Written by Joe Ballenger
@Stylopidae As a layman I know nothing about regular mosquitoes & how they bite etc. Be good to establish that.
— Jude Adamson (@smalljude) January 31, 2015
While writing my post about GMO mosquitoes, Jude Adamson made a request over Twitter I thought was a good idea. Understanding how insects feed on blood, why and why this facilitates disease transmission is a really important topic.
Blood feeding is super-common in insects, with more than 30 groups having evolved this diet independently. Lots of insects have made this jump from plant-feeders, feces-feeders, predators or even parasites of other groups. The flies were probably the first to evolve a blood feeding lifestyle, and this habit probably evolved in the late Triassic**. Mosquitoes evolved in the Jurassic, as did other species of bloodfeeding flies. All in all, blood feeding insects have been around for about 200 million years…which means that at least some of the dinosaurs in Jurassic Park lived blissfully unharassed by our bloodsucking frenemies.
So how does this process work, and how does it result in disease transmission?
As with most lifestyles, bloodfeeding fly mouthpart modifications run the gamut from relatively poorly adapted to highly specialized. There are a whole lot of adaptations which need to take place to get blood, and different groups of flies show the steps which need to occur to make this happen.
There are two types of blood-feeders: pool feeders, and capillary feeders. Pool-feeders feed a lot like the horsefly shown below: they cut a hole in the skin, and wait for blood to well up. When the animal bleeds, they lap up the blood that pools on the surface which gives them their name.
Horsefly bites are quite painful, but they tend to attack larger and clumsier animals which don’t have the dexterity needed to kill them.
Mosquitoes, on the other hand, are a different story. They are capillary feeders, which means they use their mouthparts to seek out a blood meal from one of the tiny veins below the surface of the skin. Their mouthparts are modified to form a flexible tube which they use to draw blood directly from the capillary, without alerting the host to their presence. The video below shows how this happens, and it’s not at all like inserting a needle into the skin. Instead, the proboscis is more like a tongue because it’s a sensory organ which is capable of moving on it’s own while seeking out food.
When feeding on blood, insects have two problems. First, they have to keep the host from killing them. This is a problem not only if you’re feeding on people, but also if you’re feeding on frogs. Second, you need to keep the blood from clotting. All blood feeding insects use some sort of salivary factor to keep the host’s juices flowing, which is why the man who took the horsefly video bled so much from a relatively small cut. A good example of a salivary protein is the D7 protein family, which is one of the proteins you’re probably reacting to if you’re allergic to mosquito bites.
The D7 protein family from mosquitoes act as kind of a sponge which keeps the body from secreting small molecules which trigger immune reactions. They grab up signaling molecules like histamine or eicosanoids, which tell the body that something’s wrong. By delaying the immune reaction, they keep the bite site from swelling up and pushing the mosquito out. By preventing the host from itching immediately, they keep the host from swatting it. Blood feeding isn’t just about extracting materials from a host. To get the blood, the mosquito has to put something into the host.
During an insect bite, there’s an exchange of fluids between the host and parasite. Both of these exchanges, in or out, can result in disease transmission. Loa loa, the eyeworm, will migrate to the fly’s head and then push it’s way out the mouth when the insect is feeding. Arboviruses, including Dengue Fever, will infect the mosquito’s salivary gland and then get injected along with the saliva. It’s also pertinent to mention that it’s not uncommon for the insect’s saliva to help the parasite out, because it modulates the immune system. A good example of this is the interaction with Leishmania with it’s host’s spit.
Leishmania lives inside white blood cells, which are soldiers on the front line when the body battles infection. It’s insect host, the sand fly, introduces a molecule which shuts down production of white blood cell activating compounds while telling the body to produce hormones which tell white blood cells to stand down. Besides stopping the inflammatory process which would keep the fly from feeding, the saliva ensures Leishmania parasites are free to attack the now sleeping white blood cells.
Knowing how insect saliva works is really important, because it helps us understand disease transmission. The sandfly salivary factor described above, maxadilan, is under investigation as a potential vaccine component to Leishmania vaccines. People who are bitten by a lot of sand flies are more resistant to infection…so it may be possible to increase the efficacy of vaccines by training the body to recognize the salivary factors injected with each and every insect bite.
**It’s actually likely that blood feeding evolved much earlier, but the groups we know for sure were blood feeders all appeared about the same time about 200 million years ago. However, many entomologists have hypothesized that a now extinct group called the Paleodictyoptera evolved this lifestyle before any other insect. The fossil record is spotty, but ‘transitional’ fossils which have characteristics of mosquitoes and other families have been recorded from the mid-Cretaceous.