A hot summer brings out the sunglasses, ice cream and bare feet. Unfortunately it also brings out the flying, biting pests. The UK has 7,000 species of flies, including midges, horse flies and the ones with arguably the worst reputation, mosquitoes.
The most common mosquito species in the UK is Culex pipiens. Few people are actually bitten by it, since it mainly feeds on birds, but this year there are likely to be more C. pipiens and other mosquito species than usual. This is down to seasonal conditions. Pregnant female mosquitoes hibernate and the mild winter will have resulted in greater survival – and more eggs. A wet May was ideal for the aquatic larvae. A hot summer means more fly activity and more people outside to be bitten. So the cycle continues.
Mosquitoes have needle-like mouth parts that pierce flesh so they can suck blood. They also secrete anticoagulants that prevent clotted blood blocking their mouth parts, and a local anaesthetic so you can’t feel the bite. But these bites aren’t just annoying – they’re potentially deadly.
Mosquito saliva can be a vehicle for transmission of diseases such as malaria, caused by a tiny protozoan organism called Plasmodium. In 2013, between 124m and 283m people contracted the disease and an estimated 584,000 people died from it.
As well as malaria, mosquitoes can transmit viruses including dengue fever, yellow fever, West Nile virus and chikungunya. Luckily for the UK, the species that carries most of these diseases, Aedes egypti, doesn’t live here, but it is increasing its range. It recolonised Madeira in 2004-2005 and there are concerns that it could be transported to western European countries.
Malaria in the past
There are very few recent cases of malaria transmission in the UK, although there is evidence for the disease’s presence from the 14th to the 17th centuries. Shakespeare even mentioned it in The Tempest. Malaria in the UK virtually died out by the end of the 19th century due to a combination of marsh drainage, use of quinine and better sanitation.
Aside from this, the absence of malaria in Western Europe is most likely due to its climate. Plasmodium needs a sustained high temperature to complete its reproduction in the mosquito. The lack of the species mainly involved in transmission of the disease is also crucial.
However, the UK’s freedom from dangerous mosquitoes could be set to change. Firstly, with increased globalisation and faster transport, non-native species could be introduced in sufficient numbers to establish a breeding population.
Most insects are strongly “r-selected”, meaning they have evolved to produce large numbers of offspring but live relatively short lives. That means once a species has established itself in a location it can increase very rapidly. For example, Harlequin ladybirds spread over most of southern Britain in just 10 years, with a much slower breeding rate than mosquitoes.
Secondly, temperatures are predicted to increase due to climate change. A review published in The Lancet Infectious Diseases concludes that warmer conditions and more rainfall could provide the right conditions for disease carrying mosquitoes to arrive in the UK. Existing species such as the common Culex pipiens, could spread West Nile Virus here. Another virus-carrying species, Culex modestus has already established colonies in the Thames estuary.
A further problem is the Asian tiger mosquito (Aedes albopictus), which spreads dengue fever and chikungunya. Both can be serious illnesses and have no effective treatment. This mosquito’s spread, especially in the United States, has been exacerbated by the international trade in used tyres, whose colour and structure provide ideal incubation pools for the species’ aquatic larvae.
Predictions based on a 2oC temperature rise – the commonly agreed limit before which “dangerous” climate change will kick in – could extend Tiger mosquitoes’ activity season by a month and its range by up to 30% by 2030. In the past 10 years insect-borne diseases have spread within Europe, including Greece (malaria) West Nile virus (Eastern Europe), Italy and France (chikungunya). This temperature rise could lead to outbreaks of chikungunya in south-eastern England by the second half of the century.
So as the climate warms, mosquitoes in the UK may no longer be just a pest that gets worse during the occasional heatwave. They may become a widespread, constant and dangerous health threat.