Helping mosquitoes fight back against the diseases they spread
By Debs Barber
Mosquitoes are the deadliest animals in the world — but maybe we should think of them as victims, not villains? Worldwide, an estimated 700 million people contract diseases from a mosquito bite, causing over one million deaths per year. Yet it’s not the mosquitoes themselves that cause disease: it’s the pathogens — viruses, parasites and bacteria — that use them to replicate and spread. We explore how research is searching for a way to help the mosquitoes fight back to end the cycle of transmission.
You might think you hate ALL mosquitoes, but it’s only female mosquitoes that bite humans, and she’s only looking for the nutrients she needs to produce her eggs — up to 200 at a time. Mosquito bites are certainly unpleasant and annoying, but the swelling and itchiness — caused by our immune reaction to their saliva — only lasts a few days. However, the pathogens in the saliva can be a serious problem.
Vector-borne diseases are caused by parasites, bacteria and viruses, but they are transmitted from person to person via insects that bite vertebrates for survival — such as mosquitoes.
Female mosquitoes can drink up to three times their own weight in blood.
So, while mosquitoes probably aren’t about to win the most popular insect award, they deserve our attention. Killing them isn’t an option as doing so would deprive many other insects and animals of a vital food source. Could we instead help mosquitoes fight back against the parasites, bacteria and viruses which infect them?
Hijacked
When an uninfected female mosquito feeds on a human who is infected, she picks up the pathogen through her blood meal. This pathogen — often a virus or parasite — hijacks the mosquito to replicate and travels to her salivary glands. When the pathogen has reached her salivary glands, she then passes the pathogen on again when she feeds. Now inside another previously uninfected human, the pathogen replicates again, building up its numbers until the next mosquito comes along and the cycle continues.
Malaria is the most well-known disease associated with mosquitoes. Caused by a parasite which is spread by Anopheline mosquitoes, the World Health Organization estimates that there are 219 million cases a year worldwide and more than 400,000 deaths.
Yet malaria isn’t the only disease mosquitoes transmit. Aedes mosquitoes aren’t infected by the human malaria parasite, but they are vulnerable to infection by a type of virus called an arbovirus. Dengue is the most prevalent viral infection transmitted by this species, but they are also the vectors for viruses that cause chikungunya, Zika and other diseases.
There are over 3000 species of mosquito and they don’t all carry disease — and most aren’t even interested in biting humans at all.
That’s why researchers are looking to understand what it is about the small number of disease-spreading species that makes them an ideal host for the pathogens that take such a huge toll on human health.
A sugar boost for the immune system
“I don’t really like insects, but I started to work with mosquitoes because I was fascinated by their biology and interested in finding answers that might help to fight against the diseases they carry.”
Dr Emilie Pondeville and her team at the MRC-University of Glasgow Centre for Virus Research look at the biology of the Aedes aegypti mosquito to build a picture of how mosquito biology influences transmission.
Mosquitoes may not develop a fever like humans do after being infected, but that doesn’t mean that they don’t experience any negative effects. Just like us, mosquitoes mount an immune response to try and fight off the pathogen.
We know that eating a balanced diet is good for our health. One reason for this is our immune system. Without the right amount of micronutrients — vitamins and minerals — in our diets, the immune system can’t function to its maximum capacity and we’re more likely to get sick. There’s a wealth of research in this area, exploring the effects of diet in humans and other animals. But what about mosquitoes?
Dr Pondeville and her team looked at how eating sugar affects their ability to fight off the viruses infecting them. Most mosquitoes — both male and female — feed on sugary foods such as nectar and plant juices to get the nutrients essential for their survival.
From their research they discovered that feeding on sugar has a positive effect on mosquito’s immune system — increasing and maintaining antiviral immunity in the digestive tract of Aedes aegypti mosquitoes.
“The fact that some mosquitoes do not drink on sugar in some regions will greatly influence their ability to transmit viruses,” explains Dr Pondeville.
“Further research is needed, but our study suggests that in areas where there are high levels of viral transmission, one potential strategy could be to try to push the females to drink sugar and better protect themselves against infection.”
The future for mosquitoes
We tend to think of mosquitoes as being a problem in countries across Africa and Asia which have a warmer climate than countries in Europe. However, the combination of transport, trade and climate change is causing mosquito-borne diseases to be seen in previously uninfected parts of Europe.
Over the next few decades, we are inevitably going to see an ever-increasing number of outbreaks — and not just in the warm Mediterranean countries.
The tiger mosquito, Aedes albopictus, which can transmit the same viruses than Aedes aegypti, started to colonise the south of France in 2004 and is now detected in northern France. Scientists predict by 2060 we could see mosquitoes able to transmit viruses as far up as Scotland.
By understanding the inner workings of the mosquito through the work of scientists like Dr Pondeville, we can begin to explore cost-effective solutions that complement the insect’s biology to close the door on the pathogens that target them.
Want to know more?
If you’re a UK taxpayer, your contributions help fund the work of researchers like Dr Pondeville and the MRC-University of Glasgow Centre for Virus Research via UK Research and Innovation — the funding body that allocates government funds for research — and the Medical Research Council. You can read more about what we do here. And if you liked this article, follow us on Medium, Instagram, Facebook or YouTube — or sign up for our weekly newsletter!
