They transmit disease to more than 700 million people and account for least 2 million deaths annually. The control of mosquitoes just got personal.
Mosquitoes have been responsible for more deaths worldwide than any other animal. They are the sole carriers of malaria, yellow fever, dengue fever, elephantiasis (lymphatic filiriasis) and chikungunya. There is no vaccine for any of these diseases.
For any disease, prevention is preferable to treatment. Vaccines for dengue and malaria are being developed but could be many years away. Effective mosquito control would decrease the burden of disease significantly and scientists have made huge advances in recent years.
Studies from Oxitec Ltd., a biotech company from Oxfordshire, have focussed on controlling the mosquito populations by genetically modifying the insects. Tactics to protect people in endemic areas include stopping mosquito bites using insecticides, net and repellents, developing preventative drugs and eradicating insects.
Controlling mosquitoes is no easy feat. Mosquitoes lay eggs in standing water – stagnant puddles, buckets and old tyres are prime breeding areas.
Insecticides are the best option for killing mosquitoes indoors. They are effective in the short term but mosquitoes build up a resistance after prolonged exposure. Resistance to insecticides can develop over a few generations, and since a mosquito’s lifespan is around 40 days, this can be within a year or two.
The unregulated use of pesticides for agriculture in areas where mosquitoes are endemic has been blamed for mosquito resilience spreading through populations.
Biocontrol is another method for controlling mosquitos. Natural predators are introduced to prey on mosquitos. Dragonfly naiads and the mosquitofish eat mosquito larvae. However, the predators may not be suited to the environment. Mass-breeding and release of the predators is often expensive or impossible. Although ecologically friendly, for these reasons biocontrol is not a viable scheme for large-scale mosquito control.
A more precise and efficient way to eliminate mosquitoes is needed. That is where Oxitec comes in.
Of the 3,500 mosquito species, Aedes aegypti transmits the majority of deadly diseases. The insect’s genome was sequenced in 2007. Now, the 1.4 billion pairs of letters that make up its genetic code for life might be about to bring its demise.
Research into the possibility of creating a genetically modified mosquito started as early as 1999. Reports from the BBC describe hopes that a database of malaria would help develop “mosquitoes that have been genetically-modified to make it harder for them to pass the malaria parasite on to humans”.
Now Oxitec has compiled the results from its first major field test in the Cayman Islands of the Caribbean. 18,000 GM mosquitoes were released over a period of four weeks in a small patch of jungle.
The GM mosquitoes are all male. The insects have been engineered to be infertile in a method known as sterile insect technique (SIT) (see diagram). Released into the environment, they mate with the native females and pass on their genetic fault to the offspring. The female lays eggs that develop into pupae but the faulty gene causes them to die before reaching adult stage.
Female mosquitoes only mate once in their lifetime. If they do so with the sterile males, the population will decrease.
The Cayman Islands mosquitoes were free to roam but this did not cause a problem for the researchers. “Mosquitoes only travel about 200 metres in their lifetime. They’re human-loving and tend to stay in and around humans,” saidHaydn Parry, chief executive of Oxitec.
The success of the trials is measured in two ways. Ovitraps consist of a jam jar with a panel and a layer of water inside. Mosquitoes lay their eggs around the water and on the panel. When the traps are collected, the larvae are hatched out and can be identified as being a GM offspring by engineered fluorescence from the male parent.
Adults are caught using a BG Sentinel, a device that attracts the mosquitoes using UV-light and draws them in by an air flow. Over time these two counting techniques show the ratio of male to female mosquitoes increases.
SIT past, present and future
SIT is not a new concept. Genetically modifying insects to control populations is not new either. The first GM animals were released into the environment in 2006, the New World screwworm.
This is the first time, however, that engineered mosquitoes have been let loose. The success of these trials could mean the eradication dengue, a disease that affects more people worldwide than any other.
The first SIT experiments targeted the New World screwworm, a parasitic fly that lays eggs in the tissue of warm-blooded animals such as livestock and humans. The experiments were conducted on the island of Curacao, off the coast of Venezuela in 1954.
By 1982, the US had eradicated the screwworm and the programme continued through Central America until the region was declared screwworm free in 2001.
In the screwworm SIT, insects were exposed to gamma radiation, which made them sterile. The optimal dose of radiation is crucial. Too much and the health of the male is jeopardised, which puts him at a disadvantage for mating. “You’d need an awful lot to compete with the healthy males,” says Parry. Too little and fertile insects are released, nullifying the experiment.
“Radiation in agriculture has produced a number of successful programmes, such as the screwworm and the Mediterranean fruit fly… The trouble is radiation failed to work on mosquitoes,” says Parry.
The GM approach is more precise and gets faster results. Targeting Aedes aegypti’s genome guarantees that the mutation is not transferred to other species. Unlike broad controls such as insecticides, GM mosquitoes will not affect the surrounding environment.
As with all genetically modified research, the impact on the environment is a discussion point. Eradicating an entire species of mosquito would have serious consequences for any animal that feeds exclusively on it.
Aedes aegypti has no such predators. The mosquito originated in Africa and only spread across the tropical world in the 1930s, so is not native in most countries.
Additionally, no trace of the GM is left in the ecosystem after the end of testing. The offspring of the GM mosquitoes are engineered not to reach adulthood and the males will die of their own accord.
The next step was to scale up the programme. In 2010, 3 million GM mosquitoes were released over a few months and a larger area in Brazil. Results have yet to be published but the prognosis is good. “We have reduced the Aedes aegypti population by about 80 percent,” says Parry of this new trial. “This is a hugely successful result.”