said several strategies were in place in the country to combat the disease and were still in use to date.
“Many strategies have been evaluated including the use of mass drug administration, mosquito population reducing gene drives, mosquito repellent technologies, insecticides-treated bed and ceiling nets, intermittent prophylactic treatment, vaccines among others. In use are insecticide-treated nets, indoor residual spraying and intermittent prophylactic treatment,” said the researcher.
Dr Gitaka, a lecturer at Mount Kenya University, said the research findings were an important step in the war to control and finally stop malaria transmission from mosquitoes to humans.
“The research team’s findings are significant and have potential to enhance the fight against malaria by reducing the effectiveness of mosquitoes to transmit malaria. However, more studies are needed to understand how this translates in the real world,” he said.
Dr Gitaka said there was also need to ensure that efforts to control the spread of malaria were maintained “especially during this Covid-19 pandemic” so as not to roll back the gains.