Mosquitoes Don’t Like Parasites Either (A Guest Post)

By Maranda Cardiel

A photograph of Culex pipiens, the species of mosquito that the researchers used
in their experiment. Source: David Barillet-Portal at Wikimedia Commons.

Everybody hates mosquitoes. They are annoying, persistent, and make us itch like crazy. Sometimes there are so many of them that we are afraid to go outside unless we want to risk getting covered in spots and scratching ourselves all over for the next week. And if that wasn’t enough, they can also carry dangerous diseases with the potential to kill us. However, just like us, mosquitoes don’t like to be bugged by parasites that can make them sick either. Research shows that they may even avoid interacting with hosts that might pass along parasites to them.

A group of researchers - Fabrice Lalubin, Pierre Bize, Juan van Rooyen, and Philippe Christe from the University of Lausanne in Switzerland and Olivier Glaizot from the Lausanne Museum of Zoology – wanted to see if mosquitoes would show a preference for feasting upon birds that were infected with malaria (a blood parasite) or uninfected birds. Mosquitoes find animals to snack on by sensing odors and carbon dioxide in the air that animals give off, along with using their senses of vision, hearing, and touch. In order to figure out if mosquitoes use these senses to specifically choose their unlucky victims, the researchers did an experiment with mosquitoes, malaria, and great tits (a type of bird with a funny name).

For their experiment, the researchers collected mosquito eggs that they hatched and raised in a lab. Only female mosquitoes suck blood, so only female mosquitoes were used in the experiment. The mosquitoes had never been exposed to birds before and were starved of sugar for one day to make sure that they would be hungry. The researchers also caught wild adult great tits, and they took small blood samples from each bird to test for malaria before and after the experiment.

Next it was time to see if the mosquitoes would find some birds to be more appealing than others. A special Y-shaped wind tunnel allowed the mosquitoes to choose between the odors of two birds: one that was infected with the malaria parasite and one that was not. But don’t worry, the mosquitoes could not directly contact the birds. The researchers set up the lab so that it was completely dark to mimic the natural settings of when mosquitoes feed in the wild. This also meant that the mosquitoes were blind and could only choose a bird based on the chemicals in the air. Randomly-chosen pairs of birds and new mosquitoes were used for each round of the test.

A cartoon depicting the experiment setup. A hungry female mosquito hones in on the odors
of a healthy great tit and a great tit infected with malaria parasites. Source: Maranda Cardiel

The results of the study showed that the mosquitoes had a strong preference for birds that were not infected with the malaria parasite. This was true even when the researchers took into account the body sizes and sexes of the birds. Previous studies with different kinds of birds, mosquitoes, and malaria or malaria-like parasites have found similar results. The researchers think that this may be because the malaria parasite somehow causes changes in the chemical processes in the birds’ bodies that the mosquitoes can pick up on.

Infection with malaria might change what the birds smell like to the mosquitoes or how much carbon dioxide the birds give off. There is also evidence that birds who are more susceptible to malaria infections have a different odor than birds with stronger immune systems. But why should mosquitoes be picky and choose to bite healthy birds? They certainly don’t seem like they care whose blood they suck when they are swarming around us!

Previous research has shown that mosquitoes infected with malaria parasites have problems developing their eggs and can have trouble sucking up blood from their victims. Female mosquitoes use blood to nourish their eggs, so if they don’t drink as much blood, they will not be able to lay as many eggs. This means that female mosquitoes carrying malaria parasites are less likely to produce as many healthy offspring. Thus, it makes sense for female mosquitoes to want to avoid feeding on birds that are infected with malaria.

This probably has not changed your thoughts about mosquitoes. They are still a nuisance that we all squish - or at least attempt to squish - upon sight. It might be ironic, but mosquitoes don’t like to have parasites bothering them either. Even though we hate them, maybe now you can find some solace in mosquitoes finding you attractive. It might be a sign that you are actually healthier than your peers.


Source:

Lalubin, F., Bize, P., van Rooyen, J., Christe, P., & Glaizot, O. (2012). Potential evidence of parasite avoidance in an avian malarial vector Animal Behaviour, 84 (3), 539-545 DOI: 10.1016/j.anbehav.2012.06.004