Technical| PPC96 August 2019

Traditionally mosquitoes have been treated with pesticides and insect growth regulators by pest controllers, but scientists have been looking at how we can fundamentally change our foe to make them less of a public health concern worldwide.

Dr Claire Donald, from the University of Glasgow, gives PPC readers an overview of the most promising new treatments for mosquito management.

SPEED VIEW

• Dengue and chikungunya viruses contracted from mosquitoes have been recorded in Spain, France, Italy and Croatia
• Control methods have focused on pesticide eradication using insecticides and insect growth regulators
• Sterile Insect Technique (SIT) releases a large number of sterilised males
• A Wolbachia-infected male mating with an uninfected female yields eggs that don’t develop
• Genetic modification is promising but has yet to be fully tested.

Many people have experienced a mosquito bite. For some, it’s a small itchy bump but for others, it can result in a painful, fluid-filled blister. However, being bitten by a mosquito is not just irritating, it could be dangerous.

Public health concern

Mosquitoes feed on blood as a source of protein to produce their eggs.

They insert their needle-like proboscis into your skin and hunt around until they find a blood vessel before gorging themselves.

However, mosquitoes carry a number of different pathogens which cause disease in humans. These can be passed on to us when they take their blood-meal.

There are no treatments or vaccines available for many of these infections and so the most effective way of preventing illness is to avoid being bitten.

In areas where these diseases are found, vital public health campaigns are in place to help educate at-risk communities about how these infections are spread and why it is so important to protect ourselves from mosquito bites.

Tropical countries, characterised by low socioeconomic status with hot and humid weather, are more prone to these diseases. However, changes in a number of factors (such as increasing global temperatures, global travel and deforestation) means mosquitoes are able to live in more areas.

Pathogens spread by mosquitoes

Increasingly warmer and wetter weather means that mosquitoes are able to inhabit areas outside of the tropics, while increased deforestation means that humans are frequently a convenient and accessible food source.

This means that, although historically Europe was mostly at risk of travel-associated infections, it is becoming more and more capable of supporting indigenous disease-carrying mosquito populations.

In the past 20 years, there has been a rise in the number of mosquito-transmitted infections which have been acquired locally.

For instance, cases of dengue virus and chikungunya virus contracted from local mosquito populations have been recorded in Spain, France, Italy and Croatia.

As these diseases continue to increase their incidence and geographical locations, it is more important than ever to develop effective control methods.

Disease control methods used over the last 40 years have focused on pesticide eradication strategies, such as insecticides (pyrethroids), insect growth regulators (eg methoprene, pyriproxyfen) or biological agents (eg predatory species).

However, these methods can be difficult to implement, costly and often have low coverage. In addition, their efficiency can be affected by the development of resistance, not to mention the risks to non-target organisms.

Although work continues to develop new, safe and fast-acting pyrethroid insecticides, for instance on materials that the target species are bound to come into contact with (eg bednets), alternative strategies are being investigated that aim to overcome some of the associated obstacles.

Examples of these include sterile insect technique (SIT), Wolbachia, release of insects with dominant lethality (RIDL) and gene drives.

In the past 20 years, there has been a rise in the number of mosquito-transmitted infections which have been acquired locally. As these diseases continue to increase their incidence and geographical locations, it is more important than ever to develop effective control methods.

Sterile Insect Technique (SIT)

SIT involves the release of a large number of sterilised male mosquitoes.

These males have been subjected to irradiation which mutates their sperm so that they are unable to fertilise an egg. Therefore, a sterile male who mates with a wild female will fail to produce any offspring which results in a decrease in the population.

Although this method has shown promising results with some species, there are still difficulties with costs and establishing facilities needed to mass-rear the sterilised males.

Wolbachia

Wolbachia is a natural bacteria which is found inside the cells of about 60% of insects. It is able to be transmitted from female insects to their offspring which allows it to persist within populations.

When a Wolbachia infected male mates with an uninfected female, the resulting eggs will be unable to develop due to a phenomenon known as ‘cytoplasmic incompatibility’.

This means that regular releases of males infected with a strain of the Wolbachia bacteria which is not normally found in wild mosquitoes could lead to a decrease in population size.

Further to this, Wolbachia has been shown to protect insects against viral infections and can reduce their ability to transmit them.

Controlled releases of Wolbachia-infected mosquitoes are currently being investigated for their effectiveness for controlling viruses like dengue and Zika in Australia, Malaysia and Indonesia.

Release of insects with dominant lethality (RIDL)

RIDL aims to prevent mosquitoes surviving past the pupal stage. Mosquitoes are genetically modified to express a lethal gene within their DNA.

The lethal gene has a molecular ‘on/off switch’ which allows the modified mosquitoes to be reared in a lab before they are released to mate with wild populations.

Following successful mating with a RIDL mosquito, the egg becomes fertilised and hatches into a larvae.

As the lethal gene is dominantly expressed, all offspring of a RIDL mosquito will inherit it and the gene will lead to the death of the mosquito before it is able to emerge into an adult.

This technique has been successfully employed in Brazil, Panama and the Cayman Islands to reduce the predominant disease-carrying mosquito species by 90%.

Gene drives

Gene drives are a form of genetic modification.

Using the clustered regularly interspaced short palindromic repeat (CRISPR) associated protein 9 system, transgenic mosquitoes can be produced which express particular desired traits. These traits can then spread within populations.

Theoretically, gene drive technology should be a very effective and specific method, although this has yet to be fully tested in the field.

Examples of this include the inactivation of genes controlling fertility (thus reducing population numbers) or activation of genes involved in immunity (making them resistant to infection and therefore unable to spread the disease to humans).

Theoretically, gene drive technology should be a very effective and specific method, although this has yet to be fully tested in the field.

As with all genetically modified organisms (GMOs), safety is a primary concern and several factors need to be taken into consideration before large-scale trials can be done.

In conclusion

Any method which aims to remove mosquitoes from the environment comes with risks which must be considered.

Mosquitoes provide a major food source for a number of insect-eating animals, such as birds, frogs and bats, so their eradication may result in problems within local food chains.

Furthermore, the removal of one mosquito species may provide the opportunity for a second species to replace it.

It is therefore important that control strategies consider the effect that population suppression may have on the ecosystem.

As insecticide resistance is considered to be a major hurdle which must be overcome for effective control of disease-transmitting mosquito species, it is likely that effective control measures will rely on the implementation of a combination of different strategies.

The development of alternative methods will allow for an increase in the range of options available for mosquito control and may benefit current strategies.

Further optimisation is on-going and large-scale studies are required to be completed to ensure that the methods are specific and effective.

Mosquitoes: the world’s deadliest ‘animals’

We all know mosquitoes are annoying little critters, but here are our top ten interesting facts you may not know about these vampiric blighters.

1. Mosquitoes have been around since the Jurassic period, making them about 210 million years old.
2. Mosquito is Spanish for ‘little fly’.
3. Only female mosquitoes bite; both male and female feed mainly on fruit and plant nectar, but the female also needs the protein in blood to help her eggs develop.
4. Mosquitoes don't have teeth: the females ‘bite’ with a long, pointed mouthpart called a proboscis.
5. The bumps from mosquito bites are caused by minor allergic reactions to the saliva. While one tube in the proboscis draws blood, a second pumps in saliva containing a mild painkiller and an anticoagulant.
6. Mosquitoes can smell human breath. They have receptors on their antennae that detect the carbon dioxide released when we exhale.
7. Certain smells help mosquitoes choose their victims. They are attracted to octenol, a chemical released in sweat, as well as cholesterol, folic acid, skin lotions and perfume, which all smell like dinner to mosquitoes.
8. Mosquitoes do not transmit HIV, which is actually digested in their stomachs and broken down without being passed on.
9. In the 1993 Steven Spielberg movie Jurassic Park, the cloning of dinosaurs was accomplished by extracting dinosaur DNA from mosquitoes preserved in amber.
10. Mosquitoes are considered the deadliest ‘animal’ in the world, transmitting diseases like malaria and dengue fever. Mosquito-borne illnesses infect around 700 million people a year, killing more than one million. It is thought that Alexander the Great, conqueror of nations, may have died of malaria in 323BC.

Source: PPC96