Here’s What Happens Inside You When a Mosquito Bites

ORIGINAL: Phenomena - NatGeo
by Ed Yong

Anopheles gambiae. Credit: James D. Gathany, CDC.

The video above shows a brown needle that looks like it’s trying to bury itself among some ice-cubes. It is, in fact, the snout of a mosquito, searching for blood vessels in the flesh of a mouse.

This footage was captured by Valerie Choumet and colleagues from the Pasteur Institute in Paris, who watched through a microscope as malarial mosquitoes bit a flap of skin on an anaesthetised mouse. The resulting videos provide an unprecedented look at exactly what happens when a mosquito bites a host and drinks its blood.

For a start, look how flexible the mouthparts are! The tip can almost bend at right angles, and probes between the mouse’s cells in a truly sinister way. This allows the mosquito to search a large area without having to withdraw its mouthparts and start over.

“I was genuinely amazed to see the footage,” says James Logan from the London School of Hygiene and Tropical Medicine, who studies mosquitoes. “I had read that the mouthparts were mobile within the skin, but actually seeing it in real time was superb. What you assume to be a rigid structure, because it has to get into the skin like a needle, is actually flexible and fully controllable. The wonders of the insect body never cease to amaze me!”

From afar, a mosquito’s snout might look like a single tube, but it’s actually a complicated set of tools, encased in a sheath called the labium. You can’t see the labrum at all in the videos; it buckles when the insect bites, allowing the six mouthparts within to slide into the mouse’s skin.

Four of these—a pair of mandibles and a pair of maxillae—are thin filaments that help to pierce the skin. You can see them flaring out to the side in the video. The maxillae end in toothed blades, which grip flesh as they plunge into the host. The mosquito can then push against these to drive the other mouthparts deeper.

The large central needle in the video is actually two parallel tubes—

• the hypopharynx, which sends saliva down, and 
• the labrum, which pumps blood back up. 

When a mosquito finds a host, these mouthparts probe around for a blood vessel. They often take several attempts, and a couple of minutes, to find one. And unexpectedly, around half of the ones that Choumet tested failed to do so. While they could all bite, it seemed that many suck at sucking.
The video below shows what happens when a mosquito finally finds and pierces a blood vessel. On average, they drink for around 4 minutes and at higher magnifications, Choumet could actually see red blood cells rushing up their mouthparts. They suck so hard that the blood vessels start to collapse. Some of them rupture, spilling blood into the surrounding spaces. When that happens, the mosquito sometimes goes in for seconds, drinking directly from the blood pool that it had created.

When the mosquitoes were infected with the Plasmodium parasites that cause malaria, they spent more time probing around for blood vessels. It’s not clear why—the parasites could be controlling the insect’s nervous system or changing the activity of genes in its mouthparts. Either way, the infected mosquitoes give up much less readily in their search for blood, which presumably increases the odds that the parasites will enter a new host.

Many hours after a bite, Choumet’s team found Plasmodium in the rodents’ skin, huddled in areas that were also rife with the mosquito’s saliva. The mosquito starts salivating as soon as it probes the mouse’s skin, releasing substances that 

• prevent blood vessels from constricting, 
• stop blood from clotting, and 
• prevent inflammation. 

Sometimes, Choumet could see the saliva as small bubbles that hung around the tips of the mouthparts. And even after the mosquito stops feeding, pockets of saliva linger in the lower layers of the skin. Plasmodium parasites seem to stay in the same place—perhaps they work together with the salivary chemicals to suppress the mouse’s immune system.
The team also tested “immunised” mice, which were loaded with antibodies that recognise a mosquito’s saliva. “Some people, especially in Africa and Asia, are bitten several times every day, so we wanted to know if mosquitoes behaved differently when they bit animals that were immunised against their saliva,” says Choumet.

She found that the antibodies reacted with the insect’s saliva during a bite, forming noticeable white clumps at the tips of the probing mouthparts. This clogged up smaller blood vessels, which stopped the mosquitoes from drinking from them. But the insects got around this problem by probing around for longer, and by hitting the largest blood vessels.

Beyond the stunning videos, these discoveries are unlikely to lead to new ways of preventing or treating malaria by themselves. However, they do tell us a lot more about the event that kicks off every single malaria case—a mosquito bite. It’s a resource that other researchers will undoubtedly use. “I have submitted a grant application to investigate aspects of the interactions between mosquitoes, hosts and parasites,” says Logan. “The techniques and discoveries from this paper are very exciting to me, and will be of value to future activities of my own research group.”

Hat-tip to James Logan for alerting me to the story via Twitter, and inspiring the headline!

Reference: Choumet, Attout, Chartier, Khun, Sautereau, Robbe-Vincent, Brey, Huerre & Bain. 2012. Visualizing Non Infectious and Infectious Anopheles gambiae Blood Feedings in Naive and Saliva-Immunized Mice. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0050464