Winning the battle but not the war

Mosquitoes are notorious for being the world’s biggest killers – but how can we best prevent them from spreading disease? Could it be that we are going to eradicate them entirely? And what has bacterial infection of insect sex organs got to do with all this? We delve into the front line of vector control…

Known by a plethora of names, mosquitoes have existed for at least 210 million years. Their penchant to annoy humans and animals alike has lasted millennia – references to them have been found in Aristotle’s diary from 300BC.

But of course, their interactions with other species goes way beyond annoyance. Mosquitoes, specifically Aedes aegypti, carry some of the world’s most dangerous diseases ­– malaria, Zika, chikungunya as well as yellow and dengue fever. They infect more than 700m people annually, leaving behind in their wake a death toll of more than 1m. The World Health Organisation estimates one child dies as a result of malaria every 30 seconds.

Past attempts to control or reduce mosquito numbers have produced mixed results. Some successful – modifying natural habitats and reducing contact using mosquito screens or repellents, for example – some less so. Pesticides have shown limitations as some mosquito species have become resistant. Mosquitofish – who consume large amounts of mosquito larvae ­– have been introduced worldwide, but this has been to the detriment of other species. Two methods, however, have come to prominence over the last few years; use of the bacterium Wolbachia and genetic modification of mosquitoes.

Silent but deadly

Wolbachia is a genus of Gram-negative bacteria, which has infected, according to estimates, up to 70% of all insect species¹. First discovered in 1924, numerous strains have been discovered since, the species well known for infecting the sex organs of its hosts – in male mosquitoes, it renders them sterile, so females lay eggs that don’t hatch. And with traits like that, it is no surprise Wolbachia is a prime choice for the vector control community. MosquitoMate, for example, is an American company using the bacteria as a form of vector control for mosquitoes such as Aedes albopictus (Asian tiger mosquito) and Aedes aegypti.

 

From a vector control point of view, reasons for the continued existence of mosquitoes are few and far in between.

In the past, ionising radiation has been used to sterilise insects that transmit diseases, such as the sleeping sickness carrying Tsetse fly. But it is not a technique without flaws. Dr Jimmy Mains, principal investigator at MosquitoMate, explains further: “The use of Wolbachia allows us to implement a vector control strategy without the deficiencies commonly associated with radiated sterilisation techniques. Specifically, the Wolbachia infected males utilised in our approach are equally competitive for mates compared with male mosquitoes from the field. Reports show that irradiated males are inferior competitors, due to the impacts of the sterilisation process.”

Using Wolbachia ensures species specificity, requires no chemicals, and allows integration with alternative approaches to limit mosquito populations. At first, MosquitoMate targeted Aedes albopictus, says Dr Mains, because it was a major issue in Kentucky, where the company is located. After successful primary trials in Kentucky, subsequent studies were extended to California and Florida. MosquitoMate was founded in 2010, although Dr Stephen Dobson, co-founder, has been involved in research of Wolbachia for more than 20 years. Research into how the bacteria could affect A. albopictus and aegypti populations began in 2004.

Genetic warfare

Not every method for vector control involves biological warfare. Oxitec, an Oxford based company, genetically modifies male A. Aegypti that are then released into the wild to reproduce with female mosquitoes that again, produce no offspring. The company aims to reduce numbers of Aedes aegypti to a level where they can no longer transmit disease – just like MosquitoMate.

Dr Derric Nimmo, principal scientist at Oxitec, says: “The mosquitoes that Oxitec use have a self­-limiting gene, the tetracycline transactivator (tTAV) that passes onto their offspring, causing them to die before they reach adulthood. While in captivity, the mosquitoes are fed tetracycline, which switches off the tTAV gene, so they stay alive. However, when they are in the wild and there is no tetracycline available, the tTAV gene then produces the tTAV protein ­– when produced in large enough amounts, prevents essential gene expression, causing insect death.

The mosquitoes are tagged with another gene marker –DSRed2 ­– that produces a fluorescent protein, allowing the offspring of the mosquitoes to be identified under a special light after being collected from traps laid in an area of interest. This effectively allows us to track and trace our insects in the field.”

Work on creating a genetically modified mosquito began in the late 1990s before the first self-limiting mosquito strain was developed. Several years of laboratory testing ensued before the first release of the Oxitec mosquitoes in the Cayman Islands in 2009. Since then there have been more trials in both Panama and Brazil. Dr Nimmo says: “The results of these trials deemed the technology safe for wider use. In Brazil, we have a factory that is roughly 5000sqm and is able to produce up to 60m mosquitoes weekly. We are now conducting an operational project in Piracicaba, Brazil ,treating an area with a population of 65,000.”

Winning the battle but not the war

A. aegypti numbers are very hard to control. “They’re resistant to four of the six common insecticides and can breed in very small sites – I’ve seen them breed in water in a bottle cap for instance!” says Dr Nimmo. To give an idea of how much money, time and effort goes into battling mosquitoes, in Florida Keys, more than $1m is spent every year for a population no larger than 25,000. The Florida Keys Mosquito Control District comprises of three helicopters, four planes and roughly 10 staff members ­– even so, they only achieve a reduction of 30-50% year on year. Dr Mains explains: “Despite mosquito control operators’ best efforts; it’s often unmanageable to reduce densities below that which is needed to impact disease transmission with the current tool set. Other factors include insecticide resistance, globalisation or movement of mosquito vectors and the pathogens that they transmit and the increase in human densities in our cities.”

As keeping numbers of A. aegypti at low levels presents such a challenge, maybe complete eradication would be a more viable option. The species are not native to large parts of the world – in fact, only North East Africa can be regarded as their natural home. The ability of their eggs to survive desiccation for up to 12 months has helped them spread around the world. A. aegypti do not contribute hugely to the food web wherever they may be, either as predator or prey – in fact approaching things from a purely vector control point of view, reasons for their continued existence are few and far in between.

Dr Nimmo says: “For isolated sites such as islands, we can envisage eradication there. For countries such as Brazil, there are hotspots where dengue outbreaks are common. If you can treat these hotspots effectively, you can reduce the risk of disease transmission by A. aegypti significantly without having eradication.” Due to the mosquitoes’ non-nativity in large parts of the world, eradication could be a viable option, but according to Dr Mains: “It’s unclear whether there is the political motivation for a large scale elimination effort. At this time, MosquitoMate remains focused on localised use of Wolbachia as a pesticide to control medically important mosquitoes.”

MosquitoMate are currently awaiting certification by the US Environmental Protection Agency for commercial sales of their product this summer as well as an expansion of field trials, both in scale and the numbers of areas in the US. Oxitec is planning to begin trials in India and in Florida. Last November, a referendum took place in the Florida Keys, with 31 of 33 precincts voting in favour of the trials – making the the first referendum ever held on a genetically modified organism.

Both methods, whether through harnessing a microbial warrior or directly targeting genetics are both viable ways to control and maybe even eliminate mosquitoes. However, unless there is international agreements on how best to proceed with dealing with the threat mosquitoes pose; it’s unlikely that they will be eradicated any time soon.

References:

1. Kozek, Wieslaw J.; Rao, Ramakrishna U. (2007). "The Discovery of Wolbachia in Arthropods and Nematodes – A Historical Perspective". Issues in Infectious Diseases. Issues in Infectious Diseases. 5 (Wolbachia: A Bug’s Life in another Bug): 1–14. doi:10.1159/000104228. ISBN 3-8055-8180-7.