Friday, March 18, 2011

Malaria Part 2: The vector


Last time I described the complex life cycle of the parasite that causes malaria. You'll remember from Part 1 that malaria in humans is primarily caused by four species of Plasmodium. You'll also recall that the life cycle of Plasmodium, irrespective of species, requires development in both the human and vector hosts. While we've discussed the parasite stages in both the human and vectors hosts, we have not yet discussed the vector, its ecology, and why these are important for producing malaria in humans.

The vector for malaria is once again a mosquito. However, the relevant mosquitoes are quite different to those we have covered so far in relation to dengue virus and West Nile virus. Anopheles is the mosquito genus that is capable of transmitting the malaria parasite to humans:

Anopheles gambiae

There are many, many anopheline species that are capable of infecting humans with the Plasmodium sporozoites. In fact, there are between 70 and 100 Anopheles species that can transmit malaria to people, and approximately 40 species that would be considered effective at transmitting the infection. These are the important vectors. We will not cover each of these important mosquito species, but keep in mind that the species, along with their behaviors and biologies, are many and varied, which can make vector control a very complex and difficult task in efforts to reduce malaria transmission. Here is a map of the global distribution of Anopheles species that are important vectors for malaria:



Anopheline mosquitoes, like most mosquitoes, are "vegetarian" and require a blood meal only for the production of eggs rather than subsistence. (They subsist on plant nectars, so they are not really vegetarian either.) You'll remember that among the other mosquitoes we've talked about, only females take blood meals, and only those adult females who are preparing to produce eggs. All anopheline mosquitoes are the same in this respect. Nevertheless, there can be tremendous differences among the species in preference of vertebrate host, biting and resting behavior, and selection of sites for oviposition (i.e. laying the eggs).

The anopheline mosquitoes also require a similar transition through four stages of development to complete the life cycle. Here is a comparison between the three mosquito genera we've covered at Infection Landscapes, with Anopheles on the far left. All four stages of the life cycle are depicted for each genus:



Notice the adult mosquitoes in the picture above. One of the distinguishing features of anopheline mosquitoes is the roughly 45 degree angle their abdomen forms with respect to the surface on which they land. This is unique to Anopheles and can be used to identify the mosquito. Keep in mind however that after taking a blood meal the abdomen will be heavy and weighed down form the extra mass, and so will likely no longer form this distinguishing 45 degree angle.

Soon after emerging as adults, anopheline females will mate before taking their first blood meal. They typically will mate just once and then store the male's sperm to fertilize the 200-1000 eggs that will be produced in batches of 3 to 12 over the course of the remainder of their adult lifetime. A fresh blood meal is required for the development of each batch of eggs. After hatching, the larvae will feed just below the water's surface continuing to develop over the next 5 to 15 days before pupating. The adult mosquitoes will emerge from the pupae in another 2 to 3 days. Depending on the Anopheles species, and how favorable the environmental conditions, the whole life cycle of the mosquito will require between 7 and 20 days. If temperature and humidity are high enough, the time to completion to adulthood will be reduced and the length of the adult lifespan can be extended up to a month or longer, thus providing ample time for the completion of the sporogonic cycle of the Plasmodium parasite. Keep in mind that once the sporogonic cycle of the parasite is complete in the adult mosquito, that mosquito is then capable of infecting humans with each taking of a blood meal, which typically occurs every 2-3 days for the remainder of the adult mosquito's life.

Here is a nice video that gives a quick summary of the four stages of the mosquito life cycle and also compares some differences among the three genera covered at Infection Landscapes:



Similar to past mosquitoes we have discussed, the anophelines seek out their hosts by following concentration gradients of carbon dioxide, body odor, heat, and movement. Most anophelines feed during the night, but some will also feed during the dusky hours of morning and evening. Their night preference means that humans are at greatest risk of infection during sleep, when we are at our most vulnerable. As a result, bed nets, and especially bed nets treated with insecticide, have become one of the staple approaches to intervention and control efforts to stop the transmission of malaria in high endemic areas. Here is an example of some bed nets:


While bed nets can be very effective in reducing malaria morbidity and mortality, they can also fail completely if not used properly. The complex issue of bed net effectiveness, as well as the socio-political context of  their distribution will be covered in detail in a later post.

Because we are talking about these mosquitoes in the context of causing malaria in humans, it is typical to take an anthropocentric view regarding some of their behavior. For example, those anopheline species that prefer a variety of vertebrate hosts are known as zoophilic, while those species that distinctly prefer humans are knowns as anthropophilic. Some anopheline mosquitoes prefer to take their blood meal indoors. These are endophagic. Others are exophagic, meaning they prefer to take the blood meal outside. Another important distinction among species is determined by what they do after they take their blood meal. All mosquitoes must rest for period of time after taking blood, but some are endophilic rest-ers, meaning they prefer to rest inside, while others are exophilic, which means they prefer to rest outside. This aspect of mosquito behavior is very important for mosquito control efforts, which may include residual spraying and thus would need to target inside or outside the home depending on the resting preference.

Most species of Anopheles mosquitoes prefer to lay their eggs in clean water, which is quite different to the Culex and Aedes species we have covered previously. While this helps to characterize anopheline ecology somewhat, there can still be great differences between individual Anopheles species with respect to their water preferences for oviposition. Here is a depiction of some different potential breeding sites published in: Keating J, Macintyre K, Mbogo CM, Githure JI, Beier JC. Characterization of potential larval habitats for Anopheles mosquitoes in relation to urban land-use in Malindi, Kenya. Int J Health Geogr. 2004 May 4;3(1):9. (PMID: 15125778)

Pictures illustrating the types of habitat identified by strata during this study: (A) Swimming pool in well drained tourist area; (B) Broken water pipe in well drained residential area; (C) Open water tank in poorly drained area; (D) Pond in poorly drained area; (E) Drainage channel in well drained area; and (F) Ditch and tire tracks in poorly drained area.

Let's talk a little more specifically about water preference. For example, Anopheles gambiae is the most efficient vector for transmitting malaria to humans. It prefers small sunlit pools, and it's natural habitat is tropical forest. In natural, undisturbed habitat, this mosquito is limited in abundance by the distribution of breaks in the tree canopy that allow the sun to reach the forest floor. However, when habitat is disturbed, due to deforestation or agriculture, for example, much larger areas of land cover become exposed. In this situation any pools of collected rainwater can receive direct sunlight and provide ideal and abundant breeding for A. gambiae. Such habitat disturbances often also coincide with increased human proximity, and so more and more people come into greater contact with more and more of this efficient malaria vector. Here are a few CDC pictures of some diverse land cover that A. gambiae can make use of:

Irrigation in forest ecotones
Rice fields
Tire tracks

On the other hand, A stephensi is a species quite different to A. gambiae and one which may be more suited to the "human environment". This mosquito prefers contained water sources such as tin cans and water containers around the home. If you remember, this is very similar to Aedes aegypti, which is a mosquito extremely well adapted to the human home environment. There are more differences in water preferences between all the Anopheles species, but the two species described above provide a simple contrast of some water preferences that illustrate the need for different approaches to vector control if the water environment is the target.

Control of anopheline mosquitoes typically is comprised of several domains. The first entails control of breeding sites, i.e. water sources. The second entails control of the larval stage of the mosquito as it lives and develops in the water. The third entails control of the adult mosquitoes, either prior to taking a blood meal or following the blood meal.

Control of breeding sites requires the elimination of viable water sources for Anopheles oviposition. This can be quite a daunting task because of the immense diversity in preferred water habitat across the different species of Anopheles mosquito. Nevertheless, limiting human impact on natural resources, particularly forest transformation, can go a long way. This is, however, a long-term approach that must overcome societal, governmental, and economic constraints that simply may not be amenable to change for the sake of malaria reduction. Nevertheless, more localized efforts may focus on minimizing the number of potential rainwater collection areas in and around areas close to human habitation. The task is still incredibly daunting, especially given the sheer amount of rainfall in tropical climates.

While elimination of water sources suitable to mosquito breeding may not be possible in most circumstances for Anopheles, targeting the water source can still be a viable means to control the mosquito larvae. Instead of eliminating the water source, such interventions take advantage of the larval water environment by  introducing agents that can kill the larvae. These may include chemical agents, such as pesticides, but often they may include options that do not pollute potentially important water sources with chemical substances. For example, predatory animals, such as certain kinds of fish or other insects, can be introduced to feed on the larvae and thus reduce the numbers of mosquitoes reaching adulthood.

Here is one researcher who is exploring pathogens and predators as viable non-chemical means of mosquito control:



Notice the emphasis on "natural enemies" for mosquito control. The use of natural enemies is very important as the introduction of an alien, potentially invasive, species may control mosquito populations, but may also have damaging effects on the larger ecosystem.

Control of adult mosquitoes can take various forms depending on when they are targeted with respect to taking the blood meal.

Intercepting the mosquito with insecticide treated bed nets (ITNs) before it can take a blood meal from sleeping humans has become a major staple of malaria intervention. ITNs can be quite effective, but they need to be used correctly and they need to be widely distributed, adopted, and correctly maintained in order to translate to reduced transmission. For example, some barriers to ITN effectiveness can be comfort and cleanliness. In many tropical areas, the temperatures are often quite high and the tight mesh of the ITNs typically does not allow much breeze to pass through in the night. As a consequence, sleeping under an ITN can be quite uncomfortable in endemic areas where they are most needed. People may opt for comfort over malaria protection if it improves their sleep. Similarly, the ITNs easily collect dust and become dirty, which means people want to wash the nets regularly. However, if the ITNs are washed without being impregnated again with the insecticide, they will not effectively kill landing mosquitoes and some will be able to access the sleeping person beneath.

Here is a video on ITN distribution. I would like you to view this with a critical eye. As you watch, think about what factors are at play in the distribution of these nets. Who gets the ITNs, and who is distributing them? What are the global entities that determine this distribution and why? Are any of the logistical issues mentioned above, or others, addressed as the health workers give instructions on using the ITNs? Will the circumstances described lead to equitable distribution and use of ITNs? Who are the real agents of malaria control in this context? Who is telling the story, and what are their interests? Why was this video made?



We will examine the social, economic and geopolitical constructs of malaria intervention in a later post, but it is useful to begin thinking about these issues now as the topic is introduced.

The resting mosquito offers another point of intervention. You will recall that immediately after taking a blood meal, the mosquito must rest. Some rest indoors and some rest outdoors. Targeted insecticide spraying will aim to cover the resting surfaces of the mosquitoes so that they are killed after taking the blood meal. This will not prevent infection in the person from whom the blood meal was taken, but it will stop transmission by killing the mosquito before it can infect someone else. Residual spraying on walls in the home is a particularly common control measure, though this is of course limited to endophilic Anopheles species. Exophilic mosquito targeting is more difficult because potential resting surfaces are much more dispersed in the outside environment.

It is becoming increasingly clear that top-down approaches to vector control will not be effective in many, if not most, areas where malaria is endemic. In order to provide better solutions for controlling the anopheline mosquitoes, efforts must include community-based insight and leadership. For example, community-base approaches to ITN distribution, use, and maintenance will ensure greater dissemination of the nets and their effective use. Another critical factor will be the increased emphasis of community driven approaches to mosquito control in general. Individuals will have insights into the nuance of the microgeography of their communities that outsiders will not have. This information can be use to develop and direct new efforts at controlling mosquitoes in a particular region. Moreover, resources and support should be provided to these kinds of community-directed efforts, which should also include support for ecologic research.

In the next post, I will describe the malaria disease process. Stay tuned.

10 comments:

  1. I was struck by the fact that mosquitoes are vegetarians, very interesting. It is clear that controlling these vectors can be quite complicated. While introducing new species to water sources seems like a better idea than using pesticides, I am concerned about the long term impact of these introductions. Has anyone come across information on the impact of mosquito control on biodiversity?

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    1. I've been curious about that too, Stephanie, how does killing mosquitoes affect the ecosystem? A few papers I've read say that there aren't many predators that specialize in mosquitoes. Although there are some mosquito species that prey on other mosquito species so the question would be what could killing the prey mosquitoes mean for the predator ones.
      On the other hand, from what I know, mosquitoes are not very important pollinators so controlling these vectors might not mean immense harm for plant populations.

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    2. There are about 3,500 different mosquitoes that exist. Out of that, only a few hundred bite or bother people. Mosquito and mosquito larvae are essential food for fish, other aquatic animals, bats, spiders and dragonflies (and others) and often employ the lowest level of the food chain. As we know they are also responsible for transmitting many infectious diseases. That being said... many believe f they were eliminated completely... it would have a devastating effect on the ecosystem. That being said.. some scientists believe that this niche would be filled by other insects ... resulting in a lot less infectious diseases that we need to worry about.

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  2. Allison GrossmanJuly 17, 2013 at 2:40 PM

    I was researching DDT and its use for indoor residual spraying as vector control against malaria for an environmental health class, and I was surprised by the limitation of this measure as described in this blog post. What percentage of mosquitoes in the malaria-affected parts of Africa are endophilic and how many are exophilic?

    I was also wondering about the potential ecological effects of killing off mosquitoes even with "natural" enemies. Is the idea just that we'll never really kill them all off?

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    1. Sheaba Daniel

      According to the source listed below, it’s thought that in sub-Saharan Africa, the mosquitoes are largely endophilic, though exophilic mosquitoes of the same species have also been observed.

      In terms of the potential ecological effects of killing off mosquitoes, there would be several effects to consider. Without mosquito larvae, many species of fish would have to find another source of nutrition, plant species would have to find another pollinator, but overall, these fish, birds and other insects that feed on mosquitoes would find other sources of food to survive and other insects would help in the pollination process. But eliminating mosquitoes would mean that there would be less spread of pathogens in the human population and less of a burden on human morbidity and mortality.

      Indoor resting by outdoor biting females of Anopheles gambiae complex (Diptera:Culicidae) in the Sahel of northern Senegal.
      Faye O, Konate L, Mouchet J, Fontenille D, Sy N, Hebrard G, Herve JP, J Med Entomol. 1997 May; 34(3):285-9.

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    2. Mosquitoes can have both positive and negative impacts on the ecosystem. I agree that as part of their useful role, the larvae of mosquitoes live in water and provide food for fish and other wildlife, but I believe these species will find other food source (say moths). Eradicating it would certainly be a huge relief to many people because not only are mosquitoes the deadliest (based on its disease burden) but also the most annoying insect in the world.
      While we don’t exactly know the outcome of getting rid of all mosquitoes, because we do not know everything about them yet, a possible way of eradicating them without killing any other living things, is a new plan scientists are working on, to release males with a lethal gene which prevents their offspring from surviving.

      http://www.mosquitoreviews.com/mosquitoes-ecosystem.html

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  3. This post was a very interesting and informative read. I was unaware that mosquitoes mate before their blood meal and this meal is necessary for the development of the batches of fertilized eggs. I was also surprised to learn of the many species of mosquitoes. We learned that the anopheles gambiae is the most effective species in effectively transmitting malaria to humans., making them endophilic. These mosquitoes prefer small sunlit pools for breeding and their natural habitat is tropical forests. In their natural habitat they are limited because of the lack of sunlit that hits the forest floor. Unfortunately, do to human expansion this species is able to breed because of deforestation. Once an area that is ideal for A. gambiae is disturbed the rainwater receives direct sunlight and then becomes ideal for breeding. This information is useful as it gives us an understanding of the need for differentiated vector control. Each species has a unique breeding preference and resting preference and effective malaria control has to target these ever diverse conditions.

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  4. Racquel BreretonJuly 23, 2014 at 2:20 PM

    It's very interesting to learn of all the dynamics behind mosquito bites (female restricted, blood meals used for reproduction, sustenance from plant nectar). It's also amazing how something seemingly so simple (and irritable) is a quite complex system.

    Intervention for the spread of malaria is a challenging feat as the landscape of high endemic regions often promotes mosquito abundance. Reducing the water access to mosquitoes is nearly impossible and discarding water collection containers stationed around residences may have little impact. I'm not sure if a vaccination is in progress but I think vaccination would be the best measure for to reduce malaria transmission.

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  5. Malaria is a complex disease, that includes humans, mosquitos, and the parasite it self. Controlling malaria have to be on multiple fronts, by disrupting the life cycle of the parasite in humans and also in the mosquito, by control the mosquitos population itself and by avoiding transmission between human and mosquitos.
    Having a natural predator or agent for the mosquitos, either be it flies, a virus , or a bacteria could be a promising way to control malaria without harming the environment or humans by using chemicals or medications.
    We hope that the new advancement in biotechnology help in that front by developing biological agents to control the mosquitos and larva .

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  6. One method of mosquito control that was not mentioned in this post is insect repellent. The many different forms or repellent available and the easy in which one put it on makes it a viable intervention method. Most effective mosquito repellents contain deet (N,N-diethyl-3-methylbenzamide) which is highly useful in repelling mosquitoes.
    Moreover, if spraying oneself consistently is too bothersome an may be forgotten then together application of deet can be used:
    Bracelet repellent: Although not overly effective over the entire range of the body, several bracelets can have a significant effect. Manufacture of different style bracelets can create a trend.
    Perfume/Cologne/Deodorant/Detergent: Creation of cosmetics and hygiene supplies with mosquito repellent can be useful.
    Spray Machines: Small little automatic sprayers that can be placed in rooms of households, schools, and workplaces (like the ones they have in bathrooms) can distribute repellant effectively.

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