Wednesday, May 16, 2012

Lymphatic Filariasis

This week at Infections Landscapes, I will cover the last of the nematode infections in this extended helminth series. Lymphatic filariasis, which includes the advanced form, elephantiasis (often incorrectly referred to as elephantitis), causes extensive disability in several tropical and sub-tropical regions of the world. Like onchocerciasis, lymphatic filariasis is caused by filarial worms and is vector-borne.

The Worm. Lymphatic filariasis is caused primarily by three main helminth species in the Onchocercidae family of nematodes: Wuchereria bancrofti, Brugia malayi, and Brugia timori. Approximately 90% of infections are caused by W. bancrofti, while most of the remaining infections are caused by B. malayi (~9%) and B. timori (~1%). Humans are the reservoir for W. bancrofti, while non-human hosts serve as reservoirs for Brugia species, and thus infections with the latter result from zoonotic transmission.

Wuchereria bancrofti

In the human host, the adult worms concentrate in the vessels and nodes of the lymphatic system. Gravid females release motile sheathed microfilariae, which migrate to the peripheral circulation. It takes approximately 6 to 12 months from initial infection to the time when the microfilariae begin to appear in the periphery. These peripheral microfilariae exhibit a fascinating circadian rhythm, wherein they migrate at night into the peripheral blood via the circulation, returning to the arterioles in the lungs during the day. This migration to the periphery aids in the transmission of the microfilariae to the vectors, which are primarily, though not exclusively, night-biting mosquito species. After the microfilariae are ingested by the mosquito, the sheath is lost and they migrate out of the mosquito stomach and into the flight muscle tissues of the thorax. At this loaction, the microfilariae will molt three times, developing into the infectious 3rd stage larvae (L3) after approximately 10 to 20 days. The exact time required is dependent on the density of infection in the mosquito and the local climate conditions at the time of development. Infectious L3 larvae migrate to the proboscis of the mosquito. The larvae are not directly injected into the human host by the mosquito, but rather are deposited on the skin and migrate into the bite wound. The L3 larvae then migrate through subcutaneous tissues to gain the lymphatic system, where they will develop into adults over the course of approximately 1 year. Male and female adults will mate soon after reaching maturity and gravid females subsequently release their motile microfilariae. The following graphic developed by the Centers for Disease Control and Prevention (CDC) nicely depicts the life cycle of the nematodes that cause lymphatic filariasis:

The symbiotic relationship between this helminth and a bacterium constitutes another critical, and fascinating, component to the worm's life cycleWolbachia is a genus of bacteria that commonly infects many species of insects, and a few species of nematodes. These ubiquitous bacteria can engage either parasitic or symbiotic relationships with their hosts. In the case of W. bancrofti, L3 larvae cannot complete their adult development without this bacterial infection, and, as such, the helminth's life cycle is arrested in the human host.

Because the vast majority of infections are caused by W. bancrofti, this discussion will focus primarily on bancroftian lymphatic filariasis. However, certain aspects of infections caused the Brugia spp. (i.e. brugian lymphatic filariasis) will be discussed where noted.

The Vector. Mosquitoes are the vectors for the helminths that cause lymphatic filariasis. The species capable of transmitting these filarial nematodes are many and varied, making the medical ecology of lymphatic filariasis extraordinarly complex, and the landscape epidemiology hyperlocal. W. bancrofti is vectored by Culex mosquitoes (primarily Culex quinquefasciatus) in most urban and semiurban areas of the world where this helminth is endemic. However, anopheline mosquitoes are important vectors in rural areas, especially in rural areas of sub-Saharan Africa, and Aedes mosquitoes are important vectors in southeast Asia and the endemic islands of the Pacific. The primary vectors for Brugia nematodes, however, are Mansonia mosquitoes. We'll briefly examine the ecology of each genus of mosquito below, although it is important to keep in mind that the behavior of species within genera can also vary widely, especially for anopheline mosquitoes. 

Culex Mosquitoes

Culex mosquito taking a blood meal

Culex mosquitoes seek out dirty water, rich in decayed nutrients, for oviposition, and lay their eggs on the water's surface in the form of an egg raftCulex mosquitoes bite primarily at night, and also during the dusky times of dawn and sunset. Different Culex species have very different preferences with respect to their hosts.

Culex life cycle

Culex quiquefasciatus is the most important Culex mosquito vector for lymphatic filariasis. This Culex species is highly anthropophilic, and so prefers to take its blood meals from humans, unlike other Culex vectors that are ornithophilic (e.g. Culex pipiens pipiens). This preference for humans, and stagnant water around human habitation for larval development, make this mosquito's ecology very similar to Aedes aegypti, which is another prominent disease vector for humans. C. quiquefasciatus is responsible for a substantive burden of lymphatic filariasis in South Asia.

Anopheles Mosquitoes

Anopheles gambiae

Anopheline mosquitoes are quite heterogeneous and so exhibit tremendous differences across species in their preference of vertebrate hosts, biting and resting behavior, and selection of sites for oviposition.

Of course, the anopheline mosquitoes require the same transition through four stages of development to complete the life cycle. Here is a comparison between three mosquito genera, 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.

Most anophelines feed during the night, but some will also feed during the dusky hours of morning and evening. As with with C. quinquefasciatus, the night biting preference of anophelines means that humans are at greatest risk of infection during sleep, when we are at our most vulnerable. Some anopheline species are zoophilic, while other species are anthropophilic. Some anopheline mosquitoes are endophagic, prefering to take their blood meal indoors. 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. Adding further heterogeneity to this genus, some anopheline species 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. 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 one of the most important vectors for transmitting lymphatic filariasis in sub-Saharan Africa. 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 vector of W. bancrofti. 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

Aedes Mosquitoes

Aedes aegypti mosquito

A. aegypti mosquitoes have 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 adapted to the human environment. 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 is anthropophilic so its preferred host is humans.

While, A. aegypti is a vector of lesser importance for W. bancrofti across southeast Asia, Aedes polynesiensis is a very important vector of W. bancrofti in the South Pacific Islands of Polynesia, and shares similar characteristics with A. aegypti.

Mansonia Mosquitoes

Adult Mansonia Mosquito

Mansonia mosquitoes are the primary vectors of brugian filariasis, and they are partial to stagnant water, typically in swamp, marsh, or rice field habitats. Mansonia mosquitoes have adopted a unique exploitation of these specific aquatic environments during larval development. The larvae and pupae attach and fix to the aquatic plant stems or roots below the surface of the water, acquiring oxygen directly from these plants:

The Disease. Lymphatic filariasis is comprised of a very wide spectrum of clinical disease. Most endemic infections occur in children and young adults who are asymptomatic. This level of infection is referred to as asymptomatic microfilaremia.

Acute clinical disease can present in several forms. The most common acute presentation (~97%) is acute dermatolymphangioadenitis (ADLA). Fever is a common symptom, often accompanied by some lymphedema at a site (or sites) that is typically associated with lymph centers in the extremities. The draining lymph nodes swell and can become quite sore, possibly with redness and warm skin at the affected area. Depending on the volume of infection and the degree of lymphedema associated with the acute episode, lymphangitis, lymphadenitis and cellulitis are possible sequelae in ADLA. Interestingly, ADLA attacks are largely due to secondary bacterial infections. With high grades of lymphedema, affected limbs are more susceptible to breaches in the skin due cuts, drying cracks, or fungal infections, especially between the toes. Invading bacterial pathogens take advantage of the comprised skin integrity and cause additional infection secondary to the filariasis, ultimately leading to persistent cycles of ADLA. 

Acute filarial lymphangitis occurs when the adult worms die in the host, either naturally or by pharmacological treatment. Nodules form at the lymph centers where the worms die. Lymph nodes can become swollen and painful, and large tracts of the lymphatic system can actually stand out under the skin due to the inflammatory response that follows the death of the worms. These acute episodes are rare, and typically do not present with fever as does ADLA, and neither are they associated with secondary bacterial infection.

Elephantiasis is the primary chronic manifestation of lymphatic filariasis and is strongly associated with ongoing lymphedema and the cycles of ADLA described above. This clinical manifestation is responsible for extraordinary disfigurement and disability in the host. The progression of lymphedema is graded as follows:

Grade 1: Pitting edema, reversible on elevation of the affected limb

Grade 2: Pitting or non-pitting edema that does not reverse on elevation of the affected limb; no skin changes

Grade 3: Non-pitting edema that is not reversible; thickening of the skin

Grade 4: Non-pitting edema that is not reversible; thickening of the skin, with nodular or warty excrescences:

With progressive chronic infection and advanced lymphedema, thickening of the skin also advances until extensive folding occurs in concert with increasing nodular development, and ulceration, ultimately leading to severe disability and immobility:

Hydrocele is another common clinical manifestation of chronic infection with W. bancrofti. This condition results from the accumulation of fluid in the serous membrane surrounding the testes. Swelling increases over the period of the chronic infection and, when left untreated, leads to very large growth of the scrotum. This can be very painful and another severe impediment to mobility:

The pathogenesis of these filarial helminths in the lymphatic system begins with dilation of the lymph vessels where the adult worms are located. This early, often asymptomatic, damage seems to be present among individuals with adult worms as well as those with only microfilariae present. With extended exposure to the worms, the vessel damage proliferates throughout the lymphatics of the affected limb(s). In high volume chronic infection, occlusion of the lymphatic vessels plays a role in the progression to the more profound clinical manifestation of elephantiasis. However, secondary bacterial infection, as described above, likely plays the most important role in the progression of lymphatic filariasis.

The Epidemiology and the Landscape. There are approximately 120 million prevalent infections with the filarial worms that cause lymphatic filariasis, most of which are W. bancrofti. These infections occur in at least 83 countries, where it is estimated that over 1 billion people are at risk for infection. There are approximately 40 million people who experience severe disability due to their lymphatic filariasis. Approximately 1/3 of the cases occur in Africa, 1/3 occur in India, and the remaining 1/3 occur throughout Southeast Asia, the Pacific Islands, and in the Americas. Just four countries alone, India, Bangladesh, Nigeria, and Indonesia, account for 70% of the world's total infections. The map below by the World Health Organization shows the countries that are endemic for lymphatic filarisis:

However the map below, produced by the CDC, depicts a more specific distribution of lymphatic filariasis as it occurs in endemic countries:

The disability-adjusted life years associated with lymphatic filariasis are quite significant:

Age-standardised disability-adjusted life year (DALY) rates from Lymphatic filariasis by country (per 100,000 inhabitants).
   no data
   less than 10
   more than 500

Control and Prevention. Unfortunately, the transmission of lymphatic filariasis is not limited to one vector. Rather, there are several genera of mosquitoes capable of transmitting infectious larvae to humans, and these mosquitoes demonstrate extraordinarily different behaviors in the widely varied landscapes they inhabit and ecologies they exploit. As such, while vector control should never be ruled out as a means of blocking transmission, there can also never be a universal approach to vector control that can be expected to be effective. Instead, any such vector control strategies must be hyperlocal, considering the specific mosquito species that transmit infection in the endemic local landscape.

The primary Aedes (A. polynesiensis and A. aegypti) and Culex (C. quiquefasciatus) mosquito vectors 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 lymphatic filariasis is endemic. Pesticides can be used, 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 expect some results in transmission reduction where these species are important vectors.

The logistics of Anopheles vector control are not as straightforward. 

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 lymphatic filariasis 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, and is also relevant to the control of lymphatic filariasis. 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 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 the net.

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. However, residual spraying can be quite effective against A. aegypti and C. quinquefasciatus, which are more likely to reside in or close to the human residence. Targeting of exophilic anopheline mosquitoes is more difficult because potential resting surfaces are much more dispersed in the outside environment.

Chemotherapy/chemoprophylaxis is another major component of control and prevention of lymphatic filariasis. Indeed, while vector control by itself, or in concert with de-worming drugs, is implemented in some settings, prophylaxis has become the foundation for most elimination efforts. This is largely due to the problems described above with widely varied vector control initiatives requiring hyperlocalized implementation to interrupt transmission in distinct ecologic niches. The Global Programme for the Elimination of Lymphatic Filariasis (PELF) was launched following the resolution of the World Health Assembly to eradicate this disease. Treatment and chemoprophylaxis with one of two drug regimens comprise the primary approach to this initiative. The first regimen is albendazole plus diethylcarbamazine, and the second is albendazole plus ivermectin. The implementation of these regimens, without specific universal vector control (although vector control in specific geographic locations can often supplement the de-worming campaigns), constitute the current major global eradication initiative under PELF.


  1. Although this was a little scary because of the sample pictures but it is the reality. This can be happen if we are not aware on that issue. This is the meaning of how importance of having a clean surrounding.

  2. I think a lot of infectious diseases are transmitted with various species of mosquitoes acting as vectors including flilariasis, malaria etc and the prudent thing is to break the cycle that propagates the growth of harmful organism by having clean surroundings with no pool of water collection. However, I think because of the limited infrastructure and resources, it is more common in developing countries.

  3. Of the many things about Schistosomiasis that are notable, one that attracts my attention is that the cerceriae do not require a broken skin to penetrate the definite host, it can do that through the intact skin, unlike leptospirosis. Does it probably mean that the cerceriae have a higher probability of penetrating the host once it comes in contact with them?

    Abhishek Kansari

    1. Thats a really interesting question. I think understanding the mechanisms that guide a parasite's movement toward a host would also be helpful in comparing differences in risk of contraction. Are they attracted by blood, sweat, other host specific molecules, or maybe heat?

    2. i don't think that the virulence of the organism is directly linked to its mode of entry in the host but rather to the immune system of the host....

  4. most of the cases of lymphatic filiaris are in third world country where poor sanitation continues to be a problem, these poor sanitaries conditions are of course man made. If each one of us do what we are supposed to do we can definitely protect ourselves from a lot of those diseases cause by unsanitary conditions. I was wondering if there was a screening test for lymphatic filiaris before the symptoms appear

    1. I agree, the conditions are man-made but the conditions are also a result of poverty. When a family has barely enough to survive, it would be hard to save money to buy materials to build a toilet. Also non-government organizations that do build toilets in these areas may not be able reach the remote places and so the problem persists.

    2. Stephanie SaettaJune 22, 2012 at 3:47 PM

      I think it is important to recognize that lymphatic filariasis is less a condition of poor sanitation and more so a condition created by environmental disruption. The conditions that many of the vectors for the disease thrive under are created by increases in population, agriculture, deforestation etc. How could we change farming practices, for example, to impact presence of vectors?

    3. Lymphatic filariasis has been identified as one of the 7 diseases prioritized for eradication by the CDC. I think Ricardo makes a good point about prevention; bringing communities together could help interrupt the cycle of infection. The policy-level challenges include capitalizing on existing community structures (tribal, religious, etc.) to implement interruption strategies.

      Some authors ( have identified multiple drug therapy as an effective interruption strategy. Strategies like these rely heavily on using existing community structures to generate widespread behavior changes that might prevent further infection.

    4. Stephanie makes a great point. It's easy to cite poverty as the main contributor to Lymphatic filariasis, but as Dr. Walsh and Stephanie pointed out, issues of environmental disruption also play a significant role. Of course, impoverished areas are also the areas that are the least likely to have governments that can/will take an active role in regulating development. However, there can be education campaigns to reduce the areas of standing water around residential areas. Obviously it is not a cure-all, but it's a step. Instead of focusing on what can't be done, interventions that are both feasible and economical should be more seriously considered.

  5. During college, I worked a summer in Rwanda in the public health field. As a part of our health needs assessment there, I shadowed and observed some operations in the local hospital and the first one I saw was a hydrocele operation. I had no idea it was linked to this infectious disease. I would imagine that the severe cases with visible changes in anatomy would pos an heavy disease burden in the social and functional context as well.

  6. In India, and I am sure in many developing countries, there are water cuts, specially in the summer months. People thus collect and store water in plastic buckets or metal containers to last for the entire day. These containers can very well serve as a 'fertile' domain for the Aedes mosquitoes to lay eggs and mature. In such instances, control measures become difficult. In times of water shortage or cuts, collection and storage of water becomes essential as it is needed for most of the basic human needs.

  7. I agree with Abhishek. Many countries still use plastic buckets or big jars to contain water for later usage. Maybe in these developing countries they should start publicizing using canopy nets when they sleep to reduce their risk for mosquitoes bites. Additionally, there should be a push to educate and train the public about early symptom signs for lymphatic filariasis to seek for medical attention.

  8. Russel Sharif (MPH)October 12, 2012 at 1:15 AM

    While I was reading different websites, I saw a news about Lymphatic Filariasis which I want to share with you. "Scientists at the Liverpool School of Tropical Medicine have proved that a single course of one antibiotic may hold the key to curing the parasitic worm disease Elephantiasis (lymphatic filariasis) that has been one of the most common causes of global disability since Biblical times. The discovery offers the first new treatment for this distressing disease for decades".

  9. This disease is alive and well in the United States. Just not discussed.

  10. I'm scared of getting bit and developing that stuff. I would totally off myself, because I can't live in pure misery by carrying all that huge legs or arm if it was infected, then people looking at you as if you were in a freak show! I feel really sorry for those people!

  11. If you look at the pictures of the patients with elephantiasis, their lower limbs are extremely swollen while their upper bodies are relatively normal. I looked online at more photos of patients with elephantiasis, and this seems to be a trend. Given the extensive network of superficial and deep vasculature in the legs, along with the effects of gravity, and it makes sense that the lower body is disproportionally affected by filariasis infection.

  12. It is interesting to note that mosquitoes serve as transmitters of various diseases not limited to malaria. Further research shows that Elephantitis is more intense in people who have never been exposed to lymphatic filariasis than it is in the native people who live in the tropical areas where this disease is prevalent. It is advisable to foreigners traveling to countries where lymphatic filariasis is prevalent to avoid mosquito bites by wearing protective clothing and netting.

    1. I agree Lincy. I find it fascinating that a parasite can use our lymphatic system to its advantage. This system is often put on the back burner compared to the GI system or the cardiovascular system. I tend to think of the lymphatic system as this drainer of excess fluid, but these worms find a niche there. And with the variety of mosquitoes that act as a vector for this parasite, it is no wonder the estimated number of people at risk for infection is so high. I would think that the “benefit” of sharing these common vectors with the pathogens that cause malaria and dengue would be that interventions aimed at controlling the latter two should essentially also reduce the incidence of lymphatic filiarisis. However, because reducing malaria and dengue incidence is such a daunting task, this sharing of vectors is problematic as well.

  13. Throughout my childhood I constantly heard about the dreaded elephantitis disease; now corrected to its proper name elephantiasis and that it is an advanced stage of Lymphatic Filariasis. I consider the Culex quiquefaciatus, one of the more important vectors in the life cycle of the W. bancrofiti to be a dangerous one. This mosquito prefers to lay eggs in dirty water that may contain pathogens of unknown pathogenicity. It is interesting and disturbing to learn that the primary nematode W. bancrofiti, that is responsible for the majority of infections, use humans as a reservoir. Also disturbing but fascinating, is the tag-team approach by the mosquito that is required for W. bancrofiti to reach its infectious state. The W. bancrofiti microfilariae is surprisingly crafty, changing positions in the human body to acquire maximum potential for infection. During the day, they go to the lung arterioles where they are essentially protected since their benefactor; the mosquito is participating in other daily mosquito activities other than taking blood meals from humans. Also, where better to get the most oxygen exchange than in the lungs? At night though, they are on the move, when mosquitoes are on the prowl, and are more likely to take a blood meal from the unsuspecting human. This most likely increases their chance of being infected greatly, (exponentially?) since they travel in the blood just under the skin. If not infected, the W. bancrofiti life cycle stops. The challenge is to develop effective strategies to control mosquitoes in the various regions of the world where the nematode and the mosquito collaborate to create Lymphatic Filariasis as suggested by the scientist. We need help, and we have to look for other methods to control the mosquito population while minimizing damage to the ecosystem.

    1. I understood that the mosquito responsible for lymphatic filariasis feeds during the day.

  14. Mosquitos have so many varied lifecycles, habitats, targets, and behavior, that elimination of mosquitos clearly cannot take a universal approach. There are so many different ways that people are working to reduce mosquito populations in human habitats, I wonder how much of a discussion is going on as to the collateral effects of different interventions. With so many options, I think it’s an important consideration.
    Solutions that are heavy on pesticides, and on chemotherapeutics, seem more likely to have potentially lasting negative affects, whether those be negative ecological impact, adaptive pressure on the mosquito or the helminthes to develop resistance, or even potentially a negative impact on the relationship between communities and aid organizations. By contrast, things like controlling water sources, or limiting the clearing of forests, seem as though they could have lasting positive impact. “Labor intensive” is so often used as a pejorative, but what it could mean is a real contribution to employment in a community that also helps to combat vector born disease. It seems like this is too often discounted when we look at so-called costs in public health. Moreover investigating why people are clearing forests may force us to confront issues of poverty and inequity in the regions where vector born disease are a major problem. Rather than throwing our hands up and saying “well these problems are too complex” we might point to these issues and say “not doing this is just putting a Band-Aid over a much larger issue”

  15. I noticed that much like Dengue, Aedus Aegypti are a viable vector for transmission of the helminth that carries Lymphatic Filariasis. What is it about A. Aegypti that makes it such a good vector for a multitude of different infections? Are they much more adaptable than other species of mosquitoes or is it just their preference for water has helped them live amongst humans?

  16. Hey David,
    That's a great question. I think we can certainly say that their preference for standing water and human blood count for quite a bit of their importance as disease vectors. It seems like a chicken and egg situation, to a degree: is A. aegypti a great disease vector because it has adapted to human habitats, or did it adapt to human habitats because it is such a good vector and these areas are where it can thrive? I'm sure it's a little bit of both, but it's a really interesting thought experiment.
    Of course, there's also the other aspect of your question asking about biological factors that make it able to transmit so many diseases. I would love to know this, too. Is there something about A. aegypti that makes its immune system easily subverted by various bacteria, viruses, and parasites? Or, as you were asking, is it simply that they have become so powerful from living among humans that the infectious agents have great survival opportunities in it?
    If there is a biological factor that makes A. aegypti a good vector for multiple diseases, I wonder if there would be some way to genetically engineer a subspecies that is unable to pass these diseases. Over time, would it be possible to eliminate pathogens by making them incompatible with their vectors? Would these genetically modified mosquitoes be more fit for survival and therefore replace the unmodified ones?
    Update: No way! The Wikipedia page for Aedes aegypti (forgive the source) has a section about a genetically modified version that requires tetracycline for maturation, which means that the mosquito will not be able to mature in the wild. It's not exactly what we were talking about when we were discussing modifying the mosquitoes to make them less susceptible to infection, but it's a similar idea. Very, very cool. Check out:

  17. I found it quite interesting that part of the article focused on the use of "natural" predators for vector and pathogen control. Usually, when I think about infectious disease control, I think more about environmental solutions, such as chemical spraying and elimination of stagnant water. However, these solutions can be dangerous and difficult to do. Chemicals may be toxic, and eliminating all stagnant pools may be unfeasible depending on the environment. Use of predators seems like a safer, more long-term solution to infectious disease control. However, it's important to note that this solution can be dangerous if foreign predators are used.

    Treatment for elephantiasis seems very complex. It involves DEC, albendazole, ivermectin, steroids, anti-inflammatories, appropriate timing, appropriate dosages, and good observation of the person undergoing treatment. I'm curious to know, how accessible and successful are these treatments in the populations in which elephantiasis is endemic? In addition, are the side effects of such treatments an issue to these populations?

  18. We seem to be in a catch 22 here from an environmental standpoint. Deforestation is causing more sunlight to create ideal breeding grounds for mosquitoes. Chemical spraying can have detrimental results on both beneficial insects and humans as well. Engineered chemicals are expensive thus making it cost prohibitive. Rainwater collection is often essential for survival yet a great habitat for mosquito breeding. Public infrastructure is also quite expensive so eliminating stagnant pools through proper irrigation is a pipe dream. For now, it still seems that a combination of low tech solutions like mosquito nets,fostering natural predators like bats and burning animal dung may be the only practical solution in resource poor environments. It seems that looking into vector control as opposed to the pathogens themselves may ameliorate several diseases not just Filariasis


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