Mosquitos are the perfect vectors for disease. They are superbly adapted to finding and feeding on us, are incredibly resilient, and can carry viruses and parasites deadly to us without being the slightest bit affected themselves. Their numbers also dwarf the human population -there are more than fourteen thousand mosquitos per person in the world.
One species alone, Aedes aegypti, is the main spreader of Dengue fever, Yellow Fever, Zika and Chikungunya. Most of the time, we may think of mosquitos as nothing more than a nuisance, but they kill more people every year than any other creature on the planet.
Dengue Fever is an unpleasant and potentially fatal disease whose symptoms involve fever, rash, aches and pains, and nausea. Australians don’t tend to worry much about dengue, and reported cases have become relatively rare and confined to Far North Queensland. Since the COVID-19 pandemic began in 2019, the disease has been pushed even further back in the consciousness of the general public.
However, Dengue is an increasingly urgent problem globally. Cases have increased thirty-fold since 1960, with 390 million people a year becoming infected and 40,000 eventually dying from the disease. Earlier this year The World Health Organisation (WHO) declared Dengue Fever to now be endemic in more than 100 countries, having increased by some 30 fold in the last 50 years. As is so often the case, climate change is exacerbating the problem. The geographic range and habitat of the Aedes mosquito — being the main spreader of the disease — is expanding as temperatures rise. Thus, finding an effective way of slowing the spread of dengue has become increasingly urgent.
Arming mosquitoes with Wolbachia
An initiative in Northern Queensland may provide our best chance to overcome the spread yet: using the mosquitos themselves to fight the disease.
The key to this strategy is a bacteria called Wolbachia, which prevents the Aedes mosquitoes from spreading Dengue. The bacteria exists naturally in more than half of all insect species, however not within Aedes. Years of research has been devoted to getting the bacteria into the mosquitos and their broader population.
The theory is that when the bacteria-modified mosquitoes are released, they would breed with wild mosquitos. The bacteria would then be passed down through multiple generations, establishing itself in the local population. As a result, eventually a majority population of Aedes mosquitos in the Northern Queensland area would be unable to spread Dengue.
Which is why, between 2011 and 2017, Northern Queenslanders have become accustomed to the initially bewildering sight of hundreds of thousands of mosquitos being released into the environment. Getting the local community on side has been key, given that they were being told the solution was more, not less, mosquitoes in the areas where they live and work .
Professor Craig Williams, an expert on mosquito-borne diseases at the University of South Australia, describes the consent of the surrounding community as “vital.”
“The World Mosquito Program, which has pioneered the use of Wolbachia symbionts for Dengue control, did extensive community engagement and consultation. That was almost as important as the science,” said Williams. Schools, community groups and local volunteers were instrumental in helping release the modified mosquitoes.
And the results have been promising; Dengue transmission has all but stopped in the region. For the first time in more than a century, Northern Queensland is essentially Dengue-free. Not only did the Wolbachia-infected mosquitoes remain in the population, but they drastically reduced Dengue transmission. In the areas where the strategy was implemented there was a 93 per cent reduction in Dengue cases. After Monash’s medical epidemiologist Scott O’Neill discovered Wolbachia’s resistance to Dengue, researchers in Brazil also reported similar results with Zika virus.
One advantage is that the bacteria is essentially on our side: it wants to become established in the population as quickly as possible. In order to spread more rapidly, Wolbachia stops the wild mosquitos from being able to become infected with the virus and pass it on to humans.
It’s also extremely cost and time effective. Wolbachia-infected mosquitoes come to dominate and remain in the population for years, with studies showing that even after 10 years most mosquitos still carry Wolbachia. Once they are released, the mosquitos do the rest.
Dengue remains a concern overseas
While Queensland has enjoyed incredible success, other countries are still struggling to contain the disease. Singapore is facing a dengue ‘emergency’ as its outbreak has reached 11,000 cases this year, almost doubling the numbers from last year. While Singaporeans can remain hopeful about the Wolbachia project, some experts suggest that Singapore’s high urban density means that the city may not experience the same success as Australia.
“You’ve got to flood the island with these mosquitoes, and people get annoyed,” National University of Singapore’s Professor of Medicine Paul Tambyah told Honi.
This could lead to residents killing the mosquitos before they get a chance to mate with and spread Wolbachia within a population. Even though the risk of disease transmission is significantly lower, the mosquitoes’ unpleasant bites could reduce public compliance with the program.
However, others including biotech company Oxitec, are advocating for a different strategy: the insertion of a specific gene into male mosquitos makes their offspring unviable.
Oxitec’s strategy is promising as it can be used to significantly reduce the population of many different species of mosquitoes without the introduction of additional mosquitos, thus addressing a wider range of diseases like malaria and Yellow Fever.
However, according to Oxford University’s Professor Luke Alphey, since Malaria can be spread via other mosquitoes in the Anopheles family, targeted genetic solutions are difficult to devise. Compared to Wolbachia, Oxitec’s solution is simply a means of population control. Therefore, if Oxitec’s male mosquitoes don’t reach their female counterparts, disease-bearing offspring will survive and may continue to transmit diseases.
Both of these strategies raise some interesting ethical considerations. Cost is a major issue for both methods and economic estimates can not be made until mass breeding of the mosquitos begins in large-scale factories. Cost is closely linked to issues of equity, considering prohibitive costs behind project design can lead to the elimination of disease in some countries but not in others.
Despite these concerns, scientists around the world are working hard to continually develop other alternatives to ensure an equitable and effective method of keeping disease-carrying mosquitos off humans. It is an exciting area of research which has already had a considerable promise at the regional level, whilst also showing the potential to impact a much larger population worldwide.