The mosquito invasion
You could call it a perfect storm. The wet start to the summer, followed by a hot August, has created ideal conditions for the irritating pests
Friday 10 August 2007
One of the pleasures of an English summer evening is being able to sit in the garden with a bottle of rosé and a bowl of olives and listen to the swifts as they wheel screeching about the houses at sunset. Not any more. This summer a new sound is set to ruin the idyllic scene. The shoulder-hunching whine of a million mosquitoes is heading our way.
In a normal year, England is relatively insect-free in summer. One of the advantages of holidaying within these shores is that there is no need to pack the mosquito nets, wasp killer or sting remedies that are de rigeur for travel abroad.
But this year is not a normal year. Met Office forecasts of a heatwave in August to follow the exceptionally wet start to the summer come with a bite. Scientists are predicting that weather conditions in the UK could be "extremely conducive" to the spread of the most unwelcome of guests - the parasite-infested mosquito.
From Norfolk to north Wales and London to south Devon, holidaymakers have begun to itch, scratch and complain about the insects. A quiet evening fishing beside a country stream has become a battle with clouds of biting mozzies.
Unusually high rainfall during May, June and July has left the country strewn with pools of still, stagnant water that are ideal breeding grounds for mosquito larvae. Long, hot days and warm, humid nights over the next month, if the forecast proves accurate, will encourage the movement of the hatched eggs, of which there could be millions more than is standard in the UK.
Professor Chris Curtis of the London School of Hygiene and Tropical Medicine said: "Mosquitoes need still water by day and warm air by night. We've had a lot of rain this year and, as temperatures rise the infectious parasites carried by mosquitoes will thrive". He added that sustained high temperatures over August made it "very likely" that mosquito numbers will be "markedly higher than they've been for years".
Paul Pearce-Kelly, senior curator of invertebrates at the Zoological Society of London, agreed. "It's the combination of wet weather followed by warmer conditions that encourages the greater numbers, which we're already seeing," he said. "Changing weather patterns are creating conditions more favourable to mosquito breeding. A combination of climate change, bringing with it milder winters, and increased travel - not just of humans but of cargo too - is producing an environment in which mosquitoes succeed. As a general principle, increased travel means increased risk: things can just turn up."
Mosquitoes are not hatched carrying malaria or any other parasitic disease, but can become carriers if they bite an infected human. The blood sucking insects transmit the parasite to the next victim they bite. There are known to be at least 33 different species of mosquito in the UK, but no clear estimates as to the total number of insects. Of those in the UK, the most prevalent is the Culex pipiens, which does not carry malaria. But the female Anopheles plumbeus, which does, can still be found in many parts of England, and especially in the north Kent marshes.
Scientists have warned for more than a decade that the effects of climate change and global warming could trigger a resurgence of disease in Britain. Mr Pearce-Kelly added: "There is no question that with climate change species which are not native to this country could take a toehold, and become established here." At least 2,000 Britons return each year from trips to the tropics infected with malaria, and there are fears that, if they are bitten by mosquitoes native to the UK on their return, the disease could become established in the low-lying salt marsh districts of the South-east.
Indeed, it was the return of British citizens already infected with the disease that sparked the country's last outbreak of malaria almost a century ago. In 1918, soldiers returning to England from serving abroad in the Great War prompted a mini-epidemic in the green and pleasant Kent countryside. Coming home after being diagnosed with worrying symptoms in Thessaloniki, northern Greece, the troops returned to army barracks in Cliffe, near Rochester, to recuperate.
In fact, Kent was possibly the worst place in the country to which they could have been transferred. Ronald Ross, an English academic who in 1902 had been awarded the Nobel Prize for Medicine for demonstrating the role of mosquitoes in the spread of malaria, was the British Army doctor treating the servicemen. When he spotted malarial symptoms amongst a small minority of those who were ill, he recommended they be sent back to the UK to get better. He did, however, make explicit the need for them to avoid regions where the Anopheles atroparvus mosquito - capable of transmitting Plasmoduem vivax, a milder version of the malarial parasite - thrived.
Unfortunately the north Kent marshes were - and still are - home to a large population of the Anopheles atroparvus, and the soldiers' return sparked a minor epidemic in the area. Over the next year, 500 civilians living close to the Hoo peninsula were diagnosed with malaria, although there were no fatalities.
Thanks to vast improvements in public health standards over the past century, the likelihood of the disease spreading again to the UK remains small. But the threat has not gone away altogether. Professor Curtis insists that "higher temperatures may bring other species of mosquito to the UK and, with it, other dangerous parasites".
A particular concern is the Asian tiger mosquito, so-called because of the white stripes on its legs and thorax. This species "is very good at spreading to new territory" according to Professor Curtis. He sought to quell speculation of a sighting on Wednesday at Burnham-on-Sea, Somerset, pointing out that several species of mosquito are striped. Though it cannot carry malaria, the Asian tiger does transport Dengue fever, encephalitis and yellow fever.
Reports of an increase in mosquito numbers first surfaced in Norfolk in mid-July. Since then, flood-hit areas across England have witnessed a surge in numbers of the insect. The proliferation of patio heaters has added to the problem as mosquitoes are attracted to the carbon dioxide which they produce.
This week, attention switched to two sewage treatment centres in London, where large areas of semi-stagnant water, rich in nutrients, make an ideal breeding ground for mosquitoes.
The plague of insects has become so bad that some people living close to Beckton Sewage Treatment Centre, in the east London borough of Barking and Dagenham, are already sleeping under mosquito nets.
In Hounslow, more than 1,000 residents living close to the Mogden Sewage Treatment Centre are suing Thames Water over the insect menace. Neil Stockdale of Hugh James Solicitors, representing the residents, said: "Our case is Thames Water should have realised that it had to carry out works to tackle this problem as far back as 1988 but failed to do so."
Thames Water will later this month use a technique new to the UK in an attempt to prevent the insects breeding. The still-water sections of the centre will be flooded with polystyrene beads to create a surface layer 3cm thick, suffocating larvae already in the water and stopping any other mosquitoes from getting to it. The technique has proven successful in trials conducted in Zanzibar and Tamil Nadu in southern India. Because the beads are inert, they pose no ecological threat and are considered preferable to insecticide.
Hounslow and Richmond-upon-Thames councils have distributed leaflets to residents living within a 2km radius of the Mogden Centre warning them of the problem and offering them advice on how to cope. But that is unlikely to satisfy those seeking compensation.
Guide to mosquitos
What attracts them
Only female mosquitoes bite, but both females and males detect their prey using the same three sensory mechanisms. Heat sensors direct the mosquito to those parts of the body with the greatest supply of blood. Veins that are nearest to the skin give off the most heat. Women tend to have slightly thinner skin than men, and are therefore more vulnerable. But how do the mosquitoes detect blood from afar? Chemical sensors attract them to the vapours of lactic acid and carbon dioxide that emanate from the skin, and they also have a taste for the bacteria found in sweat. It's best, therefore, to stay clean and dry. Patio-heaters, which also give off carbon dioxide, work like magnets to mosquitoes. Using their antennae to follow these vapour trails, the insects then use their eyes to home in on the victim.
The different types
Found in Asia, the US and southern Europe, it carries Dengue fever.
The most common UK species, it is widespread in London's Tube network.
Found in the UK, it prefers feeding on amphibians and reptiles to humans.
Capable of carrying malaria, it is found in the South-east.
Common in north Kent marshes, it carries malaria.
Found almost exclusively in Turkey, but could move to UK.
Found across the UK, this species is known as a salt marsh mosquito.
Led to outbreak of yellow fever in 19th century Swansea, carried by ships.
Found in trees and woodland on flood plains, common across the UK.
Fond of tree-holes. Known to breed in the UK.
Among the most widespread mosquitoes in UK.
Often seen in England, it freely attacks humans.
How to avoid them
Remove unnecessary water containers from the garden. Change birdbath water once a week and keep pools stocked with fish. Avoid patio heaters, mosquitoes are attracted to the carbon dioxide produced by them. Dress in long sleeves and trousers, don't spend too much time outdoors at dusk or dawn, and use insect repellent. If mosquitoes are troublesome at night, spray the bedroom with insecticide in the evening to kill any that may have entered during the day. When camping, use mosquito nets impregnated with an insecticide. If travelling to a country where malaria is endemic, see your GP or travel clinic. Anti-malarial products vary depending on where you are going. The Health Protection Agency and Chartered Institute for Environmental Health have an online 'Mosquito Watch' scheme at cieh-npap.org.uk/policy.asp
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