Fighting fever

Known as ‘break bone disease,’ it causes very painful joints as well as a soaring temperature and there is no vaccine or drug for prevention or cure.

While one attack may not be too bad, a second may well develop into haemorrhagic fever, an illness life-threatening to vulnerable patients such as children and the elderly.

With 55 per cent of the world’s population at risk, there are more than 100 million cases each year and 25,000 deaths.

While found primarily in the tropical and sub-tropical regions of the world, travel, freight traffic and urbanisation have spread dengue to the southern states of the United States and warm climates such as Italy where the mosquito can survive.

In a climate of warm summers and cold winters, the insect will die once the temperature drops.

Oxitec, based at Milton Park, is the only company in the world that has developed genetically modified strains of Aedes that are sterile in that their offspring die quickly before maturity.

Released in a town or village affected by dengue, these strains will dramatically reduce the native Aedes population.

Chief executive Hadyn Parry outlined how the mosquito breeds and how the disease is spread.

“Unlike the malaria mosquito, which breeds in swamps or sewage, Aedes breeds in clean water. It lays its eggs in plant pots, old tyres and bird baths.

“We call it the urban mosquito because it lives only around people, not in the jungle or bush. The mosquito does not itself have the dengue, it is what we term a vector, a carrier. It bites one human who has contracted dengue, then infects others with further bites.”

Only females bite and unlike malarial mosquitoes, this mosquito is active in daylight.

Controlling the pest is difficult. Spraying a targeted chemical effectively means reaching all the breeding sites and a general insecticide will kill everything it touches.

Dengue has recently become a problem in the Florida Keys. The response was to instigate three helicopters spraying three times per week, but this did not solve the problem.

House-to-house surveys of gardens or yards, all much the same size, revealed innumerable breeding sites in each — the record was 147 in one yard.

Oxitec has been researching and developing its sterile mosquitoes and systems for ten years.

First came the technology, followed by the insect strains and then laboratory tests. Now the company is conducting live trials in locations across the globe.

Two types of mosquito are available. All are male, as the males do not bite. One type kills only females and the second, the bisex, kills both male and female.

Choosing which type to use depends essentially on who will use them. The bisex demands a well-funded and organised operation, but does eradicate more of the mosquito population.

The type which kills females only is simpler to use, but does leave males alive, although the males are harmless.

The trials are carried out in three stages. Stage one is to release a few thousand sterile males into a locality and observe how long they live, where they go and how well they compete against wild males when mating with females.

Stage two is to extend to a larger site such as a village, release mosquitoes each week, then monitor the population.

Trials have taken place in the Cayman Islands in the Caribbean where three million ‘sterile’ mosquitoes were released into the wild.

Over a ten-hectare area, the results showed an 80 per cent drop in the mosquito population after 80 days.

Each mosquito contains a fluorescent protein, plus two copies of the gene. Any offspring will contain one copy. A population census shows up the fluorescence on a parent and any offspring will show up red.

Part of the census involves ovitraps, traps for eggs. A small container is part filled with water and a wooden spatula, like an ice lolly stick, placed within it.

Mosquitoes lay their eggs on the spatula and the eggs are then harvested and hatched. By comparing the sterile to the unsterile newly born and an initial census, the drop in population can be determined. In addition, houses are vacuumed for insects and the numbers counted.

Stage three involves scaling up the operation to a larger site such as a town. In Brazil, for example, breeding has been transferred there instead of sending out egg batches every week as the remoteness of the site makes regular deliveries difficult.

Local staff are then trained in breeding, release and monitoring techniques.

Oxitec has now been given £8m of funding via Oxford Capital, of Cumnor Hill, Oxford which first backed the company in 2005, plus new investors from South America, Saudi Arabia and Asia.

Mr Parry said: “The aim is to broaden our shareholder base in those countries where dengue is prevalent.

“We need local partners to introduce our technology and link up with health authorities to conduct trials.

“We see our mosquitoes as part of a package of measures, a package which may involve spraying as an immediate palliative, followed by our insects to reduce the native population and keep numbers low.”