Showing posts with label hematophagy. Show all posts

Wednesday, July 2, 2014

How Do Mosquitoes Find You?

Biology concepts – semiochemicals, hematophagy, proboscis, thermosensing, TRPA1

Sure, mosquitoes suck blood and pass along malaria

that kill more humans than any other infectious disease.

But would it be good to get rid of them. They provide

food for birds – one scientist suggests that elimination

of Arctic mosquitoes could reduce northern bird

populations by 50%. And mosquitoes pollinate flowers

too, like blueberries and cranberries. See, they’re

not all bad.

We can start our summer series of biology questions by continuing our discussion of taste and thermosensing. It seems that some people are bitten by mosquitoes if they peak out the front door, while other people can sit outside next to tall grass or ponds for hours with suffering a single bite. Unfortunately, I happen to be in the former category.

How do mosquitoes find some people and not others? Are some people just tastier than others?

First let’s get some common misconceptions and basic information out of the way. Do mosquitoes bite you (or any other animal)? No, they have no teeth, so they don’t bite in the traditional sense. What they do have is an elongated set of mouthparts called a proboscis. The sheath on the outside retracts as the longer parts inside pierce the skin like a hypodermic needle. Only this is a flexible hypodermic needle, small enough to go around individual cells and look for a small vein or venule.

On the left is a drawing of the mosquito proboscis parts. Most sit

in a groove of the labium, which retracts as the rest is injected

into the skin. The maxillae and mandibles are like our upper and

lower jaws. They are the sharp parts. The hypopharynx is what

delivers the anticoagulant saliva. On the right is the parts put

together. The fuzzy part is the labium and the sharp tips are

from the maxillae.

Take a look at these videos taken from a 2012 PLoS One study of a mosquito biting a mouse. The squarish objects are skin cells, and the red streaks are blood vessels. The second video in the sequence shows what happens when the proboscis finds a vessel and starts to suck out the blood. Makes you respect the mosquito a bit more – these are some determined females.

Of course it’s only the females that feed on blood. This suggests that feeding on blood is related to having babies. And it is – just not in a “gotta get the baby some food” sort of way. Most mosquito species require a blood meal in order to develop viable eggs. Females get energy from drinking nectar (full of carbohydrates), but they need protein to produce yolk for the eggs. They get the protein from feeding on blood. If the female doesn’t feed on blood, the eggs will be produced, but they won’t be able to hatch and become larva.

But here is one of our exceptions – some mosquitoes have gotten around the need for blood meals. All 92 species of mosquito in the genus Toxorhynchites (elephant mosquito) don’t need to feed on blood. Instead, their larvae feed on the larva of other mosquitoes, and the gather the proteins they need to lay viable eggs from their larval meals. They store the amino acids in their larval and pupal bodies, until they become adults and need them to lay eggs of their own.

Compare the sizes of the elephant mosquito (left) and

A. aegypti. I’m very glad that the females of the

Toxorhynchites genus don’t suck blood. They could drain

people dry! Even though their size is small, species like the

one on the right can consume 300 ml a day from every

caribou in a herd when they are swarming.

This suggests that the elephant mosquitoes could be used to combat disease spreading mosquitoes, like the Aedes aegypti mosquitoes that spread dengue fever, yellow fever and the current disease of interest, chikungunya fever. And the elephant mosquito has been used as a natural biocontrol agent. What's weird is that A. aegypti females actually help the situation.

A. aegypti, and many other mosquitoes that lay eggs in water, have larvae that eat bacteria. So they want to lay eggs where there are a lot of bacteria. Well, the eating of larvae by Toxorhynchites species leaves lots of little pieces of mosquito larva in the water, and this provides bacteria with a lot of food. A June 2014 study showed that A. aegypti females actually prefer to lay eggs in water that contain predators for their larva, because it increases the bacterial numbers so much. Thos that survive have lots of bacteria to feed on. It’s a calculated behavior – risk being eaten or risk starving.

So some mosquitoes will go a long way and risk death in order to get a good meal for their potential offspring. They’re looking for mammals usually, but even here there are exceptions. Some mosquito species, like Culiseta melanura, feed almost exclusively on bird blood – they say it tastes like chicken.

The picture represents the transmission cycle for

eastern equine encephalitis virus (EEEV). It can’t be

transmitted from mammals to other animals, so they are

called dead-end hosts. But it can produce disease in

them. Humans most often will present with a limiting or

subclinical disease, but horses have a hard time with it.

The major source is in birds, and the transmission from

bird to bird is by mosquitoes that rarely bite humans. The

way into dead-end hosts is by a mosquito that normally

bites mammals occasionally biting a bird, or the rare

occasion that a bird specific mosquito will bit a mammal.

But just because they feed mostly from birds doesn’t mean they aren’t important disease transmission. They are – for horses. Eastern equine encephalitis virus is passed from bird to bird by C. melanura, so the birds, especially cardinals, are a reservoir of virus. Then, when another species of mosquito that is less particular about its host species bite a bird then bites a horse or person, the disease can be spread. There are even cases where a C. melanura will occasionally feed on a human and spread the disease directly.

With this background, we still need to answer our question of the day – how do mosquitoes find a blood meal. Believe it or not, your socks help answer the question. For many years it was assumed that mosquitoes followed the heat of warm-blooded animals in order to find a meal, but this was an assumption that was not tested rigorously.

Then it was discovered that carbon dioxide (CO₂) is a strong cue for mosquitoes seeking sustenance. CO₂ means respiration, and respiration possibly means mammals. The mosquitoes have taste receptors in their antennae and mouths that will sense changes in CO₂ and they will follow the path of more carbon dioxide right to your nose and mouth (see this post).

Large people and pregnant women tend to exhale more CO₂, so they will be more attractive to mosquitoes. But there are large individuals who never get bothered by mosquitoes. Maybe there’s more to it.

Semiochemicals are part of the answer. Semio- comes from the Greek meaning signal, like in semaphore flags. So semiochemicals are molecules emitted by organisms for communication. Pheromones are the most famous of the semiochemicals – and we know that these are used in many animals, from helping to guide ants to follow the path of their predecessors, to influencing mate choices in many animals.

Semiochemicals might be attractants or repellants. In some cases, they can be both. Take human body odor – it contains dozens of semiochemicals, people find body odor repulsive, but mosquitoes enjoy it like the smell of fresh apple pie. Of course, body odor is only offensive nowadays; before the advent of deodorant, daily or three times daily baths and showers, perfume, aftershave, and of course Axe products – everybody smelled like that guy that lives under the bridge.

On the top of this image is a general idea of semiochemicals.

If they work on members of the same species (like mating

signals), then they are called pheromones. If they work on

other species, they are called alleochemicals. Each can be

either attractive or repellant. On the bottom is a homemade

mosquito trap. You might be able to see that it has been

baited with old shoes and grimy socks.

Bacteria feed on the sweat, sugars and proteins that mammals exude, and they give even more semiochemicals. This can make you more or less attractive to mosquitoes. In general, people with many types of bacteria on their skin are less attractive, while those with mostly a few attractive species will get bitten more often. Having a high number of bacteria is a turn off too, probably because that would expose the mosquitoes to more possible pathogens as well. Is it possible to be so disgustingly colonized that even mosquitoes won’t land on you?

Mosquitoes are attracted to several different semiochemicals, including octenol, CO₂, and nonanal. On mosquito antennae, especially the female antennae, there are receptors in the sensilla (see this post) for at least 27 different chemicals in human sweat.

Studies have shown that old socks are a good experimental attractant for mosquitoes. Instead of using an arm or other body part, scientists will compare the attractive ability of someone’s old sweat socks to individual chemicals or mixtures of chemicals. Of course, whose socks you use matters as well. Some people are classified as high attractors (HA) and some as low attractors (LA), so studies often include comparisons of chemicals or mixtures to both HA and LA socks.

But there are other considerations as well. People with blood type O secrete different semiochemicals and are more attractive to many species of mosquitoes. Go ahead, try to change your blood type so you’re less attractive to mosquitoes.

Different species may aim for different body parts. Some seem to prefer feet and ankles, but this may be because they are closer to the ground. If convection currents created by the body heat rising suck the mosquitoes in from below, then it is really the fact that they are following their noses and not going after feet particularly. A small 1998 study showed that mosquitoes that went after feet and ankles preferentially did not do so when the volunteers lied on their backs and raised their feet high in to the air. But, what we have stumbled across in this discussion is body heat.

This is part of a complex figure from a 2011 scientific paper.

In addition to the pretty colors used, the message is that these

researchers identified TRPA1 ion channels on the proboscis

of a species of mosquito. They don’t just sense heat with

their antennae, but also their sucking parts. I wonder if the

interior parts also have TRPA1 to help them find a vessel

when the proboscis is inserted into the tissue.

But what was old is new again…. Scientists are again looking at heat as an attractant for mosquitoes. As compared to HA or LA socks, heat isn’t a strong attractor, but warm socks attract more mosquitoes than cold socks. On the other hand, a 2010 study says that heat and moisture is a greater attractor than heat alone, so it would seem that people working outside in the heat would be the perfect attractors for mosquitoes.

Since heat does seem to be an attractor, it would follow that female mosquitoes would have a receptor for heat. Voila, a new study shows that mosquitoes have sensilla on their antennae and palps that house TRPA1 ion channels. A 2011 study even showed that one malaria-carrying mosquito has TRPA1 receptors on its proboscis. We have talked before about how many mammals use this receptor to sense noxious cold as well as chemicals that cause irritation or pain.

On the left is a species of tick. You wouldn’t believe how big they

can get when feeding on blood. Look it up – I dare you. On the

right is a bedbug. The bedbug is not that closely related to the

tick, since the tick is an arachnid. Count the legs on each – spiders

(arachnids) have eight legs, insects have six. Both these animals

feed on blood, but no one has identified a heat sensor in them.

But in birds, reptiles and insects, TRPA1 is a heat sensor. The 2009 study showed that the TRPA1 were expressed on the female antennae only. But that isn’t to say that only female mosquitoes have TRPA1. A 2013 study indicates that A. gambiae mosquito larvae have functioning TRPA1 so that they can sense water temperature and stay in the most comfortable water.

So mosquitoes (most female mosquitoes) are finding suitable hosts for blood meals by using their senses of taste, smell, sight, and infrared detection. There are other vampire insects as well, ticks, bedbugs, etc. I wonder if they are using heat sensing too. These have yet to be reported on.

Next week, a related question – just how and why do mosquito repellants work?

Maekawa E, Aonuma H, Nelson B, Yoshimura A, Tokunaga F, Fukumoto S, & Kanuka H (2011). The role of proboscis of the malaria vector mosquito Anopheles stephensi in host-seeking behavior. Parasites & vectors, 4 PMID: 21272298

Albeny-Simões D, Murrell EG, Elliot SL, Andrade MR, Lima E, Juliano SA, & Vilela EF (2014). Attracted to the enemy: Aedes aegypti prefers oviposition sites with predator-killed conspecifics. Oecologia, 175 (2), 481-92 PMID: 24590205

Olanga EA, Okal MN, Mbadi PA, Kokwaro ED, & Mukabana WR (2010). Attraction of Anopheles gambiae to odour baits augmented with heat and moisture. Malaria journal, 9 PMID: 20051143

Liu C, & Zwiebel LJ (2013). Molecular characterization of larval peripheral thermosensory responses of the malaria vector mosquito Anopheles gambiae. PloS one, 8 (8) PMID: 23940815

Wednesday, June 25, 2014

They Can See The Blood Running Through You

Biology concepts- thermosensors, TRPV1, hematophagy, taste sense, alternate splicing, echolocation

All three species of vampire bat live in Central to South

America, the common vampire bat (Desmodus rotundus),

the hairy-legged vampire bat (Diphylla ecaudata), and

the white-winged vampire bat (Diaemus youngi).

Any idea what the picture to the left shows? A hint – this may be the most sophisticated piece of machinery ever devised by nature. Together with the organism to which it’s attached, this piece of evolutionary engineering is capable of almost everything a billion dollar jet can do.

It’s the nose of the common vampire bat (Desmodus rotundus). These bats belong to the family Phyllostomidae, one of three families of leaf-nosed bats (Rhinolophidae and Megadermatidae being the other two families). One of the exceptional skills mediated by this nose makes use of the same receptor that makes our mouths burn when we eat chili peppers. Vampire bats can detect the hot blood in your veins from far away!

It’s the noseleaves of the vampire bat that are so amazing, but maybe we should include the rest of the bat head as well. The ears, teeth, mouth, and eyes all work with the nose to give this bat some jet fighter skills.

Leaf nosed bats come in some very odd varieties. The picture on the below and right will give you some idea of the shapes and sizes possible. The question is – what’s the reason for these bizarre growths and why isn’t one odd shape enough? The answer will best be found if we know what their function is, because in biology – form follows function (except for proteins, see this post).

Here are some different leaf nosed bats. Top middle is the Ridley’s leaf

nose bat; bottom left is the Honduran white bat. Top right is the

Commerson’s leaf nose bat and the middle bottom is the greater

spear-nosed bat. The bottom right image is of the cleaf nosed bat of

Vietnam, a more newly discovered variety. It was first described in

2008, but it took 4 years to determine if it was a new species or just

a variant of another species.

Two basic needs of the bat are to find food and find its way. Whether it's a fruit bat, an insectivorous bat, or a vampire bat, a bat must be able to negotiate obstacles within its environment and find a source of nutrition.

To accomplish these tasks, especially given that most bats are nocturnal, they use echolocation. They send out a high-pitched sound, and it bounces off objects and returns to their ears. This is very much like the radar used in airplanes. But this isn’t all they use. Bats can see just about as well as humans; the phrase “blind as a bat” might as well be “blind as a Bob.”

We said most bats are nocturnal. This is Livingstone’s fruit bat

or Comoro flying fox. It is a fruit eating bat of the Comoros

Islands in the western Indian Ocean (just northwest of Madagascar)

and is at least partially diurnal. Other bats may be seen during

the day, but it almost always because they have been disturbed

in their hiding place or they were disturbed in their

feeding the night before.

Bats can also smell their way to food, especially fruit bats. But to answer the question about the noseleaves we have to return to echolocation. Vampire bat sounds emitted for echolocation come through their nose, not their mouth! According to a 2010 study, the leaves aren’t used to gather the returning sound, but to focus the outgoing sound so that the “pictures” formed by the returning echos will be most accurate.

Different shapes help to increase the difference in the reflectivity of objects in the area of focus as opposed to those in the periphery. This allows the various species to hone in what they need to discern and dismiss those things that are uninteresting. Different backgrounds and different needs require different nose leaf shapes.

This answers the question about the wild shapes of noses, but it brings up another question. If vampire bats find their food by echolocation, sight, and smell, then why do they have heat sensors?

To answer this new question, consider the sizes of the vampire bat and its intended prey. The bat weighs about 2.5 oz (71 g), but it needs blood for food (mammalian blood for common vampire bats, bird blood for hairy legged and white-winged species). In fact, vampire bats are the only mammals that completely depend on hematophagy (blood meals). Because of this, they often feed on animals that are over 1000x their size.

Pigs are a favorite source of blood for vampire bats. Here, a

sleeping pig has been bitten on the snout. Why the snout – read

on. Notice the bat can hold its weight with its wings, and that

there seems to be more blood than you would expect from

such a small bite. Again, read on to know why.

To get to the blood of say a cow or horse, vampire bats would have to have teeth so large that they couldn’t lift themselves for flight. No, they have to be very careful about where they bite a victim; somewhere that will bring enough blood to feed on, but won’t cause the huge animal to kill them. Vampire bat teeth are so sharp that prey animals rarely feel the bite, and since the bats are nocturnal, the victims are most often asleep at the time and stay asleep during the feed.

The bats need to locate a place on the sleeping animals where blood vessels are near the surface. This is where the heat sensing comes into play. Vessels close to the surface will give off the most heat to the environment, and vampire bats can “see” these vessels from up to 20 cm away!

The vessels in question need to be covered with less hair, so the bat almost always goes for the lower leg or snout. They will land on the ground, and walk or run up to the prey from behind the animal to make the bite. Vampire bats wings are much stronger than most other bats, so they have an easier time moving along the ground, supporting some of their weight on their wings.

On the left you can see the incisors of the vampire bat. The cheek

teeth and canines are used to shave off any hair from the site, but

the incisors do the cutting. The lack enamel, so they are always

razor sharp. On the right, the tongue is being used to take in the

blood. The tongue is deeply grooved, so the anticoagulant saliva

runs down into the wound and more blood can easily be lapped up.

Their teeth then cut a 5 mm x 5 mm gouge in the victim and they lap up the blood that comes out. It isn’t too common, but vampire bats do feed on humans. This leads us to another amazing skill.

Instinct tells the vampire that a good feeding once will probably mean a good feeding again – if they can find the same animal. So how do they find the same animal several night is a row? They hear them.

A 2006 study showed that vampire bats do tend to feed on the same individual (be it human or cow) for several nights in a row. They can distinguish their previous victim by the sounds of their breathing! Every animal has a unique breathing pattern and sound profile, and the vampire bat can distinguish between individuals to find the one that matched a previous good meal. Imagine if we could find our favorite meal again by listening for the clinking of the right pans!

Returning to a good feeding spot each night, the vampire bat searches for a surface vessel to drink about 1-2 teaspoons of blood (4-5 ml). This isn’t enough to harm the animals, and is what allows them to go back several nights in a row.

Rabies can be spread by bats, and they don’t have to bite you.

When a bat bites an infected animal, it takes in the virus. The

virus grows in the animal and gets distributed to the saliva as

well. Startled bats sometimes spit, and if this gets into your

eyes, mouth, nose, or an open wound, you could contract the

infection. It’s rare overall, but rabies kills about 60,000

people a year.

This doesn’t mean that feeding by vampire bats is without negative consequences. One, the idea of being fed on gives me the heebie jeebies. Two, the vampire bat is a common vector for rabies virus. And three, in the cattle of Latin America, repeated feeding by vampire bats is associated with reduced milk production in dairy cattle and reduced mass gain in beef cattle. So if you wake to find a vampire bat licking your ankle, best to shoo him away and try to breathe differently tomorrow night.

How do vampire bats locate that ankle vessel they need to feed on? Back we go to that amazing nose. The heat sensors of bats are called pit organs, just like in the pit vipers we talked about last week. There are three to four of these organs in the noseleaves of the bat, and a couple across the upper lip as well.

As opposed to the pit vipers, vampire bats have adapted a heat sensor, not a cold sensor to use as their infrared detector for blood vessels. TRPV1, the same receptor that is used for the capsaicin burn and heat regulation in mammals, is present in very high numbers in the neurons of the pit organs.

But this is no ordinary TRPV1. Mammals can’t detect heat from 20 cm away with a regular TRPV1 – this is a modified TRPV1. A 2011 study found that this version of the protein is missing the last three amino acids on the carboxy terminus (the end produced last). This small change increases the sensitivity of the receptor from 43˚C all the way down to 30˚C, so that small differences in heat can be noted from almost a foot away.

One more amazing fact - the bats have regular TRPV1 too. The two version of the protein come from the same gene and the normal one is used throughout the bat’s body for all the things we use TRPV1 for: heat regulation, reproduction, cancer inhibition, etc. Only in the neurons of the pit organs is the mRNA altered after it is transcribed from the gene (alternately spliced) to make the slightly shorter, more sensitive protein.

Here is a cartoon of how blood clots. On the bottom flow chart,

the first anti-co line is where desmolaris and draculin work.

The third line is where desmoteplase acts.

Now our bat friend has located a victim, found a surface vessel and taken a bite to let the blood flow. There’s yet another problem. Mammalian blood clots to prevent loss. The bats must either keep biting, which might wake their prey, or have a way to keep the blood flowing.

Their mouths have specialized salivary glands that make anticoagulants so no clot is formed. There is one anticoagulant that someone with a sense of humor named draculin. It acts to prevent blood clot formation. We have mentioned a second anticoagulant before, called desmoteplase. One of our Halloween posts talked about how it may be good for people that have had strokes. It dissolves any clots that may form.

A 2014 clinical trial is showing that desmoteplase is better than the tissue plasminogen activator clot busters now being used (rtPA), since they have a half-life of four hours (as opposed to 5 minutes for rtPA) and it’s breakdown products aren’t as toxic to nerves and the blood brain barrier as compared to rtPA.

A newer anticoagulant is called desmolaris. A 2013 study showed that it works on yet another part of the clotting system to prevent clot formation. And this isn’t all of them. A 2014 protein survey suggests that there may be dozens more anticoagulant proteins in vampire bat saliva.

Which flying machine is more complex and cool?

Lets add up the vampire bat’s technologies and compare them to an F16. The bat can fly and turn better. The bat has radar and infrared heat detection. It has high powered listening devices that can discriminate between two individuals. Finally, it has biological weapons that allow it to do its work without alarming the target.

All that in a “machine” that can fit into the palm of your hand. Defense aeronautical engineers must feel so embarrassed.

Next week, let’s take it just a bit further. Female mosquitoes aren’t just looking for you, they’re tasting and feeling for you. They use CO₂ gradients as well as my prodigious heat to find me on a warm picnicking evening.

Vanderelst D, De Mey F, Peremans H, Geipel I, Kalko E, & Firzlaff U (2010). What noseleaves do for FM bats depends on their degree of sensorial specialization. PloS one, 5 (8) PMID: 20808438

Patel R, Ispoglou S, & Apostolakis S (2014). Desmoteplase as a potential treatment for cerebral ischaemia. Expert opinion on investigational drugs, 23 (6), 865-73 PMID: 24766516

Ma D, Mizurini DM, Assumpção TC, Li Y, Qi Y, Kotsyfakis M, Ribeiro JM, Monteiro RQ, & Francischetti IM (2013). Desmolaris, a novel factor XIa anticoagulant from the salivary gland of the vampire bat (Desmodus rotundus) inhibits inflammation and thrombosis in vivo. Blood, 122 (25), 4094-106 PMID: 24159172

Gröger U, & Wiegrebe L (2006). Classification of human breathing sounds by the common vampire bat, Desmodus rotundus. BMC biology, 4 PMID: 16780579

Gracheva EO, Cordero-Morales JF, González-Carcacía JA, Ingolia NT, Manno C, Aranguren CI, Weissman JS, & Julius D (2011). Ganglion-specific splicing of TRPV1 underlies infrared sensation in vampire bats. Nature, 476 (7358), 88-91 PMID: 21814281

For more information or classroom activities, see:

Leaf-nosed bats –

http://animaldiversity.ummz.umich.edu/accounts/Phyllostomidae/

http://animals.jrank.org/pages/2868/American-Leaf-Nosed-Bats-Phyllostomidae.html

http://sandwalk.blogspot.com/2012/02/ugliness-of-leaf-nosed-bat.html

http://www.speciesconservation.org/case-studies-projects/ridleys-leaf-nosed-bat-ridleys-round-leafed-bat/3049

http://animals.jrank.org/pages/2860/Old-World-Leaf-Nosed-Bats-Hipposideridae.html

http://animaldiversity.ummz.umich.edu/accounts/Hipposideridae/

http://www.inaturalist.org/taxa/71378-Hipposideridae

http://www.ehow.com/about_6579902_leaf_nosed-bat.html

http://animaldiversity.ummz.umich.edu/accounts/Rhinolophidae/

http://kids.nationalgeographic.com/animals/vampire-bat.html