Preserve and protect? Exploring mosquito communities in urban mangroves

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This is a special guest post from Dr Suzi Claflin. Suzi found herself in Sydney, Australia, (via Cornell University, USA) in 2015 to undertake a research project investigating the role of urban landscapes in determining mosquito communities associated with urban mangroves. She was kind enough to put this post together to celebrate the publication of our research in Wetlands Ecology and Management!

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Sometimes you’ve got to make hard choices for the greater good. These situations can arise anywhere, but here – as usual – we are concerned with mosquitoes. There’s a balancing act carried out by public health officials and wetland managers trying to both preserve endangered habitat and protect human health. In this guest post, I’ll explain the science behind research I recently published in collaboration with Dr Cameron Webb, and suggest one way forward for addressing human and environmental health concerns in urban wetlands.

During my PhD, I studied how the landscape surrounding small-scale farms affects the spread of a crop virus and the community of insect pests that carry it. When I came to Australia to work with Cameron, I was surprised to find myself applying the same type of landscape ecology to mosquitoes and mangroves in urban Sydney.

The misfortune of mangroves

Mangroves are real team players. They provide a range of services to the surrounding ecosystem and to the humans lucky enough to live near them. Mangroves are extremely effective at protecting the shoreline (but this can sometimes be a problem). They prevent erosion by gripping the soil in their complex root systems and buffer the beach by serving as a wave break. By filtering sediment out of the water that flows over them, mangroves also prevent their neighbouring ecosystems, such as coral reefs and seagrass forests, from being smothered.

Despite all their good work, mangroves have an almost fatal flaw; they prefer waterfront property. Unfortunately for them, so do humans. Urban and agricultural development has eaten away at mangroves, leaving them highly endangered.

The mosquito menace

Mozzies are a public health menace, because they spread human diseases like Ross River virus (RRV). Because of this, public health officials rightly spend time considering how to supress mosquito populations in order to reduce the risk of disease transmission.

Here’s where things get tricky: mangroves are great for mosquitoes.

That leaves public health officials and wetland managers in a difficult position. On the one hand, mangroves are delicate, at-risk ecosystems that need to be preserved. On the other, mangroves and surrounding habitats potentially harbor both the animal carriers of the RRV (e.g. wallabies) and a load of mosquitoes, which means that people nearby may need to be protected.

How can we do both?

 

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Dr Suzi Claflin trapping mosquitoes in the mangroves along the Parramatta River, Sydney, Australia.

 

The potential power of prediction

This is a hard question to answer. One approach is prediction: using measurements of the environment, like rainfall and tide level, to estimate what the mosquito community will look like in a given region. The mosquito community determines what management actions, like spraying an insecticide, need to be taken, based on the threat it poses to public health.

We set out to explore how the way we use land (e.g. for residential areas or industrial areas) near urban mangroves affects the mosquito communities that live in those mangroves. The project involved dropping over retaining walls, slipping down banks, and tromping through muddy mangroves along the Parramatta River in Sydney. We set mosquito traps (billy cans of dry ice with a container on the bottom) and left them overnight to capture the mozzies when they are most active. We did this at two points in the summer, to see if there was any change over time.

We found that yes, the way we use land around a mangrove makes a difference. Mangroves with greater amounts of bushland and residential land in the surrounding area had fewer mosquitos, and fewer species of mosquitos. On the other hand, mangroves with greater amounts of industrial land surrounding them had a greater number of mosquito species, and those surrounded by greater amounts of mangrove had more mosquitos.

And, just to muddy the waters a bit more (pun intended), several of these relationships changed over time. These results show that although prediction based on the surrounding environment is a powerful technique for mangrove management, it is more complicated than we thought.

Another way forward: site-specific assessments

Our work suggests another way forward: site-specific assessments, measuring the mosquito community at a particular site in order to determine what management approaches need to be used. This is a daunting task; it requires a fair number of man-hours, and mangroves are not exactly an easy place to work. But it would be time well spent.

By assessing a site individually, managers can be confident that they are taking the best possible action for both the mangroves and the people nearby. It turns out that the best tool we have for striking a balance between environmental and public health concerns, the best tool we have for preserving and protecting, is information. In mangrove management—as in everything—knowledge is power.

Check out the abstract for our paper, Surrounding land use significantly influences adult mosquito abundance and species richness in urban mangroves, and follow the link to download from the journal, Wetlands Ecology and Management:

Mangroves harbor mosquitoes capable of transmitting human pathogens; consequently, urban mangrove management must strike a balance between conservation and minimizing public health risks. Land use may play a key role in shaping the mosquito community within urban mangroves through either species spillover or altering the abundance of mosquitoes associated with the mangrove. In this study, we explore the impact of land use within 500 m of urban mangroves on the abundance and diversity of adult mosquito populations. Carbon dioxide baited traps were used to sample host-seeking female mosquitoes around nine mangrove forest sites along the Parramatta River, Sydney, Australia. Specimens were identified to species and for each site, mosquito species abundance, species richness and diversity were calculated and were analyzed in linear mixed effects models. We found that the percentage of residential land and bushland in the surrounding area had a negative effect on mosquito abundance and species richness. Conversely, the amount of mangrove had a significant positive effect on mosquito abundance, and the amount of industrial land had a significant positive effect on species richness. These results demonstrate the need for site-specific investigations of mosquito communities associated with specific habitat types and the importance of considering surrounding land use in moderating local mosquito communities. A greater understanding of local land use and its influence on mosquito habitats could add substantially to the predictive power of disease risk models and assist local authorities develop policies for urban development and wetland rehabilitation.

Dr Suzi Claflin completed her PhD at Cornell University exploring environmental factors driving the spread of an aphid-borne potato virus on small-scale farms. She is now a postdoctoral research fellow at the Menzies Institute for Medical Research in Hobart, TAS. In her spare time she runs her own blog, Direct Transmission, focusing on disease and other public health issues (check it out here). To learn more about her doctoral research, follow this link!

Could a mosquito bite make you a Jedi?

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On the eve of the release of Star Wars The Force Awakens, I’ve been revisiting bits and pieces of the previous six Star Wars movies. You know what stands out? I’ve been thinking about the possibility that wielding “The Force” may just be a symptom of parasitic infection!

Growing up with the original trilogy of Star Wars films, my perspective on “The Force” was entirely defined by Obi-Wan Kenobi’s explanation to Luke in A New Hope, “The Force is what gives a Jedi his power. It’s an energy field created by all living things. It surrounds us and penetrates us. It binds the galaxy together.”

I always thought of it as more a spiritual, rather than a scientific, thing. If I thought about it really hard, I could snatch that tv remote into my hand from across the room. Apparently it doesn’t quite work that way. I’d need to be infected with a parasitic pathogen. Midi-chlorians.

Midi-chlorians were explained to a young Anakin Skywalker by Jedi Master Qui-Gon Jinn in The Phantom Menace as “Midi-chlorians are a microcopic lifeform that reside within all living cells and communicates with the Force. We are symbionts with the midi-chlorians. Life forms living together for mutual advantage. Without the midi-chlorians, life could not exist, and we would have no knowledge of the Force. They continually speak to you, telling you the will of the Force.”

So, these intracellular microbes speak to us? Do they control the host? Are Jedi just doing the dirty work of midi-chlorians? I know midi-chlorians have sparked a sea of frustration among some Star Wars fans but lets just assume for a moment that the only reason Force users can do what they do is because they’re infected with special strains of these parasites.

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How many times have you imagined you could snatch the remote from coffee table through nothing more than concentrated thought alone?

Can pathogens and parasites control their hosts?

We know that in a galaxy not so far away and not so long ago, parasites have been described that alter the behaviour of their hosts. See here  and here and here. From mind-controlling fungi to parasites that break down your fear of predators there are plenty of examples of how parasites change the behaviour of hosts to their own benefits.

There are insect symbiotes and mostly they’re beneficial. But when it comes to mosquitoes, microbes including viruses, bacteria, fungi and protozoa can influence their role in disease outbreaks.

Viruses, such as dengue, may even influence the behaviour of mosquitoes to increase the chances of their transmission to new hosts. Infection with a virus may also change the way mosquitoes respond to insect repellents. But when some mosquitoes are infected with an intracellular bacteria, a bacteria that’s not naturally found in this mosquito, their ability to transmit dengue viruses is blocked. It can also disrupt their blood feeding and reduce their lifespan.

Now, if mosquitoes are capable of transmitting blood-borne pathogens, could they also transmit midi-chlorians? If a mosquito bites you after its taken a blood meal from Yoda, Obi-Wan Kenobi or Luke Skywalker, could you start using the force? Is the ability to use the force a symptom of vector-borne disease?

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Itchy yet? Dozens of mosquitoes drinking in a mosquito researcher’s blood! (Image: Alex Wild)

Are there Medical Entomologists in the Star Wars universe?

I don’t know how many medical entomologists will be featured in The Force Awakens. According to Wookiepedia there are mosquitoes (or at least mosquito-like insects called Msqitoes) in the Star Wars universe but how would you go about working out if theses or any other blood feeding arthropods could transmit midi-chlorians between Jedi and us normal non-Jedi folk?

When investigating outbreaks of suspected mosquito-borne disease, there is a number of things to consider. Are there clusters of cases that may suggest localised exposure to mosquitoes? If there is widespread disease with little or no link to environments associated with mosquitoes, it is usually enough to suggest the pathway of pathogen transmission doesn’t include mosquitoes. But where there is evidence to suggest mosquitoes are playing a role, where to next?

Collecting and testing field collected specimens can provide some evidence. There are mosquito-borne pathogen surveillance programs in many parts of the world. They rely on collecting and testing adult mosquitoes but can also involve taking blood from wild or domestic birds to determine the circulation of mosquito-borne pathogens in the environment. However, just because a mosquito tests positive to a pathogen, it doesn’t mean it is spreading it to people.

There are thousands of mosquito species worldwide but relatively few effectively transmit mosquito-borne pathogens. For example, in Australia around 40 mosquitoes have been implicated in the transmission of Ross River virus but only one mosquito can transmit dengue viruses. Unless there is a specific relationship between the pathogen and the mosquito, the pathogen wont be transmitted.

Complex path of pathogens from host blood to mozzie spit

Mosquitoes aren’t like dirty syringes. They don’t transfer infected droplets of blood between people. Mosquitoes must become infected with the pathogen before it can then pass it on. Taking a virus as an example, the virus must be ingested by the mosquito together with a blood meal from an animal and then the virus must pass into and out of the cells lining the insect’s gut before spreading throughout the body of the mosquito. Once the salivary glands are infected (a process that takes about a week), the virus is passed on through the spit of the mosquito when it next bites (when feeding on an animal, a mosquito will inject some saliva to get the blood flowing).

Not all mosquitoes can transmit all mosquito-borne pathogens. Mosquitoes cannot transmit non-mosquito-borne pathogens. That’s why mosquitoes don’t spread Ebola virus.

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For a mosquito researcher, surely Dagobah is top of the list of fantasy field trip destinations! There has to be plenty of mozzies on this swamp covered planet! (Image: Star Wars)

How would you test the ability of mosquitoes to transmit midi-chlorians?

Firstly you need some blood feeding mosquitoes. If there was anywhere in the Star Wars universe where mosquitoes could be found, surely it is Dagobah. The whole planet is essentially covered in wetlands. Once you’ve got the mosquitoes, now all you need is some midi-chlorian-filled blood.

Expose caged mosquitoes to a mix of blood and midi-chlorians (or ask a Jedi to kindly volunteer their arm). For those mosquitoes that feed, they can be put aside and kept alive for as long as possible. Every few days, mosquitoes are removed and tested for infection. Legs and wings can be removed from the mosquito and tested separately from the body. If they’re positive, it indicates the virus has spread through the body of the mosquito.

Next, the proboscis of the mosquito can be inserted into a micro-capillary tube filled with growth media. Once inserted into the liquid, the mosquito will instinctively feed and expectorate saliva. The saliva and growth media mix can then be tested for the presence of virus. If this mix is positive, it indicates the mosquito is transmitting the pathogen.

vcexperiments

Known as “vector competence” experiments, studies investigating the ability of different mosquito species to transmit different pathogens help build up a profile of each mosquito and its potential role in the spread of local and exotic mosquito-borne pathogens.

Studies of this nature have been conducted for many, many mosquito species worldwide and many, many different pathogens. Here is a recent example of a study conducted to determine how the transmission of a strain of West Nile virus by mosquitoes influenced an outbreak of mosquito-borne disease in horses.

Things can start to get a little more complicated when you take into account the interaction between the host (e.g. an animal) and the pathogen, the level of viremia that may develop and the propensity of the mosquito to bite people. For example, some mosquito species may be very susceptible to infection and effective at transmitting a pathogen but if those mosquitoes prefer to only feed on birds, the risk to people are generally much lower.

The take home message from all these studies is that the interactions between pathogens, mosquitoes and the animals and people carrying the pathogens are complex. When assessing the risk of mosquito-borne disease outbreak, designing more effective mosquito-borne pathogen surveillance programs or developing strategic public health responses, it is critical to understand the ability of local mosquitoes to transmit pathogens.

In theory, a mosquito taking a blood meal from a Jedi whose blood contains a high concentration of midi-chlorians could potentially pass on some Force-inducing infections to other people. Makes you wonder why Yoda was hanging out in a swamp planet?

Disclaimer: There is little doubt the issue of midi-chlorians has been discussed, dissected and the implications for the ability to wield the force determined within Star Wars Expanded Universe/Legends literature. I know I’ve taken a little artistic license here but if you’re a Star Wars aficionado, please take this article for what it is, a fun way to communicate the science behind mosquito and mosquito-borne disease research.

Why not join the conversation on Twitter?

I’ll leave you with this great CDC video on immunization.

UPDATE! [9 January 2016] So, it seems that not only do mosquitoes potentially help make Jedi in the Star Wars universe but, as the The Force Awakens demonstrates, mosquito-inspired aliens are out and about in the bars and cantinas of the galaxy! Just in the remote chance you haven’t seen the movie, I won’t describe when and where these creatures pop up but they’re known as the “Dengue Sisters” and represent a sentient species of small insectoids known as Culisetto. They look pretty awesome (see photo below from “Star Wars: The Force Awakens Visual Dictionary” by Pablo Hidalgo)

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Dengue Sisters from Star Wars: The Force Awakens Visual Dictionary (Thanks Cybergosh for the tip off)!

Perhaps I need to petition Hasbro to create a “Dengue Sister” action figure!