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Attacked by flies but what kind?

Also – what is the real threat behind the tiger mosquito as it spreads to France.

Your correspondents the Northrops ask about persecution by flies (letters Aug 2008). Unfortunately there is no picture or details of these 'flies'. As an entomologist myself I am well aware of flies during the warm months. But I'm also aware that true flies are not active at night. Yes there's often an irritating buzz around the bedroom just before I settle down to sleep. But as soon as the light goes off - silence. If I could have specimens of these 'flies' I could answer their questions.

I also noticed the sentence on page three concerning the asian tiger mosquito, Aedes albopictus. As is typical in the press the emphasis is on the chikungunya virus, showing an almost total ignorance of what is at stake here. Chikungunya is not usually life-threatening and it is usually treatable. You don't necessarily need A. albopictus to transmit it and the tiger mosquito can transmit malaria, dengue, yellow fever, you name it it's not particular.

A. albopictus has been transported around the world, in particular to the USA - for the moment, by the transport of old tyres. This insect can breed quite happily in the small amount of water which usually gathers inside old tyres left outdoors. If you have ever tried, you will find that it is practically impossible to empty the water which has collected in this way.

This mosquito is not gigantic, it never more than 10mm long, which compares with pretty much all native French mosquitos. It's black and white striped legs and abdomen make it relatively simple to identify although some North American species are similar - albeit sometimes larger!

We are still at the early stages of any infestation but this insect is potentially extremely dangerous, not because of chikungunya, but because of its behavioural traits in general.

I hate anthropomorphisms but nevertheless it can only be described as aggressive. Agressive because it is active throughout the day and not just crepuscular as are most mosquitos.

It is aggressive because, the males - which do not bite by the way - copulate forcibly with any species of mosquito they may encounter; thus preventing them from mating with their own species. Once an invasion has started the trend therefore is for the tiger mosquito to quickly become the dominant species of mosquito in that locality.

It is aggressive because it - a female that is - is happy to seek a blood meal from any warm-blooded animal it encounters. This is contrary to the behaviour of most other mosquitos. These usually have a preference for a particular bird or animal as host; which shows up in the anaesthetic they use to conceal the action of biting; it being most effective on the preferred host.

The consequences of this behaviour are people are going to be much more likely to be bitten in any area where A. albopictus becomes established and moreover there will be a hugely increased likelihood of transfer of animal diseases to humans.

Most human diseases came originally from other animals. HIV arose from a mutation of a simian immune difficiency virus coming from African monkeys that were eaten as bushmeat. If there is going to be a bird flu epidemic, the avian strain will have to infect a suitable human host and then mutate into a form transmissible between humans.

A.albopictus biting birds and then humans increases enormously the risk of starting a bird flu epidemic. Currently infected humans have to be in physical contact with birds, which is normally rare. With this mosquito present, it is going to be a whole lot less rare and who knows what other deadly diseases are lurking out there to be passed on.

It's not so much then that this insect transmits known diseases. Yes malaria is a huge problem in Africa, but it is largely unknown in Europe and North America because infected people are treated and isolated, making it nigh on impossible for a mosquito to bite twice (to transmit a disease it has to feed once on an infected host, lay its eggs before biting a second time, on this occasion a healthy victim - a process common in Africa but difficult to achieve in a country with first class health care.)

The problem lies with transmission of diseases we don't yet know about.

Terence Hollingworth

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