One of the ways parasites can get from one host to another is by hitching a ride on a biting insect. For example, the malaria parasite, called Plasmodium, is carried by mosquitoes and transmitted in the insect's saliva every time it feeds on blood. But Plasmodium doesn't just settle for relying on the mosquito's natural feeding habits: scientists have discovered that it also alters the insect's appetite, making it more attracted to certain human smells and increasing the rate of transmission. Khalil Thirlaway went to the London School of Hygiene and Tropical Medicine to hear how...
Khalil - I'm here in my home city of London standing outside the London School of Hygiene and Tropical Medicine. Inside that building is the world's deadliest double-act and I'm about to meet it face to face. But first, I'm going to learn a little bit more from someone who knows this deadly duo inside and out.
Vicki - I work on malaria. Malaria is a disease that kills half a million people every single year. So, it's a huge disease and it affects people by invading their blood cells and bursting them which causes fever and it causes illness. One of the main targets in the fight against malaria is actually the vector, the mosquito that transmits the disease and that's where I come in.
Khalil - I hear that you have a room full of mosquitoes here.
Vicki - That's true actually. Do you want to see?
Khalil - Yes, please. That would be great.
Vicki - Let's go into the insectary.
Khalil - We've stepped into a small dungeon-like room with lots of clear plastic mesh boxes all around. On closer inspection, they're actually full of hundreds of mosquitoes feeding from an electronically warmed blood pack that stop to the top of the mesh box.
Vicki - That's actually my blood....
Khalil - Wow, suffering for your work. That's great! Before we continue, I have to ask, are any of these carrying malaria?
Vicki - Absolutely not. Luckily for you, this is a clean room. So, all of the mosquitoes in here are waiting to be infected.
Khalil - So, we're nice and safe?
Vicki - We're nice and safe here at the moment, yeah.
Khalil - What are you doing with this room full of blood sucking mosquitoes?
Vicki - For years, people have been combating malaria. But most of these control methods, they focus on insecticides or insecticide treated bed nets. But we're coming at it from a new approach. So a mosquito has a really good sense of smell. They could pick up really tiny amounts of certain compounds that are given off by a human body and that's how they find a human to feed on to take a blood meal.
Khalil - What part does the parasite play in this?
Vicki - Well, that's a fascinating part actually. Once the mosquito is infected with Plasmodium, lots of different aspects of this behaviour can change. There are lots of different things such as it is maybe more likely to take a blood meal or it's more likely to take longer to feed. All of these things can increase the chance that it successfully pass on that parasite to a new host. The bit that I'm particularly interested in is that the mosquito is actually more sensitive to specific aspects of human smell and that's what's really fascinating.
Khalil - How do you determine which components of the human smell are attractive or repellent to mosquitoes?
Vicki - Essentially, what we did was electroantennography or EAG and this technique actually involves cutting off mosquito's head and mounting it on an electrode. We pass compounds that we are interested in over the antenna and we measure the electrical impulses when there's no fire and that's how we know whether the compound is exciting to a mosquito.
Khalil - How could this understanding be used in the fight against malaria?
Vicki - One possible application of our work could be to design traps that are more effective and because they're based on those really attractive compounds we identified from people. Potentially, we could even be designing traps that are specific for infected mosquitoes if we are seeing differences and that sort of attraction. Another option could be to design repellents based on certain compounds we've identified that are unattractive to mosquitoes. We know that people give off these sorts of compounds naturally already.
Khalil - Is that why some people get bitten more than others?
Vicki - Exactly. There's a huge variety in how people smell to mosquitoes and some people are very, very attractive and some people are very unattractive. And we're looking at the differences between those people and what sorts of smell they give off themselves. Most importantly, we now understand a lot more about how mosquito behaviour changes once it's infected with malaria. This sort of knowledge will feed into a huge variety of control methods and lots of other areas of research as well as we understand the system work.