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Blood lust and the mosquito

Today is World Mosquito Day, marking the 1897 discovery by the British doctor, researcher and military officer Sir Ronald Ross that the female Anopheles mosquito spreads malaria. Here our senior press officer Claire Hastings looks at what makes this ‘little fly’ one of the most dangerous organisms that has ever lived.

A drawing of a mosquito with a blood-filled stomach (Wellcome Library, London)

A bellyful of blood (Image credit: Wellcome Library, London)

No one likes mosquitoes. They bite. They’re difficult to swat. To make matters worse, they inevitably appear on the rare pleasant days when we’re trying to enjoy the Great British Summer Time. Luckily, mild to moderate irritation is the worst outcome we can expect from a mosquito bite in the UK. But in the tropics a single mosquito bite can result in anything from an itchy ankle or mild fever to organ failure and death.

Tools of the trade

Despite their blood-sucking reputation, mosquitoes primarily feed on nectar from plants. Only the female mosquito needs additional protein in her diet to allow her eggs to develop; and the blood of animals is her favourite source.

Without this blood lust, female mosquitoes wouldn’t need to bite. Over the years she has evolved a few tricks that make her rather good at drinking our blood. Stealth is key ― if it hurt then we’d know about it, and the mosquito wouldn’t have time to find the perfect place to drink.

The mosquito’s proboscis ― a sinister hybrid of a drinking straw and a syringe ― has evolved to be covered in tiny bristles so that when it pierces the skin, it makes the minimum amount of contact with our nerves and we feel no pain, no danger signal. The mosquito also prefers to feed at night time, as we lie unconscious in our beds, and are less likely to disrupt their meal.

Another key adaptation is contained within the mosquito’s saliva. It probes the skin to find the ‘right’ blood vessel from which to drink. Once it finds the right spot, it spits ― via its proboscis ― into the wound. This saliva contains chemicals that stop blood from clotting so it may drink freely until full.

Mosquitoes are unwitting killers. The transfer of mosquito saliva into the human body is what makes the mosquito so deadly. Over time, opportunistic microbes have evolved to take advantage of this feeding habit, using the mosquito as a vessel to transport them from one person to another.

After entering the mosquito’s stomach, the microbes then make their way to the salivary glands to await their arrival at a new victim. Female mosquitoes typically lay three rounds of eggs before they die ― meaning multiple feasts on human blood, so multiple chances to spread an infection.

Dangerous hitchhikers

The list of disease-causing microbes that take advantage of female mosquitoes’ blood lust to spread from person to person is impressive.

Viruses such as yellow fever, West Nile, dengue, chikungunya and various viruses that can cause brain swelling, are all passed on by mosquitoes. So are the tiny worms that cause lymphatic filiariasis (more commonly known as elephantiasis). But none is more deadly than Plasmodium ―the parasite that causes malaria. Around two billion people are at risk of malaria, there are more than 200 million infections each year and approximately one million deaths, most of which are children under five years old.

Mosquito-borne viruses are a growing problem and dengue fever is a prime example. In the 1970s only nine countries experienced epidemics, but now over 100 countries report cases of dengue each year. The World Health Organisation (WHO) estimates that there are between 50-100 million infections every year.

Targeting the mosquito

So far, targeting the mosquito through the use of insecticides and bednets, has proven the best way to reduce the number of infections and deaths caused by mosquito-transmitted diseases.

However, mosquitoes have begun to develop resistance to insecticides and malaria-carrying mosquitoes have started to adjust their feeding patterns to feed during the day instead of night time.

Now scientists are developing new ways to target mosquitoes to prevent disease. One strategy is to block transmission by treating humans with a drug or vaccine that would be effective inside of the mosquito when it takes up the treated human’s blood.

Dr Mara Lawniczak, an MRC Career Development Fellow at Imperial College London, is using genome-wide association studies (GWAS) to look at why some mosquitoes carry malaria and some appear to be resistant to it in order to design transmission-blocking treatments.

Dr Lawniczak says: “We still don’t know which genes in the mosquito are important in preventing infection by malaria parasites. By sequencing the genomes of resistant mosquitoes and comparing them to the genomes of susceptible mosquitoes, we hope to identify new drug or vaccine targets to prevent mosquitoes being infected, and thus stop them from spreading the disease from person to person.”

“For example, if the gene that appears most important for resistance is a receptor on the stomach of the mosquito which the parasite needs to successfully develop inside of the mosquito, then drugs or antibodies can be developed to block the receptor and this would stop the mosquito from spreading the disease.”

When Sir Ronald Ross discovered that malaria was spread by the Anopheles mosquito, our eyes were opened to the deadly consequences of a mosquito’s blood lust.

World Mosquito Day aims to remind us that, although we are seeing progress, the death toll from infected mosquito bites remains far too high. Hopefully research such as Dr Lawniczak’s will produce the tools we need to change that.

Claire Hastings

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