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Are we at risk from growing UK mosquito numbers?

12:00am | & News

Part of the news coverage in the run-up to the Rio Olympics has focussed on the potential threat of the Zika Virus in Brazil, spread through mosquito bites.

Mosquitoes are well-known as a spreader of diseases in hotter climes, notably malaria, yellow fever and dengue fever. But over the past 20 years there have been growing numbers of mosquitoes in the UK, so do we need to be on our guard against these diseases?

The answer, at least for now, is probably not. NHS records show that complaints of mosquito bites in the UK are almost three times higher than they were before 1996. Of course some might not be bites from mosquitoes at all, but from midges, horse flies, bed bugs and even fleas. It could also be that people are increasingly aware of mosquito bites, both from travelling abroad more and from news coverage. 

But other studies do confirm growing numbers of mosquitoes in the UK, particularly in the south of England, where changing weather conditions are increasingly hospitable to them. Wet weather in the spring and early summer, combined with generally warmer summers, are encouraging growing numbers of UK mosquitoes, with around 30 species known to exist here.

A number of schemes have been set up to establish where mosquitoes live and breed and if their numbers are growing, including a Mosquito Recording Scheme set up by the Health Protection Agency. Generally mosquitoes swarm over pools of standing water, such as lakes and ponds, even bowls of drinking water left out for pets.

In towns and cities they are also attracted to sewers, which only enhances their image as spreaders of disease. During the heat of the day they will rest in a cool place, but emerge at dawn or dusk to feed.

But while mosquitoes certainly transmit serious and even deadly diseases in many parts of the world, they do not cause much harm in the UK. As anyone who has been bitten by a mosquito will tell you, it can leave a nasty swollen and raised lump, called a wheal, which is itchy and irritating.

Here's how it works (and don't read this if you're squeamish!): It's your blood that mosquitoes are after, and in most species it is only the females which feed on other animals, including humans. Females need the nutrients from a "blood meal" in order to produce eggs, while the males are content to feed on plant juices and nectar.

When a mosquito "bites" it actually stabs, piercing your skin with a pointed proboscis. Before sucking your blood, the mosquito first injects its saliva into you – a process which continues throughout its feeding. This saliva acts as an anticoagulant, preventing your blood from clotting and keeping it flowing freely until the mosquito has drunk its fill.

It is this injected saliva which causes the itchy wheal as your body tries to fight off the protein left by the attacking insect. It is also the saliva which carries and spreads infectious diseases, putting them directly into your bloodstream.

However, for a disease to be effectively spread by mosquitoes, it must first be present in a significant portion of the local population. So, for example, in countries where malaria is commonplace, it will be spread further by mosquitoes feeding on infected people or animals and then passing on the infection when it feeds on others.

While there may certainly be more mosquitoes in the UK, they cannot easily spread diseases which are not already here. In rare cases, disease-carrying mosquitoes are accidentally "imported" from other countries on board aircraft, in travellers' luggage or in imported goods. A bite from such a mosquito could infect its victim, but other mosquitoes in the UK would not "catch" the disease in the way that people catch a cold from each other. Another mosquito would only pick up and spread the disease if it then fed on an infected person.

In short, mosquitoes will spread and amplify what's already there, but in countries like the UK with an advanced health system and effective disease control, they do not yet pose a serious threat. A mosquito bite here in the UK is certainly a nuisance, but it is unlikely to cause anything worse than a few days of itching.

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