Thursday, January 20, 2011

Dengue Part 2: The mosquito and its ecology


Last time at Infection Landscapes we discussed the dengue virus, how the disease manifests, and the global burden of dengue disease. This served as a reasonable introduction to dengue fever, but we have not yet discussed the most important aspect of infection transmission, which is the mosquito vector. In this post I will give an introduction to mosquito biology, in general, and the dengue mosquito vector, in particular, while also describing the physical landscape and ecology that will hopefully elucidate this arthropod's effectiveness in transmitting virus.

There are many, many species of mosquitoes. There are roughly 3,500 species known, but the total species may approach or even exceed 4,000. There is staggering diversity among mosquitoes; indeed, there are scores of mosquito genera (41 in total) within the the mosquito family, Culicidae. Mosquito behavior can vary drastically even among different species of the same genus. These differences can have profound consequences for the control of mosquito-borne diseases because different mosquito species may exploit their environment in very different ways, even in instances where they may be transmitting the same infectious disease. The different mosquito species will then require different public health interventions to interrupt human infection. For our discussion on dengue fever, we are focusing on one genus in particular: Aedes.  

There are two species of mosquito that are of particular importance for transmitting dengue virus, Aedes aegypti, and Aedes albopictus, although there are two additional Aedes species that are capable of transmitting the infection. A. aegypti is by far the most efficient vector of all the mosquitoes that transmit dengue virus. This mosquito is the primary vector in areas where dengue fever is endemic. A. albopictus has become important because of a unique ecologic niche it has been able to exploit and so I will also give some description of this mosquito.

So. Here's the major culprit:

Aedes aegypti mosquito

Before delving into the specifics of this critter, and why it is so good at giving us the dengue virus, let's step back for a moment and think about mosquitoes in general and what they do.

Mosquitoes, regardless of species or genus, follow a life cycle. This life cycles follows four distinct stages: eggs, larvae, pupae, and adults. To complete this life cycle, two essential elements are always necessary (see the exception below). One element is vertebrate blood, and the other is water. If you can remember these two requirements you will always have a basis for being able to control a mosquito vector. Here's a picture of what these stages look like, in general, for your average mosquito:


I should note, this depicts Culex mosquitoes, not Aedes mosquitoes, which is apparent from the egg "raft" that this mosquito is positioning on the water surface (Aedes mosquitoes lay their eggs just above the water line so that they dry out). But no matter, the picture is good at giving a general depiction of the 4 stages of the life cycle in any mosquito. Here is nice little video highlighting the 4 stages:



So the mosquito is dependent on these two fluids for survival. The water is the necessary physical environment  for the overall development from egg to larva to pupa to adult, and the blood is necessary for making eggs.Yes, mosquitoes use blood ONLY for making eggs. Based on this, you immediately know two things: 1) mosquitoes don't subsist on blood, therefore they take blood meals much less frequently than the plant nectar they use to generate energy and stay alive, and 2) only FEMALES bite vertebrates for their blood, because the blood is used to make the eggs, and only females lay eggs.

There is an exception to this blood requirement among mosquitoes. The genus Toxorhynchites actually does not require blood. Instead, these mosquitoes develop by feeding on other mosquitoes. These mosquitoes have been suggested as a possible vector control mechanism for mosquitoes that transmit disease.

The mosquito is also highly dependent on climate. Take a look at this picture:


There is more information in this image than we need for our general discussion of mosquitoes because this is highlighting the effect of climate on the mosquitoes that cause malaria. So "sporogonic" development and "gonotropic" development refer to the parasites that cause malaria and can be ignored here. But the image is still very useful to get a sense of the various parts of the mosquito life cycle and its physical landscape that are affected by climate. And we will return to this image later in the series when we cover malaria specifically.

Temperature is a critical parameter to mosquito development. The warmer the water, the quicker the mosquito transitions through its life cycle from egg to adult. The warmer the air (for many species) the longer the mosquito survives as an adult to take blood, develop eggs, and lay those eggs in an aquatic setting. Also, the level of precipitation is critical because this creates new sources of water, maintains water tables, and increases soil moisture. Fundamentally, the landscape of mosquito life is one comprised of aquatic environments that are intimately connected to climate. There are other aspects of the landscape, such as the degree of vegetation cover, the topography of elevation, the density of human habitation, the availability of natural or artificial containers that can collect rainwater, and agricultural land use to name just a few of those that can be important. But the degree of importance played by each of these additional factors varies by mosquito species and the kinds of habitats and behaviors to which the different species are adapted. However, climate and water are fundamental to all mosquitoes regardless of behavior or ecological niche. The landscape of the mosquito has dire consequences for its ability to transmit infections and our ability to control those infections. We will see how this relates to dengue fever below.

Here's an interesting question: how do mosquitoes find you? Well, it turns out that they are very good at sensing two things that you emit: carbon dioxide and heat.Carbon dioxide is a bi-product of cellular respiration and we exhale it when we breathe. There's nothing we can do to stop ourselves from emitting CO2 because we'd have to stop ourselves from breathing, and I don't think anyone would choose assured death over a mosquito bite! So the ability of mosquitoes to identify and locate animals based on an unavoidable part of their natural physiology is an extraordinarily effective adaptation. You simply cannot escape them, they will find you. So, why is heat important if the mosquitoes are so effectively adapted to finding you by way of your breathing? Simple, once the mosquito finds where you are physically located in space, it needs to be able to determine the best surface on your body for accessing a rich blood supply, since it will likely get only one opportunity to make a withdrawal before a hand (or tail) comes swatting. Mosquitoes are capable of detecting fantastically minute differences in temperature across the surface of your skin. These micro-temperature differences are usually due to differences in the distribution of the capillary beds beneath the skin's surface. The mosquito will begin to pick up on the temperature gradients as it approaches a selected host. If it selects the area with the greatest accessibility to blood, then it has the best chance of getting a sample. Of course, the mosquito has an additional arsenal to aid in its blood collection: it's saliva. Mosquito saliva is a chemical cocktail of vasodilators and anticoagulants (to help increase blood flow to and through the mosquito's proboscis, respectively), as well as other proteins that circumvent platelet aggregation and enhance inflammation.

These are the basics, and in fact there are many other cues that mosquitoes use to identify their hosts. For example, octenol is another chemical that is released by humans during breathing and sweating and is an attractant for mosquitoes. There are also unique signatures among difference hosts of the same host species that mosquitoes may find preferable. We've all experienced this when we've been outside during the summer months and certain people among us are constantly getting bitten, while others are barely touched. Much to the disgust of my friends, I am usually in the latter camp. There are also myriad environmental cues that can account for differences in biting behavior between different mosquito species. For example, many mosquitoes are most active in taking a blood meal during the dusky hours of dawn and evening. But many anopheline mosquitoes feed exclusively at night, and Aedes aegypti mosquitoes prefer to feed during the day.

Let's turn now to the dengue vector itself, the Aedes genus, and that most important species of them all, Aedes aegypti. Historically, A. aegypti became famous for another disease that it is capable of transmitting: yellow fever. Yellow fever is also caused by a virus and also causes a hemorrhagic disease but with a higher case fatality than dengue fever or DHF. The realization that A. aegypti was a major problem with respect to yellow fever came from Cuba. In fact, since yellow fever was the first infectious disease identified as being transmitted by way of mosquito, the mosquito hypothesis of disease transmission, and thus vector-borne disease transmission, was established through work done in Cuba. Often, Walter Reed is credited with the work in Cuba that established the vector relationship and led to the elimination of much of the yellow fever endemic there in the late 1800s. However, it was the Cuban doctor, Carlos Finlay, who identified and proposed the mosquito as the vector for yellow fever in 1881, and which Walter Reed was able to mobilize his significant United States military resources to test extensively. Walter Reed, himself, assigned the credit for the discovery properly to Dr. Finlay. But Western history remembers it differently, probably because Western history was not written by the Cubans.

Carlos Finlay

Following the recognition of A. aegypti in transmitting yellow fever, a massive campaign was undertaken to eliminate the mosquito from the surrounding tropical forests during the building of the Panama Canal, with modest effect. However, in 1947 the Pan-American Health Organization (PAHO) began a massive program to eliminate this mosquito from the whole of the Americas. And this time the campaign was extraordinarily successful. By 1972, A. aegypti had been eliminated from 73% of the habitat that it occupied prior to 1947, primarily without the use of insecticides. However, when it was discovered that a jungle cycle of yellow fever existed that could not be eliminated by the campaign, tragically the intensive program was abandoned. By the close of the twentieth century, A. aegypti had newly colonized more geographic area than it had prior to 1947, and occupied a much greater global distribution as well. This map published by Emerging Infectious Diseases tells the story best:

Shaded areas indicate presence of A. aegypti

And this map demonstrates the current global distribution of this mosquito:

Red: dengue is endemic. Red+Blue: A. aegypti is present


Let's think about this PAHO program for a moment. What was so effective about the campaign that reduced the A. aegypti mosquito so extensively throughout the Americas? What unique aspects of this mosquito's ecology make it such an efficient vector for dengue (and yellow fever)? And how can we change that ecology to reduce dengue transmission?

First, the unique aspects of A. aegypti. This mosquito has a very particular preference for the water environment it selects for laying its eggs. It likes SMALL containers that collect rainwater. And the mechanics work as follows. This mosquito does not lay its eggs either in the water or on the surface of the water, as most other species do. Instead, A. aegypti lays its eggs above the water on the interior wall of the vessel containing the water so that when the water vessel is refilled, from the water line at which the mosquito laid its eggs to the lip of the vessel, the eggs will have enough time to complete their developmental cycle to adulthood before evaporation depletes the water source. A truly incredible evolutionary adaptation.

This mosquito is originally adapted to a forest habitat wherein it would seek out holes in trees that would regularly collect rainwater. Tree holes are much more ubiquitous than you might think in a forest (think woodpeckers), and so this is quite an effective niche for this mosquito. As humans encroached more and more on forest habitat establishing agriculture, and building increasingly dense communities and living conditions, A. aegypti readily adapted to the new circumstances. The mosquitoes found an abundance of new and highly effective small containers strewn in and around households that can easily collect, or are intended to store, water. The mass production of plastics has been a major factor in the proliferation of potential water containers. Today A. aegypti is just as much an urban mosquito as it is a forest mosquito and probably more so. As such, A. aegypti is now uniquely adapted to the human environment. Unlike other mosquito species, they will often live in the household with humans, and can complete their whole life cycle here. They also bite during the day, so they have unlimited access to humans for taking blood meals. And finally, this mosquito's preferred host, as you may have already guessed, is humans. Because this mosquito is so very effective at exploiting the human environment, it is also very effective at transmitting any viruses that it is capable of carrying and which are infective to humans. So, with the global redistribution of A. aegypti, the stage was set for the transmission of dengue virus within the tropical and subtropical latitudinal lines of the globe. 
     
The PAHO program was so effective in eliminating A. aegypti because it utilized a campaign to eliminate all open water containers in and around the household in every community.While this was an extremely labor intensive public health intervention, it was simple to employ and it only needed to target places of human habitation. Stopping the sylvan, or wild, mosquitoes in their original forest habitat was not nearly as important as eliminating the mosquitoes around the home...at least that is what the case would be for dengue fever. However, for yellow fever, which was the impetus for the PAHO campaign to eliminate A. aegypti in the 1940s, the identification of a jungle cycle that could still make yellow fever viable resulted in the abandonment of the intensive campaign. Unfortunately, the very dangerous situation now facing the world with potentially billions at risk for the lethal form of dengue, i.e. dengue hemorrhagic fever (See post Dengue Part 1), could have been obviated if the PAHO program had not been abandoned.

Nevertheless, the take home message is clear. This mosquito can be controlled with vigilant maintenance of open water containers in the home and its surroundings. The emphasis must be placed on "vigilant control" because it takes everyone in a community to be dedicated to eliminating this water source to reduce transmission in most places in the world where dengue is endemic. Pesticides can be used (e.g. they are the primary mode of vector control for controlling dengue in the US now that it is emerging here) and they are effective as well, but their application is cost prohibitive to control efforts in most places in the world. Instead, by changing the landscape of the mosquito where that landscape intersects with the human landscape, we can yield dramatic results in dengue transmission reduction.

Next week in our last installment of the dengue series, I will give a description of dengue fever in The United States, which now occurs through autochthonous transmission, i.e. NOT imported.We will also discuss how the global redistribution of another dengue mosquito vector, Aedes albopictus, has perhaps reconfigured the landscape for how far north dengue may be able travel. This temperate climate mosquito may have implications for spreading the disease into the northeastern, northwestern, and upper midwestern United States. But it is unclear how this actually will play out because the habits of A. albopictus are different. Finally, we will touch on how climate change may also be reconfiguring the landscape of dengue fever.

19 comments:

  1. Bio-engineers are in the process of developing genetically modified mosquitoes, to be released in Malaysia, as a means of reducing the Aedes mosquito populations. Pretty wild stuff. Check it out:

    http://www.huffingtonpost.com/2010/10/11/dengue-fever-to-be-fought_n_757839.html

    -Becca

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    1. Interesting. Genetic engineering is fascinating. Engineers come together to develop modified versions of mosquitoes by altering genes of the original but no one, or no group of people, has ever successfully created an original mosquito, or fly, or even a flea. Very humbling.

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  2. I read about a biological control program in Vietnam, using crustaceans that feed on the larvae of A. aegypti as well as another program in Cambodia which uses long-lasting insecticide treated netting covers used for water containers around the home as vector control measures for dengue. Have these programs proven to be effective and if so are they feasible in other areas where dengue exists?

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  3. Becca-

    That sounds like a very cool project if it works. The thing that always seems short-sighted in my mind about attempting to eliminate a particular insect vector is that it is part of a food chain-- other animas eat mosquitoes. This seems like it would both perhaps help against dengue and not have as many ecological problems.

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  4. The project in the link placed by Becca sounds amazing but I am confused as to why don't we just use Toxorhynchites. The blog mentioned that "these mosquitoes have been suggested as a possible vector control mechanism for mosquitoes that transmit disease". Why genetically engineer something to take care of the situation when we already have something that could be the solution.

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  5. Post: Dengue Part 2. The efficiency of the mosquito is quite outstanding. But the mosquito with the genus Toxorhynchites that feeds off other mosquitoes and not humans is also interesting to me. How successful have the efforts been as a control mechanism for the disease?

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  7. I hate mosquitoes not because they're annoying but because of their possible effects on our health. I always consider insecticides or repellents to get rid of them.

    heat treatment for bed bugs

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    1. I hope you live in a colder climate, it makes it less likely to have adverse health effects, due to their inability to carry the pathogens founds in warmer climates.
      Yet! i assume you are not as concerned about yourself as you are with "our health" primarily focusing on the populations that are at greatest risk around the world, such as in warmer climates. in such circumstances repellents dont always do the job - or some people do not have access to the repellents.

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  8. This article from 2009 (http://www.cdfd.org.in/lmg/PDF/APJMBB2009b.pdf) considers the potential risks of the GM mosquitoes released into the Aedes egyptii ecosystem, now a fait accompli in Malaysia. It cites several negative potentialities: increased in disease transmission of Aedes Albopictus (by increasing their competitive advantage and population size by curbing the Aedes egyptii population, I imagine); disruption of the food chain; enhanced immunity to insecticides for the GM mosquitos; and soil and water quality degradation. The outcome of the trial release of GM mosquitoes in Malaysia might be highly encouraging to supressing dengue transmission if these concerns proven to be only minimally important.

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  9. It's amazing that the PAHO intervention in the 1940s was so successful at reducing rates of dengue cases. Getting those kind of results in any intervention is very rare. It made me think of the traditional public health practice; mobilizing people to improve the health of the population. An intervention like this seems almost too simplistic in today's world, but it certainly was effective. I wonder why PAHO decided to halt the program upon discovering the jungle cycle of yellow fever. It was a low-cost program producing great results. Although this mosquito is ubiquitous in the home, that works to our advantage because the life cycle can be interrupted before the larval stage. With malaria, it seems most initiatives tackle the adult stage, which is what makes it much harder to control these vectors.

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  10. As several others have made mention, I find the prospect of using genetically modified Aedes mosquitos a very interesting proposal for the control of dengue fever (as well as, for that matter, other viruses that use this vector). Indeed, it seems like Brazil has recently joined Malaysia in releasing GM mosquitos into the environment, in efforts to eradicate the A. aegypti species. However, as several others have posted, elimination of any species (even a pestilent insect), while effective and likely of general benefit, does have an unsavory feel to it (ie. will it effect the rest of the ecosystem).

    As an alternative, I ask why can’t mosquitos be genetically modified without the intention to kill off the species, but with the goal of making them a nonviable vector? For example, I read that years ago, recombinant Sindbis virus were used to tranduce a specific genetic sequence into female A. aegypti, ultimately making them unable to support replication of DEN-2 virus in their salivary glands, and thus curtailing transmission of the virus. With today’s advance in genetic manipulation, if we could introduce such “vector negative” GM mosquitos into the environment, and ensure that they could pass on their genetic modification to their offspring, perhaps this might prove to be additional, eco-friendly arm through which we might combat the spread of dengue.

    Finally, if we were to couple this method with other viable ideas (ie. resurrection of the preventative measures used the PAHOS program, or introduction of such predators as the Toxorhynchites genus mentioned in post), a multi-pronged approach might prove both efficient and potentially cost-effective at curtailing the further spread of dengue fever (a fear that become more and more real here for us in the US, as temperatures continue to rise).

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    1. I can appreciate this! It seems like the trend to create GM insects in efforts to kill off other insects, but not every idea is perfect as yours and other posts have mentioned. I agree that the PAHOS program should be revived especially since it is now more common for certain vector transport to span long distances due to shifts in and changes in climate. I think the revival of such a program and increased awareness on the possible impending emergence to be a very important endeavor to embark on in the present.

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  11. It is fascinating to learn that the population of A. aegypti was reduced so effectively after 1947 and that the population adapted so well to urban human environments after the program was abandoned. Given the prevalence of Dengue, the risk of people getting DHF and now the threat of Zika too, there is an urgent need to resurrect the PAHO program of getting rid of standing water and rain collecting containers. This would involve community outreach and education, but it is a simple message which could be communicated easily and I think it might be adopted readily given how aware people are of Zika at the present time. It’s not often there is a simple solution like this that could make such a huge difference so I hope it is tried out. Vaccine development is another route of attack, but this is hampered by the need to cover all serotypes at once.

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  12. When considering the evolution of A. albopictus to thrive in temperate climates rather than in the tropical and sub-tropical environments required for A. aegypti, I was wondering if any genomic studies had been considered to compare genomes between the subspecies? Also, could A. albopictus attribute its survival to a certain gene or subset of genes in the genome that have become activated due to some sort of environmental pressure?

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  13. Though PAHO has discontinued its campaign against A. aegyti because of the jungle cycle, wouldn't it still be pertinent to restart a similar intervention effort? In class, it had been mentioned that there is a tremendous population that is at risk for dengue hemorrhagic fever because of possible geographic migration of the different types of Dengue, so I find it interesting that more has not been done as a preventative measure.

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  14. We covered this disease in class and I find the spatial patterns of the mosquito especially intriguing. It is already known that those infected with the other serotypes are at an increased risk for the disease. It would be interesting to know the magnitude of risk and likelihood of concurrent serotype co-infection.

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  15. The mosquito has a lifespan of 3 weeks. the warmer the climate the quicker the mosquito transitions through the lifecycle. Therefore, for the mosquito to harbor a disease, it need to make it through the cycle before it dies. Therefore, diseases such as Dengue are found primarily in warmer climates where the mosquito can make it through the lifecycle in time.

    With global warming (or just warmer seasons), disease can spread to new parts of the world that were too cold previously. this is a major public health concern as diseases are spreading to new areas and new populations.

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    1. Shmuel, this is a good point-- especially for the A. aegypti. With the A. albopictus vector being able to thrive in temperate climates and global warming combined, the ease at which dengue could be transmitted in the 21st century is certainly an important issue to be considered in public health-- the seriousness of the matter cannot be understated.

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